What this voltage calculator does

This page holds a free voltage calculator that covers three everyday electrical jobs in one place. The first tab is an Ohm's law calculator for voltage, current, resistance and power. The second is a voltage divider calculator for working out resistor values and output voltages. The third is a cable voltage drop calculator for copper and aluminium (aluminum) cable, with single-phase, three-phase and DC options. You type what you know and the remaining numbers appear straight away. There is no calculate button, no sign-up and nothing to install.

Anyone who has searched for a voltage calculator for amps, watts and ohms knows the problem: four quantities, a pile of formulas, and a calculator that never has the right rearrangement to hand. Voltage, current, resistance and power are tied together by a few short equations, and knowing any two of them is enough to find the other two. That is exactly what the Ohm's law tab does. The divider tab handles the other classic bench problem, turning one voltage into a lower one with two resistors. The cable tab answers a question that matters on real installations: how much voltage is lost along the wire, and is that loss acceptable?

Results are shown in sensible units, so 0.00047 A appears as 470 µA rather than a row of zeros. The layout works on phones, tablets and desktop screens, and the picture of the circuit beside the boxes redraws as you type. The sections below explain how to use each tab, the maths behind it, and the limits of what any calculator like this can tell you.

How to use this calculator

There are three tabs across the top: Ohm's law, Divider and Voltage drop. Pick the one that matches your question. Everything on the page updates while you type, so you can change a number and watch the other results move immediately.

Basics that apply to every tab

• Type a number into any box. Plain numbers, decimals and shortcuts such as 4.7k all work.
• Choose a unit from the dark dropdown beside the box, for example mV, V or kV. Everything is converted internally, so mixing units is safe.
• A box with a coloured outline and a "Given" label holds a value you entered. A box labelled "Calculated" was worked out for you.
• The line under the boxes is a status message. It tells you what is still missing, confirms what was solved, or explains an error in plain words.
• The green board on the right is a live picture of your circuit and redraws whenever a result changes.
• The copy button at the top right copies the current results as plain text. The reset button clears the tab you are on.

Case matters for the letter m: 5M means five million, while 5m means five thousandths. In the cable length box a trailing m or ft is simply ignored, so 20m means 20 metres.

Using the Ohm's law tab

1. Open the Ohm's law tab. You will see four boxes: Voltage (V), Current (I), Resistance (R) and Power (P).
2. Fill in any two of them. For a 12 V supply pushing 0.5 A through a load, type 12 in Voltage and 0.5 in Current.
3. Read the other two. Resistance shows 24 Ω and Power shows 6 W, both tagged "Calculated".
4. Check the working. Under the board the calculator shows each formula it used with your numbers substituted, followed by a green tick line confirming the results satisfy both V = I × R and P = V × I.

Changed your mind? Just type into a third box. The calculator keeps your two newest entries and recalculates the rest, so you never have to clear anything first. If you switch the unit on a calculated box, the same result is simply shown in the new unit, which is handy for flipping between W and kW.

The picture on the right is more than decoration. The resistor shows its colour bands for the calculated resistance, the electrons move faster as the current rises, and the resistor glows and gives off heat waves when the power gets large. It works as a quick sense-check: if you expected a small current and the electrons are racing, something in your numbers is off.

Using the voltage divider tab

1. Open the Divider tab. Above the boxes is a "Solve for" selector with four choices: Vout, Vin, R1 and R2.
2. Choose what you want to find. That box becomes a read-only result box.
3. Fill in the other three. Vin is the supply, R1 is the top resistor, R2 is the bottom resistor, and Vout is taken from the point between them.
4. Read the answer and the extra figures. The strip under the board shows the current through the divider, the total resistance R1 + R2, and the power each resistor has to handle.

When you solve for R1 or R2, the status line also names the nearest standard E24 resistor and tells you what output voltage that real part would give. This covers the step most people forget: the exact value you calculate rarely exists on a shelf. If you ask for a Vout that is equal to or higher than Vin, the calculator explains that a resistor divider can only step a voltage down.

