Health calculators

Quick health and fitness numbers, with the caveats made plain. Everything on these pages is a rough guide, not a diagnosis. If a number here worries you or seems off, talk to a GP who can actually look at you.

What health calculators can and cannot tell you

Health calculators are population statistics dressed up as personal answers. The maths behind BMI, BMR, target heart rate, and macro splits comes from studies of large groups of people, averaged out to produce a formula that fits "most" adults reasonably well. That is genuinely useful for a starting point, and genuinely not the same thing as knowing what is going on inside one specific person, which is to say you. Used as a rough guide, these tools earn their keep. Used as a verdict, they overreach.

None of the content on this site is medical advice, and none of the calculators are diagnostic. If you have a real health question, your GP is the right place to take it. The point of the tools here is to help you turn up to that conversation with the basic numbers already worked out, so the time with a clinician can go on the bits that actually need a clinician.

Where the inputs come from

Most health calculators take height, weight, age, sex, and activity level. Two of those are awkward.

  • Weight. Use a weight from the morning, after the loo, before food or drink, with the same scales each time. Day-to-day weight bounces around by a kilo or two for reasons that have nothing to do with body composition (water, salt, glycogen, what you ate yesterday). One reading is a snapshot. A seven-day average is a measurement.
  • Height. Measure barefoot, against a wall, with something flat across the top of your head. Adults can lose a centimetre or two over a long day on their feet, so morning is more reliable. The BMI Calculator accepts metric or imperial, but stick to one set of units to avoid sneaky rounding errors.
  • Activity level. The hardest one to be honest about. "Moderately active" in the calculator sense usually means a structured exercise session three to five times a week on top of normal daily movement, not "I went for a walk on Saturday". Most people overestimate this by a notch, which inflates the calorie target and quietly stalls progress. Be conservative. If you are not sure, pick the level below the one you were going to pick.
  • Pace and distance for runners. Use the GPS distance from your watch or phone, not the route distance from a planning tool, because the GPS will have measured what you actually did. The Running Pace Calculator takes either, but you want consistency between sessions.

Common mistakes people make with health numbers

Treating BMI as a body composition score is the classic. BMI is a height-and-weight ratio, full stop. A muscular rugby player and a sedentary office worker can land at the same BMI for very different reasons, and the two of them clearly do not have the same health profile. BMI is most useful for tracking changes in one person over time, not for comparing one person to another.

Treating a calorie target as a precise daily allowance is another. The Mifflin-St Jeor and Harris-Benedict equations have margins of error of a few hundred calories either way, even before you factor in how rough the activity multiplier is. Use the number from the TDEE Calculator as a starting point for two or three weeks, then adjust based on what your weight and energy levels actually do, not what the equation promised.

Skipping protein when cutting calories is the third. Lower calories with adequate protein tends to preserve lean mass; lower calories with low protein tends not to. The macro tools here default to a sensible protein floor for that reason.

How to choose between the tools here

If you want one number to anchor everything else, start with TDEE. From there, the macro split tool turns it into grams of food, and the BMI tool gives a sanity check on where you are sitting on the height-and-weight chart. Runners can skip straight to pace. Anyone pregnant should treat the due-date tool as a planning aid, not a clinical estimate, and rely on scan dates from a midwife or sonographer for anything that matters. We will keep adding tools where the gap is real and the maths is settled enough to be honest about.

  • BMI Calculator

    Work out your body mass index in metric or imperial units. You will get the number, the WHO category, and an honest note on what BMI can and cannot tell you.

  • Calorie Calculator

    Find your Basal Metabolic Rate (BMR) and Total Daily Energy Expenditure (TDEE). Set a goal (lose, maintain or gain) and get a daily calorie target with a rough macro split and a time estimate for reaching your goal weight.

  • Running Pace Calculator

    Work out your running pace from distance and time, your finish time from pace and distance, or distance covered from pace and time. Metric and imperial, with race time predictions for 5K, 10K, half marathon and marathon.

  • UK Marathon Course Difficulty Calculator

    Drop in a GPX file for any UK marathon and a target flat pace. Get a terrain-adjusted finish time, the toughest sections of the course flagged with the time you'll hit them, and a difficulty rating. Strava-style Grade-Adjusted Pace model with late-race fatigue weighting.

  • UK Sportive Difficulty Calculator

    Drop in a GPX file for any UK sportive, set your weight and target flat-road speed (or sustained power), get a physics-based finish time, the categorised climbs flagged with arrival times, and your top descent speed. Standard cycling-power equation with rider-type CdA and Crr presets.

  • Pregnancy Due Date Calculator

    Work out your estimated due date from your last period, conception date, or IVF transfer date. See your current gestational age, trimester, and the dates of key milestones like the anatomy scan, viability, and full term.

  • TDEE Calculator

    Work out your Total Daily Energy Expenditure and BMR using Mifflin-St Jeor, Harris-Benedict or Katch-McArdle. Includes a calorie target table (cut, maintenance, bulk) and a sensible starting macro split.

  • Macro Split Calculator

    Turn a daily calorie target into grams of protein, carbs and fat. Goal presets (balanced, high-protein cut, endurance, keto) or a custom three-slider ratio that always sums to 100. Per-meal breakdown included.

Frequently asked questions

Are these health calculators medical advice?

No. They are population-level rules of thumb wrapped in a form. Useful for getting your bearings before a conversation with a clinician, not for replacing one. If a number on any of these tools worries you, take it to a GP who can look at the rest of the picture.

Why do health calculators give different answers from each other?

Different equations. BMR can be calculated with Mifflin-St Jeor, Harris-Benedict or Katch-McArdle, and each one gives a slightly different number for the same inputs. The TDEE Calculator on this site shows all three so you can see the spread, which is usually a few hundred calories. None of them is "right". They all carry a margin of error of about five to ten per cent before activity-level guesswork is layered on top.

Which calculator should I start with if I want to lose, maintain or gain weight?

TDEE. It is the anchor number for everything else. From there, the Macro Split Calculator turns the calorie target into grams of protein, carbs and fat. The BMI Calculator gives a sanity check on where you sit on the height-and-weight chart. The Calorie Calculator combines BMR, TDEE and a goal in one form if you would rather skip straight to a daily target.

Do these calculators support metric and imperial units?

Yes. Every health tool here accepts either, so kilograms and pounds, centimetres and feet/inches, kilometres and miles all work. Stick to one set of units within a single calculation to avoid sneaky rounding errors.

Does any of my data leave the browser?

No. All the maths runs in JavaScript on your device. You can disconnect from the network and the numbers still update as you change inputs. Nothing is uploaded, logged or stored.

Why "health calculators" need caveats

A lot of health numbers online are sold with more confidence than they deserve. BMI, calorie targets, ideal weight, target heart rate: they are all population-level rules of thumb applied to individual people. That is an uneasy fit. A formula can tell you roughly what a generic adult of your size and age tends to look like, but it cannot tell you what your body is doing right now.

The tools here are built with that in mind. You will get the number, the working, and a plain-English note on when the number is a good fit and when it is not. If you want a definitive answer about your own health, go and see a doctor. If you want a starting point for that conversation, these are fine for that.

Nothing here is medical advice

Informational only. Not a substitute for a GP, nurse, pharmacist, or anyone else with actual training and a view of your whole situation.