Health Calculators
Body measurements, nutrition and general wellbeing estimates.
- A1C Calculator Convert HbA1c to estimated average glucose and back, in % or mmol/mol and mg/dL or mmol/L — with the wide scatter behind the estimate shown honestly.
- Alcohol Units Calculator UK units, grams of alcohol and US and Australian standard drinks for any drink — because a 'unit' means something different in every country.
- Baby Weight Percentile Calculator Where your baby's weight sits on the WHO growth standards, from birth to two years, by exact day of age — with corrected age for premature babies.
- BAC Calculator Estimate blood alcohol with the Widmark formula, the realistic range around it, and how long until it reaches zero — never whether it is safe to drive.
- Blood Pressure Calculator Classify a blood pressure reading, or the average of three, under both the US and European guidelines — which disagree about where hypertension starts.
- Blood Sugar Converter Convert blood glucose between mg/dL and mmol/L, then read it against the right thresholds for fasting, after a meal or a random test — and for low readings.
- BMI Calculator Body mass index with the healthy weight range for your height, the lower Asian thresholds, and the height bias built into the formula itself.
- BMI Percentile Calculator for Children A child's BMI placed on the WHO percentile for age and sex, from 2 to 19 years, with the WHO weight categories — because adult BMI cut-offs do not apply.
- BMI Prime Calculator BMI as a fraction of the healthy upper limit, so 1.0 is the edge — a rescaling that adds legibility and, deliberately, no information.
- BMR Calculator Resting metabolic rate from three different equations, with the spread between them — because every one of them is a population regression, not a measurement.
- Body Fat Calculator Body fat from tape measurements, with how much a centimetre of waist error moves the answer — usually more than a fortnight of real change.
- Body Surface Area Calculator Body surface area by four formulas, with the scaling test that separates the two that are dimensionally exact from the two that are not.
- Caffeine Calculator Add up a day's caffeine from coffee, tea, cola and energy drinks, check it against the advised limit, and see how much is still active at bedtime.
- Calorie Calculator Daily calories for a goal, with the plateau built in — because the 3,500-calorie rule draws a straight line and weight loss is a curve.
- Calorie Deficit Calculator Work backwards from a target weight and a date to the intake that actually reaches it — and find out when the target is not reachable at all.
- Carb Calculator Carbohydrate targets by training load, plus how much glycogen you can actually store — because carb loading has a ceiling.
- Child Height Predictor Predict a child's adult height from the parents' heights using the mid-parental method, with the range most children of those parents actually fall in.
- Cholesterol Ratio Calculator Total-to-HDL ratio with non-HDL cholesterol beside it, LDL estimated where valid, in mg/dL or mmol/L — and the one ratio that changes with units explained.
- Conception Date Calculator Work back to the likely conception date from a due date, last period or birth date — with an honest range, since conception is not the day of sex.
- eGFR Calculator Estimate kidney function from creatinine, age and sex with the 2021 CKD-EPI equation — the version without a race coefficient — and see the KDIGO stage.
- Fat Intake Calculator Daily fat in grams, with the part that is genuinely essential separated out — it is about a quarter of the usual recommendation.
- Fiber Intake Calculator Fibre from the figure the flat targets are actually derived from — 14 g per 1,000 calories — so the target scales with how much you eat.
- Ideal Weight Calculator Four ideal weight formulas side by side, what each was actually built for, and the healthy BMI range — the only one of them meant as a health guide.
- Lean Body Mass Calculator Lean mass from body fat or from height and weight — with the James formula refused past the point where it reports lean mass falling as you gain.
- Macro Calculator Protein, carbs and fat in grams — with protein checked per kilogram of bodyweight, because a percentage is the wrong unit for it.
- Max Heart Rate Calculator Your maximum heart rate from every well-founded formula side by side, with the individual scatter shown — it is bigger than the formulas' disagreement.
- Ovulation Calculator Estimate ovulation and your six-day fertile window by counting back from your next period, for this cycle and the next two.
- Period Calculator Predict your next six periods from your last one and your cycle length, with ovulation, fertile days and whether your cycle is in the normal range.
- Pregnancy Due Date Calculator Your due date from your last period, conception date or IVF transfer, with the term window around it and the key dates of the pregnancy laid out.
- Pregnancy Week Calculator How many weeks and days pregnant you are on any date, from your last period or due date — with the age since conception shown alongside.