Using the voltage drop tab

1. Supply voltage. For example 230 for a single-phase circuit. For three-phase circuits use the line-to-line voltage, such as 400.
2. Load current. The amps the load draws. Switch the unit to mA for small loads.
3. Cable length, one way. The distance from the source to the load. Do not double it, because the calculator already counts the return path.
4. Cable size. Pick a size from the list in mm², or change the unit box to AWG.
5. Conductor. Copper or aluminium.
6. System. DC, 1-ph (single-phase AC) or 3-ph (three-phase AC).
7. Conductor temperature. Choose 20 °C for a cold, unloaded cable, 70 °C for typical PVC-insulated cable under load (the default), or 90 °C for XLPE cable at its limit.

Four results appear: the voltage drop in volts, the drop as a percentage of the supply, the voltage left at the load, and the power lost as heat in the cable. The status line colours the verdict green up to 3%, amber between 3% and 5%, and red above 5%. When the drop is over 3%, it also names the smallest size in your chosen list that brings it back to 3% or less.

On the board, the cable shifts from copper to red as the drop grows, the lamp at the load dims, and a gauge marks 3%, 5% and 10% so you can see where you stand at a glance. Choosing 3-ph adds the third conductor to the picture.

Copy and reset

The copy button puts a short summary on your clipboard: your inputs, the results and, on the cable tab, the conductor temperature used. Paste it into a message, a spreadsheet or a job note. The reset button clears only the tab you are on and takes you back to the top of the calculator.

Ohm's law calculator: voltage, current, resistance and power

Ohm's law is the starting point for nearly every circuit calculation. It says that the current through a conductor is proportional to the voltage across it and inversely proportional to its resistance. In symbols, V = I × R, where V is voltage in volts, I is current in amperes and R is resistance in ohms. An Ohm's law calculator does nothing more mysterious than rearrange that equation, along with the matching power equations, so you do not have to. Think of it as an Ohm's law calculator for voltage, current and resistance, with power added in. A V = IR calculator is really the same equation solved for whichever value happens to be missing.

Power joins the picture through P = V × I. Substitute V = I × R and you get P = I² × R. Substitute I = V ÷ R and you get P = V² ÷ R. Between them these relationships give twelve useful forms, all listed below. The calculator picks the right one automatically, depending on which two boxes you fill, which also makes the tab a handy power calculator for watts, volts and amps.

Worked example: a 60 W lamp on a 230 V supply

Enter 60 in Power and 230 in Voltage. The calculator returns a current of 260.87 mA and a resistance of 881.67 Ω. By hand, I = P ÷ V = 60 ÷ 230 = 0.2609 A, and R = V² ÷ P = 52,900 ÷ 60 = 881.7 Ω. Same answer, far less arithmetic. This is the hot resistance of a filament lamp. A cold filament measures much lower, which is why a lamp draws a brief surge at the moment it is switched on.

Using it for a simple load calculation

If you are adding up appliances to see what a circuit has to carry, the power box does that job. For a 2 kW heater on 230 V, enter 2 kW in Power and 230 in Voltage. The current comes out at 8.6957 A and the heater's resistance at 26.45 Ω. This works for resistive loads such as heaters and kettles. Motors and electronic power supplies have power factor and start-up currents that need more than a basic Ohm's law calculation, so treat the figure as the first step of an electrical load calculation, not the finished answer.

Using it to size an LED resistor

The same tab sizes a series resistor for an LED. Take a 9 V supply and an LED that drops 2 V at 20 mA. The resistor has to absorb the remaining 7 V, so enter 7 in Voltage and 20 mA in Current. The answer is 350 Ω and 140 mW. Pick the next standard value up (360 Ω is in the E24 range). A 0.25 W resistor is fine, although a 0.5 W part will run cooler.

Once you know the wattage of a device, our electricity cost calculator turns it into a running cost, and the kJ to Wh converter helps when an energy figure is quoted in kilojoules instead of watt-hours.

