Type "how much water should I drink" into Google and you'll get a different answer from almost every result — two litres, eight glasses, half your body weight in ounces, a gallon a day if you believe some of the wellness influencers. None of those numbers account for the fact that a 55kg office worker and a 95kg builder on a July building site need very different amounts of fluid. The calculator above works it out properly, using your actual weight, activity level and the day's weather rather than a single rule that's supposed to fit everyone. This page walks through where those figures actually come from, what each setting in the calculator does, and a few things about hydration that don't get explained very well anywhere else.

So how much water should you actually drink in the UK?

There are two official reference points floating around, and people often mix them up without realising they're not quite measuring the same thing.

The NHS Eatwell Guide sticks to a simple, practical message: 6 to 8 glasses of fluid a day. It counts water, lower-fat milk and sugar-free drinks — tea and coffee included — because the NHS is talking about what you actually drink, not water hidden inside your lunch. That works out to somewhere between 1.5 and 2 litres, depending on how big your glasses are.

The European Food Safety Authority looks at the bigger picture. Its adequate intake figures — 2.5 litres a day for men, 2.0 litres for women — cover total water, meaning everything you get from food as well as drinks. Roughly 70 to 80% of that total normally comes from what you drink, and the rest comes from food (soups, fruit, vegetables and so on are surprisingly water-heavy). That's why the calculator above shows you two numbers: the amount to actually drink, and the bigger total once food is factored in.

Neither figure is "wrong" — they're just answering slightly different questions. Most UK dietitians bridge the gap with a practical weight-based formula: somewhere between 30 and 35ml of water for every kilogram you weigh, which is exactly what powers the calculator on this page.

What actually changes your personal number

The 6-to-8-glasses line is a population average, and averages flatten out the differences that actually matter for an individual. Here's what genuinely moves the needle:

• Body weight. More body tissue means more cells to keep hydrated and more metabolic waste to flush out through the kidneys — this is the single biggest factor, which is why it's the first thing the calculator asks for.
• Age. Children and teenagers need more water per kilogram of body weight than adults because they're growing and have a higher metabolic rate relative to their size. Past 65, the body's thirst signal becomes noticeably less reliable, so older adults are often mildly dehydrated without ever feeling particularly thirsty.
• Activity level. Sweat loss during exercise can run from half a litre to well over a litre an hour depending on intensity and how hot it is.
• Weather. The UK doesn't get scorching heat often, but a warm spell or a stuffy office with the heating cranked up genuinely increases how much fluid you lose without you noticing.
• Pregnancy and breastfeeding. EFSA sets specific increments for both — an extra 300ml a day during pregnancy, and a more substantial 700ml while breastfeeding, since breast milk is roughly 87% water.
• Caffeine, at high volumes. More on this below, because it's more nuanced than most articles make out.

How weight changes your baseline

To make this a bit more concrete, here's what the calculator's own formula produces for a 30-year-old at different body weights, comparing a sedentary day against a moderately active one (mild UK weather, no other adjustments). It's a good way to see roughly where you'd land before you even open the calculator.

How to use this calculator

The tool is built to be quick, but it's worth understanding what each field is actually doing, because a couple of them matter more than people assume.

Weight

This is the main driver of your result, so it's worth getting right. You can switch between kilograms, pounds and stone using the small toggle — and if you pick stone, you get two separate boxes for stone and pounds rather than an awkward decimal, so "11 stone 6" goes in exactly as you'd say it.

Height

Height plays a smaller role, but very tall or very short people do have slightly different needs than the weight formula alone accounts for, so the calculator nudges the result a little at the extremes. Switch between centimetres and feet/inches with the toggle above the box.

Age

Under-18s use a higher per-kilogram figure to reflect growth and a faster metabolism. Adults up to 65 use the standard dietitian formula. Past 65, the figure drops slightly per kilogram — not because older adults need less water in absolute terms, but because body composition changes with age. The thirst-signal point above still applies, so don't take a slightly lower number as licence to drink less if you're in this age bracket.

Gender, and pregnancy or breastfeeding

Selecting "Female" reveals a second option for pregnancy or breastfeeding status, which applies the EFSA increments mentioned earlier directly to your result.

Weather today

A simple mild-or-hot toggle. On a hot day the calculator adds extra to cover the fluid you lose through sweating that you might not consciously notice, particularly if you're not used to warm weather.

Activity level

Five tiers, and it's worth picking the honest one rather than the flattering one:

• Sedentary — a desk job, minimal deliberate exercise.
• Light — exercising roughly 1 to 3 days a week.
• Moderate — structured exercise 3 to 5 days a week.
• Active — exercising 6 to 7 days a week.
• Athlete — intense daily training, the kind that leaves your kit properly soaked.

Tea/coffee today

This is where the calculator does something most others skip entirely — see the section below for why it matters.

Calculate My Target, and your result

Once you hit calculate, you'll see your daily drinking target as the headline number, with the total water figure (including food) shown just underneath for context. Below that sits Today's Progress — a small live tracker where you can log glasses and bottles as you actually drink them through the day. It resets automatically at midnight and saves in your own browser, so nothing is sent anywhere or shared with anyone.

Reset

Puts every field back to its starting point if you want to run the numbers for someone else, or just start again.

Does tea and coffee really dehydrate you?

This one refuses to die, but the NHS, the British Nutrition Foundation and EFSA all say the same thing: no, not in any way that matters for most people. Caffeine has a mild diuretic effect, meaning it can make you produce slightly more urine, but the water content of the drink itself far outweighs that effect. A cup of tea or coffee still leaves you net positive on fluid.

Where it does become worth thinking about is at the very high end — the NHS advises keeping caffeine under 400mg a day for most adults, which is roughly four to five cups of coffee. Go well beyond that regularly and the calculator adds a small buffer to your target, mainly as a nudge to keep an eye on total intake rather than because a cup of tea is doing you any harm.

Signs you might not be drinking enough

The single easiest check doesn't need an app or a calculator at all: look at the colour of your urine. Pale straw is a good sign. Dark amber means it's time to drink more.

A few ways to actually hit your number

• Keep a bottle within arm's reach — people drink noticeably more when water is visible and easy, and noticeably less when it means a walk to the kitchen.
• Have a glass before each meal rather than trying to catch up with one big session in the evening.
• Use the tracker above rather than trying to remember — it takes about two seconds to tap "+1 glass" and it's far more accurate than guessing at 9pm.
• If plain water bores you, sparkling water, herbal tea and well-diluted squash all count toward your total.
• On hot days or heavier training days, don't wait until you're thirsty — by that point you're already a little behind.

Frequently asked questions

Sources

NHS Eatwell Guide · NHS Live Well · European Food Safety Authority (EFSA) Scientific Opinion on Dietary Reference Values for Water · British Nutrition Foundation hydration guidance.

This article and the calculator above are for general information only and don't replace medical advice. If you have kidney disease, heart failure, or any other condition affecting how your body handles fluid, speak to your GP before changing how much you drink.