Enter any chemical formula to calculate exact monoisotopic mass, molecular weight, and full isotopic abundance breakdown.
Exact Mass Calculator
Monoisotopic mass, MS adducts and isotope pattern for any formula
Every value on this page — exact mass, isotope abundances, and adduct m/z — is calculated from atomic mass data published by NIST and CIAAW (the IUPAC body that sets official atomic weights), and run through a full isotope convolution rather than a rounded estimate. That's why the numbers here match what you'd get from professional mass spectrometry software, down to the fourth decimal.
If you work anywhere near a mass spectrometer — running LC-MS, checking a synthesis, or just trying to confirm a formula from a journal — you already know that "molecular weight" isn't the number your instrument actually shows you. The exact mass calculator above solves that specific problem: type in a chemical formula and instantly get its monoisotopic mass, its full isotope pattern, and a ready list of MS adducts, without opening a spreadsheet or digging through an isotope table.
This guide explains exactly what exact mass means, how it's calculated, how to read an isotope pattern, and — most importantly — how to get the most out of every feature built into the calculator on this page.
Quick answer: Exact mass (also called monoisotopic mass) is the mass of a molecule calculated using only the single most abundant isotope of each element — 12C, 1H and 16O, for instance — instead of the natural blend of isotopes used for average molecular weight. It's the number a high-resolution mass spectrometer is actually built to measure.
What Is Exact Mass? (Monoisotopic Mass Explained)
Every element on the periodic table exists in nature as a mix of isotopes — atoms with the same number of protons but a different number of neutrons. Carbon, for example, is about 98.93% 12C and 1.07% 13C. When chemists talk about a compound's average molecular weight, they're averaging over that natural mix. When they talk about exact mass (or monoisotopic mass), they mean something much more specific: the mass you'd get if every single atom in the molecule happened to be its lightest, most common isotope.
That distinction matters because a mass spectrometer doesn't weigh a mole of your compound — it measures individual ions, one molecule at a time. The tallest peak in a typical high-resolution spectrum corresponds almost exactly to this monoisotopic form, which is why "exact mass" is the number you match against a formula, not the average molecular weight printed on a bottle label.
| Element | Reference isotope | Natural abundance | Exact mass (Da) |
|---|---|---|---|
| Hydrogen | ¹H | 99.9885% | 1.007825 |
| Carbon | ¹²C | 98.93% | 12.000000 |
| Nitrogen | ¹⁴N | 99.636% | 14.003074 |
| Oxygen | ¹⁶O | 99.757% | 15.994915 |
| Sulfur | ³²S | 94.99% | 31.972071 |
| Chlorine | ³⁵Cl | 75.76% | 34.968853 |
| Bromine | ⁷⁹Br | 50.69% | 78.918337 |
These are the reference values the calculator above uses for every element — sourced from NIST and CIAAW, not rounded textbook numbers.
Exact Mass vs. Molecular Weight — What's the Difference?
These two numbers are close for small molecules but drift apart as a formula gets heavier, especially with more carbons, sulfurs, or halogens. Here's glucose (C6H12O6) as a side-by-side example — try typing it into the calculator above and you'll see these exact figures:
| Value | Result for C6H12O6 | What it's based on |
|---|---|---|
| Exact (monoisotopic) mass | 180.0634 Da | Lightest/most-abundant isotope of every atom |
| Nominal mass | 180 Da | Each atom rounded to its nearest whole-number mass |
| Average molecular weight | 180.1560 g/mol | Standard atomic weights — the natural isotope mix |
As a rule of thumb: use average molecular weight for weighing out reagents on a balance and making up solutions — that's a bulk, mole-based measurement. Use exact mass whenever you're looking at a mass spectrum, matching a peak, or reporting HRMS data for publication.
How to Calculate Exact Mass (Step-by-Step)
You rarely need to do this by hand once you have a calculator that does it properly, but it helps to understand the mechanics — it's exactly what happens behind the scenes above. Take glucose, C6H12O6, again:
| Element | Count | Isotope mass (Da) | Contribution (Da) |
|---|---|---|---|
| C | 6 | 12.000000 | 72.000000 |
| H | 12 | 1.007825 | 12.093900 |
| O | 6 | 15.994915 | 95.969488 |
| Exact mass | 180.0634 | ||
- Break the formula into elements and counts — C6H12O6 becomes C×6, H×12, O×6.