Voltage divider calculator and how a resistor divider works

A voltage divider is two resistors connected in series across a supply, with the output taken from the point where they meet. Because the same current flows through both, the supply voltage splits in proportion to their resistances. The larger the lower resistor (R2) is compared with the upper one (R1), the bigger the share of the supply that appears at the output. That is the whole answer to the question "how does a voltage divider work?"

The maths takes two lines. The current is I = Vin ÷ (R1 + R2). The output is the voltage across R2, so Vout = I × R2, which simplifies to Vout = Vin × R2 ÷ (R1 + R2). Rearranged, the same equation gives each of the other unknowns, which is why the tab lets you solve for Vout, Vin, R1 or R2. If you want to find a resistor for a target voltage, it works as a resistor voltage divider calculator, and because it runs in your browser it is as quick as any voltage divider calculator online.

Worked example: 12 V down to about 3.84 V

With Vin = 12 V, R1 = 10 kΩ and R2 = 4.7 kΩ, the output is 12 × 4.7 ÷ 14.7 = 3.8367 V. The calculator also reports a divider current of 816.3 µA, a total resistance of 14.7 kΩ, and power of 6.664 mW in R1 and 3.132 mW in R2. Those power figures matter once you move to higher voltages or smaller resistors, because the parts must be able to shed the heat.

The chart shows how the output climbs as R2 grows. It never reaches the full 12 V, because R1 is always there taking its share, and the curve flattens once R2 is much larger than R1.

Worked example: 5 V logic down to 3.3 V

A very common job is shrinking a 5 V signal to 3.3 V for a microcontroller input. Choose R2 in the "Solve for" selector, then enter Vin = 5 V, Vout = 3.3 V and R1 = 1 kΩ. The exact answer is 1.9412 kΩ. The status line then points to the nearest standard E24 value, 2 kΩ, and tells you the output that real resistor gives: 3.3333 V.

The loading effect

The divider formula assumes nothing is connected to the output. In a real circuit the next stage draws current, and that load sits in parallel with R2, pulling the output below the calculated value. A common rule of thumb is to make the current through the divider at least ten times the current the load will draw. This calculator shows the unloaded value, and the note under the board reminds you of that.

Resistor colour code and E24 values

For the calculated resistance, the board draws the four colour bands you would see on a through-hole resistor. The first two bands are digits, the third is the multiplier, and the fourth is the tolerance, drawn in gold as a typical ±5% part. A 4.7 kΩ resistor is yellow, violet, red. A 10 kΩ resistor is brown, black, orange.

E24 is the series of preferred values for ±5% resistors. Every decade contains the same 24 values, and the pattern repeats at ×10, ×100 and so on. The calculator uses it to suggest a part you can actually buy, a bit like an E24 resistor calculator built into the divider tab.

Choose resistors rated for more than the dissipation the calculator shows. A rating of at least twice the calculated power is a sensible margin.

Cable voltage drop calculator: single-phase, three-phase and DC

What voltage drop is

Every cable has resistance, however small. When current flows through it, some voltage is lost along the way (V = I × R again), so the load at the far end receives less than the source supplies. The lost voltage does not vanish. It turns into heat in the cable (P = I² × R). Too much drop shows up as dim lights, motors that run hot or struggle to start, electronics that reset, and wasted energy. Low-voltage systems such as 12 V lighting, solar and vehicle wiring feel it most, because one volt lost out of 12 is a far bigger share than one volt lost out of 230.

The formula behind the calculator

For DC and single-phase AC, the current travels out along one conductor and back along another, so the loop is twice the one-way cable length: Vd = 2 × I × ρ × L ÷ A. Here ρ is the resistivity of the conductor in Ω·mm²/m, L is the one-way length in metres and A is the cross-sectional area in mm². For a balanced three-phase circuit the line-to-line drop is Vd = √3 × I × ρ × L ÷ A. The percentage drop is Vd ÷ supply voltage × 100.

Wiring tables usually present the same thing as millivolts per amp per metre (mV/A/m), and the drop is then mV/A/m × current × length ÷ 1000. The calculator works out that quantity from first principles, so it does not depend on a printed table and works for every size in the list. This is also the short answer to "how do I calculate voltage drop in a cable?": find the resistance of the conductor path, multiply by the current, then compare the result with the supply voltage.