- Protein Intake Calculator Protein in grams a day, set against lean mass where bodyweight would overstate it — and measured against the RDA, which is a floor rather than a target.
- Resting Heart Rate Calculator Turn a pulse count into beats per minute, see how precise that count really is, and read the result against both the textbook range and a better one.
- Sleep Calculator Bedtimes and wake-up times in whole sleep cycles, checked against the hours you actually need — so a neatly timed six hours is never passed off as enough.
- Sodium Intake Calculator Sodium and salt converted properly — the 2.54 factor comes from atomic masses, and mistaking one for the other is out by two and a half times.
- Target Heart Rate Calculator Moderate and vigorous heart rate zones by the Karvonen method or percent of maximum, kept apart, with the weak link — the estimated maximum — made visible.
- TDEE Calculator Daily calorie burn, and which half of the estimate carries the error — the activity guess is worth several times more than the equation you pick.
- Waist to Height Ratio Calculator Keep your waist under half your height. One threshold for everyone — no sex adjustment, no population table, no percentile chart.
- Waist to Hip Ratio Calculator Waist divided by hips against the WHO thresholds, plus waist-to-height — because a ratio of two girths cannot see how big you are.
- Water Intake Calculator How much to drink, separated from total water intake — because roughly a fifth of it arrives in food, which is what eight glasses a day forgot.
- Weight Loss Percentage Calculator Work out what percentage of your body weight you have lost, how far you are toward your goal and your weekly rate of loss.
- Weight Loss Timeline Calculator How long to reach a goal weight at a given calorie deficit, simulated week by week as your energy needs fall — not the straight line the 7,700 kcal rule draws.
A Guide to Health Calculators
Health calculators have a reputation problem. Most of them take two numbers, divide one by the other, and hand back a word like "overweight" as though it settled something. The number is usually right. The certainty is usually not.
The forty-one calculators in this category are built on the opposite principle: show the figure, show the arithmetic, and say plainly where the figure stops being useful. Several of them will refuse to answer rather than guess, because for some questions there is no honest single number. This guide explains what each group measures, which calculator answers which question, and where the well-known formulas quietly mislead.
Body composition: what the weight is made of
Weight on its own tells you very little. These eight calculators each attack the same problem from a different angle, and they disagree with each other in instructive ways.
The BMI Calculator is the one everybody knows, and it carries a bias worth understanding before you read your result. BMI divides weight by the square of height, but bodies scale by volume, which goes with the cube. Take any person and enlarge them by ten percent in every dimension, keeping their proportions exactly: their weight rises by about a third, their height by a tenth, and their BMI rises by a tenth as well. A taller person of identical build scores higher. That is arithmetic, not a judgement about their body, and the calculator shows you what your own build would score at a different height so you can see the size of the effect. It also offers the lower WHO thresholds used for South and East Asian populations, where cardiometabolic risk appears at a lower BMI, and tells you how your figure reads on both scales.
The BMI Prime Calculator expresses the same figure as a ratio to the upper healthy limit, which is easier to act on: 1.12 says you are twelve percent above the top of the range, where a BMI of 28 requires you to remember what the limit was.
If BMI's blindness to muscle is the problem, the Body Fat Calculator and the Lean Body Mass Calculator address it directly from girth measurements. The lean body mass figure has a quirk that most implementations hide: the James equation does not increase indefinitely with weight. For a woman of 165 cm it peaks at about 98 kg and then falls, which is physically meaningless, so the calculator tells you when you have passed the turning point instead of reporting a number from the wrong side of it.
The Waist to Height Ratio Calculator is the one to use if you only want one number. Keeping waist under half your height needs no assumption about build at all, because it measures where weight is carried rather than how much of it there is. The Waist to Hip Ratio Calculator adds the distribution, which matters more for cardiovascular risk than the total.
The Ideal Weight Calculator runs four published formulas side by side rather than picking one, because they genuinely disagree and the disagreement is the useful part. The spread between them is not constant: it is about 8 kg at five feet, narrows to roughly 1 kg near 169 cm, and widens again to 15 kg at six foot six. Showing one formula's answer as "your ideal weight" hides all of that.
The Body Surface Area Calculator exists mainly for medical dosing, where it matters more than weight. Four accepted equations are offered. Two of them, Du Bois and Mosteller, satisfy the scaling test exactly, which is a good reason to prefer them when nothing else decides.