- Look up the exact mass of each element's most abundant isotope (the table above, or the NIST/CIAAW reference data).
- Multiply each isotope mass by how many atoms of that element are present.
- Add every contribution together — the total is the molecule's exact mass.
For simple organic formulas this is a five-minute job with a calculator. For anything with brackets, hydrates (like CuSO4.5H2O), or metals with several significant isotopes, doing it by hand gets error-prone fast — which is exactly the gap this tool fills.
Understanding Isotope Patterns (M, M+1, M+2)
A real mass spectrum almost never shows just one peak for a compound. Because every element is a mix of isotopes, a molecule actually exists as a population of very slightly different masses — and the spectrum shows all of them. Mass spec chemists label these peaks relative to the monoisotopic mass:
- M — the monoisotopic peak (every atom is its lightest isotope). Usually the tallest peak for small-to-medium organic molecules.
- M+1 — one atom is one mass unit heavier, almost always due to a single 13C (each carbon contributes about 1.1% to this peak).
- M+2, M+4... — two mass units heavier, most dramatically caused by chlorine or bromine, which each have a second isotope two units heavier at high natural abundance.
C6H5Cl) into the calculator above to see this pattern calculated exactly.Bromine behaves differently: its two isotopes (79Br and 81Br) are almost equally abundant, so a single bromine gives an M : M+2 ratio close to 1:1 — one of the most recognisable fingerprints in all of mass spectrometry. Two chlorines or two bromines spread the pattern further, into M, M+2, and M+4 peaks with predictable ratios (roughly 9:6:1 for two chlorines, 1:2:1 for two bromines). The calculator above works these out precisely for any formula, using the real isotope distribution rather than a simplified approximation.
Common Mass Spec Adducts, Explained
In electrospray ionisation (ESI), a molecule almost never shows up in the spectrum as its bare neutral mass. It picks up or loses a charge-carrying species first — these are called adducts, and the peak you actually see is the adduct's m/z, not the plain exact mass.
| Adduct | Mode | Mass shift from M | When you'll see it |
|---|---|---|---|
[M+H]⁺ | ESI+ | +1.0073 | The default positive-mode adduct for most organic molecules |
[M+Na]⁺ | ESI+ | +22.9892 | Common with alcohols, sugars, and polymers |
[M+NH4]⁺ | ESI+ | +18.0338 | Frequent when the mobile phase contains ammonium |
[M+2H]²⁺ | ESI+ | +2.0146 (z=2) | Multiply-charged peptides and larger molecules |
[M−H]⁻ | ESI− | −1.0073 | The default negative-mode adduct, especially for acids |
[M+Cl]⁻ | ESI− | +34.9694 | Occasional in negative mode |
[M+HCOO]⁻ | ESI− | +44.9982 | Common with formic-acid or ammonium-formate mobile phases |
The Adducts tab in the calculator above generates every one of these automatically for whatever formula you type in — including the less common radical ions [M]⁺• and [M]⁻• — with a one-tap copy button next to each m/z value.
How to Use This Exact Mass Calculator
Everything in the tool above is built around one goal: type a formula, get every number you'd normally have to calculate by hand. Here's what each part of it does.
1 Type or paste a formula
Click into the formula box and type a standard chemical formula — capital letters for elements, numbers for counts, brackets for groups, and a dot for hydrates. All of these are valid: C6H12O6, Ca(OH)2, CuSO4.5H2O, Fe2(SO4)3. The calculator understands nested brackets and multi-digit counts, and it's case-sensitive on purpose — Co (cobalt) and CO (carbon monoxide) give very different results, and the tool tells them apart correctly.
If you type something it can't parse — an unsupported element, an unmatched bracket — it explains exactly what's wrong instead of failing silently.
2 Try a quick preset
Don't want to type a formula from scratch? The row of pills below the input (Glucose, Caffeine, Water, Aspirin, and more) loads a ready formula with one tap — a fast way to explore the tool or double-check a familiar compound.