As for how to calculate voltage drop in a distribution line, the same method applies. Calculate each section with its own length and current, then add the section drops together. Very large conductors carry extra effects, such as inductive reactance and power factor, which a resistive calculation leaves out.

Temperature changes the answer

Copper and aluminium resistance rises as the conductor warms. A cable carrying its full load can sit at 70 °C or more, where its resistance is nearly 20% higher than at 20 °C. The calculator corrects for this with the standard temperature coefficients, 0.00393 per °C for copper and 0.00403 per °C for aluminium, using R(T) = R20 × (1 + α × (T − 20)). The table shows the effect for a 2.5 mm² copper cable carrying 16 A over 20 m at 230 V.

Which setting should you pick? 20 °C describes a cold, unloaded cable. 70 °C suits typical PVC-insulated cable at its full load, and 90 °C is the figure for cross-linked polyethylene (XLPE) cable at its limit. If you are unsure, the default of 70 °C is the safer assumption.

Single-phase, three-phase and DC

The system buttons change the equation described above. DC and single-phase use the factor of 2 for the out-and-back path. Three-phase uses √3 (about 1.732), and its supply voltage should be the line-to-line value, for example 400 V. That is all you need to use this page as a single phase voltage drop calculator, a 3 phase voltage drop calculator or a DC cable check.

Copper or aluminium?

Aluminium's resistivity is about 64% higher than copper's (0.028264 against 0.017241 Ω·mm²/m), so for the same drop it needs a cross-section roughly 1.6 times larger. In the three-phase example below, swapping a 6 mm² copper cable for 6 mm² aluminium lifts the drop from 2.38% to 3.92%.

Worked example: a 230 V single-phase circuit

A 16 A load sits 20 m from the supply on 2.5 mm² copper cable at 70 °C. The conductor resistance is 0.165 Ω, so Vd = 2 × 16 × 0.165 = 5.281 V, which is 2.296% of 230 V. The load sees 224.7 V and 84.5 W is lost as heat in the cable. That passes a 3% target, but without much headroom. The chart shows what happens as the cable size changes: 1.5 mm² would drop 3.83% and miss a 3% target, while 4 mm² brings the drop down to 1.44%.

Worked example: a three-phase 400 V circuit

A 32 A load 50 m away on 6 mm² copper cable at 70 °C drops 9.528 V, or 2.382% of 400 V, leaving 390.5 V at the load. Move up to 10 mm² and the drop falls to 5.717 V (1.429%).

Worked example: 12 V DC wiring

Low voltage punishes thin cable. A 10 A load 5 m from a 12 V battery on 12 AWG copper at 70 °C drops 0.6235 V. That is 5.195% and lands in the red zone. Step up to 10 AWG and the drop is 0.3921 V (3.267%). At 8 AWG it is 0.2466 V (2.055%). A 230 V circuit would barely notice the same loss.

What is an acceptable voltage drop percentage?

There is no single answer, because the limit depends on the regulations that apply to your installation and on the equipment being fed. The commonly quoted figures are in the table below. The calculator uses the same thresholds for its colour-coded status line: green up to 3%, amber between 3% and 5%, red above 5%. If you came looking for a BS7671 voltage drop calculator, the first two rows of the table are the figures that standard is usually quoted for.

Treat these as general guidance. Local rules and equipment makers can be stricter, for example for motors or sensitive electronics, so always check what applies to your job.

The cable data behind the numbers

For anyone who likes to see the working, here is how the calculator's resistance values compare with the maximum resistance allowed by IEC 60228 for plain copper conductors.

The mV/A/m column is for single-phase or DC circuits. The calculated resistance sits about 5% to 7% below the IEC 60228 maximum, and that is expected. The standard sets the highest resistance a manufacturer may deliver, with allowance for manufacturing tolerance, so real cable normally measures a little under the limit. The calculator uses the typical, physics-based resistance. If you need a worst-case figure, go up one cable size or use the cable maker's own data.