Energy needs: how much you actually burn
Six calculators cover the chain from resting metabolism to a dated weight-loss target, and they are meant to be used in that order.
Start with the BMR Calculator for what your body uses at complete rest, then the TDEE Calculator to add activity. The gap between them is larger than people expect: at a sedentary multiplier you burn roughly a fifth more than your BMR, and at the top of the scale nearly double it.
The Calorie Calculator turns that into an intake target for a goal, and the Calorie Deficit Calculator works the other way, from a deficit to a rate of loss. Both are built against the familiar rule that 3,500 calories equals a pound of fat, and both tell you where the rule breaks. It assumes your maintenance requirement stays fixed while you get lighter, which it does not. Over a full year of a steady deficit the simple rule overpredicts loss by about 7 kg.
That is why the Weight Loss Timeline Calculator recalculates your requirement as your weight falls, so the projected date moves out rather than arriving and being wrong. The Weight Loss Percentage Calculator reports progress as a share of your starting weight, which is the figure clinical guidance is written in: five and ten percent are thresholds that mean something, where "nine kilos" depends entirely on where you started.
Nutrition: dividing the total up
Seven calculators split an intake into its parts. The Macro Calculator is the one to start from, and the Protein Intake Calculator, Carb Calculator and Fat Intake Calculator go deeper on each one, with the ranges that published guidance actually gives rather than a single prescriptive figure.
Three more cover intakes that are usually ignored until a doctor mentions them. The Fiber Intake Calculator scales with energy intake rather than being a flat 30 g for everyone. The Sodium Intake Calculator converts between salt and sodium, which is where most confusion in this area comes from: the factor is 2.542, derived from the atomic masses of sodium and chlorine, so a 6 g salt limit and a 2,360 mg sodium limit are the same limit stated twice. The Water Intake Calculator accounts for body weight and activity instead of repeating the eight-glasses figure, which has no basis in the literature.
Heart and circulation
Five calculators here, and the ordering between them matters. The Max Heart Rate Calculator gives the ceiling, the Target Heart Rate Calculator turns it into training zones, and the Resting Heart Rate Calculator provides the figure that makes those zones personal rather than generic.
The Blood Pressure Calculator classifies a reading against current categories and explains why the two numbers are not interchangeable. The Cholesterol Ratio Calculator computes the ratio rather than the total, because the ratio carries more information than either figure alone.
Blood markers
Three calculators for numbers that arrive on a lab report and are rarely explained on it. The A1C Calculator converts between A1C and estimated average glucose in both directions. The Blood Sugar Converter handles mg/dL and mmol/L, the two conventions that cause genuine confusion between countries. The eGFR Calculator uses the CKD-EPI 2021 equation, which is the version that removed the race coefficient; older calculators still in circulation use the 2009 one and give a different answer.
Pregnancy
Five date-based calculators: Pregnancy Due Date, Pregnancy Week, Conception Date, Ovulation and Period. Each states the assumption it is working from, usually a 28-day cycle and a 14-day luteal phase, and each adjusts when you give it your own figures. The assumption is the whole calculation, so it is stated on the page rather than buried.
Children
Children are not assessed against adult thresholds, because normal body composition changes substantially through growth. The BMI Percentile Calculator for Children and the Baby Weight Percentile Calculator work in percentiles against age and sex, which is the only meaningful way to read them. The Child Height Predictor uses parental height and reports the range rather than a single figure, because the uncertainty on this one is several centimetres wide and a point estimate would be false precision.
Lifestyle
Four calculators for things that are measurable but rarely measured. The Alcohol Units Calculator converts drinks into units, and the BAC Calculator estimates blood alcohol concentration with a clear statement of why the estimate should never be used to decide whether to drive. The Caffeine Calculator works from the half-life, so you can see how much of an afternoon coffee is still in your system at bedtime. The Sleep Calculator works backwards from when you need to wake, rounding to complete cycles.
What none of these can do
They are calculators, not diagnoses. Every one of them works from a published equation fitted to a population, and you are an individual. A BMI cannot tell muscle from fat. A BAC estimate cannot account for what you ate. A percentile is a position in a distribution, not a verdict on a child.
Where a calculation rests on an assumption, the page states the assumption. Where two accepted methods disagree, it shows both. Where there is no honest answer for the values you entered, it says so instead of returning a number. That is the most a calculator can responsibly do, and it is a long way short of medical advice. For anything that matters, take the figure to someone who can examine you.