3 Switch ionisation mode
The Neutral / ESI+ / ESI− toggle at the top right changes how the result is reported. Neutral shows the plain molecule. ESI+ shows it as [M+H]⁺, the way it would typically appear in a positive-mode spectrum. ESI− shows [M−H]⁻ for negative mode. The isotope pattern chart updates to match automatically.
4 Read the result screen
The large number is the exact (monoisotopic) mass in daltons, with the formula shown underneath. Beneath that sits the isotope pattern chart — tap or hover any peak (M, M+1, M+2...) to see its precise mass and relative abundance.
Below the chart are six quick stats, each with a small "i" you can tap for a plain-English explanation: Average MW (the natural-abundance average weight), Nominal (the whole-number mass), Mass defect (exact mass minus nominal mass — a useful fingerprint in high-resolution MS), DBE (degree of unsaturation — rings and double bonds), Abundance (what share of all molecules of this formula are exactly monoisotopic), and Heavy atoms (the non-hydrogen atom count).
5 Explore the four tabs
Elements breaks the formula down atom by atom, with each element's exact mass contribution and percentage of the total. Adducts lists every common MS adduct for the current formula and ionisation mode, with a tap-to-copy m/z value next to each one. Isotopes is a searchable reference table of exact isotope masses and natural abundances — tap any row to add that element straight into your formula. History automatically keeps a local list of formulas you've calculated, so you can jump back to an earlier result without retyping it.
6 Copy your result
The copy icon next to the formula box copies a full text summary — formula, exact mass, average MW, and both [M+H]⁺ / [M−H]⁻ values — ready to paste straight into a lab notebook, report, or email.
The whole tool is built to work the same way on a phone as it does on a laptop, so it's just as usable checking a formula at the bench as it is at a desk.
Frequently Asked Questions
What is monoisotopic mass?
Monoisotopic mass is the mass of a molecule calculated using only the single most abundant, naturally occurring isotope of each element — for example carbon-12, hydrogen-1, and oxygen-16. It's the same thing as "exact mass," and it's the value that corresponds to the tallest peak in a high-resolution mass spectrum for most small and mid-sized organic molecules.
Why is exact mass important in mass spectrometry?
High-resolution mass spectrometers can measure mass accurately enough to distinguish between formulas that would look identical at unit (nominal) resolution — for example C6H12O6 and C7H16O4 are close in nominal mass but clearly different in exact mass. Comparing a measured mass against the calculated exact mass, usually within a few parts per million, is one of the main ways chemists confirm a molecular formula.
What does M+2 mean in a mass spectrum?
M+2 is the peak sitting two mass units above the main (M) peak. It usually appears because one atom in the molecule is a heavier isotope two mass units up from its usual one — most commonly 37Cl, 81Br, or 34S. The relative height of the M+2 peak compared to M is a quick way to spot chlorine or bromine in a compound directly from the spectrum.
What's the difference between exact mass and molecular weight?
Exact mass uses only the lightest, most abundant isotope of each element. Molecular weight (average MW) uses each element's standard atomic weight, which already averages over the natural mix of isotopes. For light molecules the two are close; for heavier ones, or ones with elements like chlorine, bromine, or sulfur, the gap grows.
Is this exact mass calculator free to use?
Yes — the calculator on this page is completely free, works directly in your browser, and doesn't require any signup or installation. You can use it as many times as you like.
How accurate is this calculator?
All isotope masses and natural abundances are taken from NIST and CIAAW reference data — the same sources used in professional mass spectrometry software — and isotope patterns are calculated with a full isotope convolution rather than a simplified estimate, so results are accurate to four decimal places, including for multi-halogen and metal-containing formulas.
Can it calculate isotope patterns for chlorine and bromine compounds?
Yes. Chlorine and bromine (along with other multi-isotope elements like sulfur, silicon, and many metals) are fully supported, and the calculator will show the correct M, M+2, M+4 pattern — including cases where a mixed isotope peak, rather than M itself, turns out to be the tallest peak in the spectrum.
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