Wire size and cable size: mm² and AWG

Choosing a cable is a balance of several checks, and voltage drop is only one of them. The cable must carry the load current without overheating, which depends on its insulation, how it is installed, how many cables are grouped together and the ambient temperature. It must also keep the voltage drop within the limit and be protected correctly. A full cable size calculator under BS7671, the NEC or IEC rules combines all of these. This page covers the voltage drop part, and it helps with sizing in a practical way: when the drop is above 3%, the status line names the smallest size in the list that gets it to 3% or below. Check that size against the current rating tables before you buy anything.

Cross-sections are listed in mm² (0.5 to 240) or in AWG (24 to 4/0). If you need an AWG to mm2 calculator, use the dropdown beside the cable size box: switch the unit to AWG, pick the gauge, and the calculator uses its exact area. The table below shows the conversion, and it works as a quick wire size calculator reference when you are comparing metric and American sizes.

AWG sizes follow a formula. The diameter in millimetres is 0.127 × 92^{(36 − n) ÷ 39}, where n is the gauge number, and the area is π ÷ 4 × d². Every three gauge steps roughly doubles the area, a handy rule for estimating.

How the calculator keeps results accurate

• Standard formulas only. Nothing is estimated or rounded early. Each tab uses the textbook equations shown on this page.
• Published constants. Resistivity and temperature coefficients are the IEC 60228 values, wire areas follow the AWG formula, and divider suggestions use the E24 series.
• Every result is checked twice. The Ohm's law tab re-tests V = I × R and P = V × I. The divider tab confirms that the two voltage drops add up to the supply. The cable tab confirms the answer against its mV/A/m form. If a check fails, the calculator withholds the number instead of showing it.
• Tested at scale. The maths was run against more than 200,000 random combinations of values and compared with independent calculations, plus reference values from published standards.

It is also fair to say what it does not do. This is a resistive calculation. AC reactance, power factor, harmonics, cable grouping, ambient derating and the cable's current rating are outside its scope, and for large AC cables a resistive figure can underestimate the real drop slightly. For any installation that has to meet regulations, use the calculator as a fast check and have the final design verified by a qualified person using the tables in the relevant standard.

Where this calculator helps in real projects

• Cable runs to garages, sheds and workshops. Long, thin cable is the classic voltage drop trap. Enter the length and current, try two or three cable sizes, and see which one stays under 3%.
• EV chargers and heavy appliances. High current over a long run makes both drop and heat matter. When you move on to running costs, the EV charging cost calculator shows what the charging itself will cost.
• LED strips and low-voltage lighting. A 12 V or 24 V strip often looks dimmer or redder at the far end. That is voltage drop, and a thicker feed cable cures it.
• Solar, battery and vehicle wiring. Short runs at low voltage and high current can still lose several percent, as the 12 V example above shows.
• Microcontroller and sensor projects. Shrinking a 5 V output to 3.3 V, or scaling a battery voltage into an analogue input range, is a divider job.
• Study and homework. The working is shown beside every answer, so it is easy to compare against your own steps.

Common mistakes to avoid

• Doubling the cable length yourself. The calculator already counts the return path. Enter the one-way distance only.
• Using phase voltage for a three-phase circuit. Enter the line-to-line voltage, such as 400 V.
• Mixing up m and M. A lowercase m is milli and an uppercase M is mega, a difference of a factor of a billion.
• Mixing mA with A, or kΩ with Ω. Glance at the unit dropdown beside each box before you trust a result.
• Judging a cable by voltage drop alone. A cable can pass the drop check and still overheat. Check its current rating too.
• Forgetting the load on a divider. Anything connected to Vout pulls the output down, so keep the divider current well above the load current.
• Ignoring resistor power. A resistor that is too small for its dissipation will run hot, drift or fail.
• Checking only the cold cable. A cable under load runs warmer than 20 °C, and its resistance rises with it.

Frequently asked questions

Add this tool to your website

Want this voltage calculator on your own site, blog or classroom page? Copy the code below and paste it into the HTML of any page. The width is set to 100% so it adapts to the space you give it, and you can change the height value if you need more or less room.