Body composition calculators that tell you the whole story
Every result says why this number, what it cannot tell you, and which tool to open next. We print the classification systems side by side and the error band right beside the figure.
Four steps
From a tape measure to a daily calorie target
Run the tools in this order and each result hands the next one your numbers, already filled in.
Measure
Height, weight and waist. BMI gives you the band, and waist to height gives you the part BMI misses: where the weight sits. Under 0.5 is the healthy line.
Composition
A tape reading turns weight into fat mass and lean mass using the US Navy method. We print the plus or minus 3 to 4 point spread against a DEXA scan next to it, because that spread is the story.
Energy
Your TDEE is a Mifflin-St Jeor basal rate times an activity multiplier. We show it as a range of roughly plus or minus 12.5 per cent, not a single confident integer.
Pace
Then a daily target with the rails already in the maths: at most 1 per cent of bodyweight a week, no lower than 80 per cent of your TDEE, and floors of 1500 kcal for men and 1200 kcal for women.
03
How we work
A clear path from first conversation to finished work.
Body composition calculators
BMI, Asian BMI, body fat, skinfold, relative fat mass, lean body mass, FFMI, ideal and healthy weight range, frame size and recomposition tools, each printing the classification systems side by side with stated error bands.
Fat distribution calculators
Waist circumference, waist to height ratio, waist to hip ratio, body roundness index and a visceral fat tape proxy, mapped to published thresholds and clearly labelled as tape measures rather than scan volumes.
Calorie and nutrition calculators
BMR, TDEE, calorie deficit and surplus, macros, protein, water, fibre, weight-loss time, check-in, reverse diet and alcohol tools, with intake floors, weekly rate caps and no finish-date countdowns.
Fitness calculators
Calories burned, walking and running energy cost from published MET values, heart rate zones, VO2 max field tests and one rep max estimates, shown gross and net where that distinction matters.
Most calculators give you a number and let you guess what it means. We show the classification systems together, the error band beside the figure, and the safety limits inside the arithmetic rather than in a footnote nobody reads.
Start here
Five body composition calculators worth opening first
One line each, so you can pick the tool that answers the question you actually have.
BMI calculator
Your body mass index in kg/m2, with the WHO standard table and the WHO Asia-Pacific table shown together instead of one being quietly chosen for you.
Body fat percentage calculator
The US Navy tape method from neck, waist and hip, printed with its plus or minus 3 to 4 point accuracy band against a DEXA reading.
Waist to height ratio calculator
Waist divided by height, checked against the under 0.5 rule. The quickest honest read on where your weight is carried.
TDEE calculator
Daily calorie needs from five basal metabolic rate equations and five activity multipliers, shown as a range rather than one tidy number.
Calorie deficit calculator
A safe daily target with the 1 per cent weekly cap, the 80 per cent of TDEE limit and the kilocalorie floors applied before you see a figure. No finish date.
05 — Common questions
What people ask before they trust a calculator
Short answers to the eight questions we get most often about the maths, the storage and the limits.
Frequently Asked Questions
- Which calculator should I start with?
- Start with BMI, because height and weight are the two numbers you already know. Then run the waist to height ratio, since it picks up the fat distribution that BMI cannot see. If you have a tape measure and want fat mass against lean mass, the body fat calculator is next. After that, the TDEE calculator turns your measurements into a daily calorie range.
- Why do two classification systems disagree about my BMI?
- Because they were drawn from different populations. At the same index, average body fat and health risk differ between groups, so the WHO Asia-Pacific thresholds sit lower: overweight begins at 23.0 rather than 25.0. A BMI of 24 therefore reads as healthy weight on one table and overweight on the other. We print both rows instead of picking for you.
- How big are the accuracy bands on these numbers?
- Larger than most sites admit. The US Navy tape method typically lands within 3 to 4 percentage points of a DEXA scan, in either direction. TDEE from an equation and an activity multiplier is worth about plus or minus 12.5 per cent for most people. We show those bands beside the result, so you read the range and not a false decimal.
- What do you store when I use a calculator?
- Nothing. There are no accounts and no saved measurements. Your figures live in the page while you use it, in the query string of the link if you share or bookmark it, and your unit preference stays in your own browser. Close the tab and the numbers are gone from our side, because they were never sent anywhere to keep.
- Is any of this medical advice?
- No. These are calculators and explainers, not a diagnosis or a treatment plan. There is no contracted medical reviewer on the site, and we say so plainly rather than printing a badge. If a result concerns you, or you are pregnant, unwell, very lean or very heavy, take the number to your GP or an accredited practising dietitian and let them interpret it.
- How often should I re-measure?
- Weigh at the same time of day, under the same conditions, and read the weekly average rather than any single morning. Measure your waist every two to four weeks, because a tape moves more slowly than a scale and carries less water noise. Body fat estimates from a tape are worth re-running monthly at most. Trends are the signal; daily readings are mostly fluid.
- Which home body fat method should I trust?
- None of them on its own. Tape methods, skinfolds and bathroom impedance scales each carry a few points of error, and they disagree with one another. Pick one method, keep your technique identical, and watch the direction it moves over months. A consistent method that trends is more useful than chasing the number a different device gave you yesterday.
- Can children use the adult calculators?
- No. Adult BMI cut-offs are not valid under 18, because height and body composition are still changing. Children and teenagers need BMI percentiles based on CDC growth reference data, which compare a child to others of the same age and sex. Use our child tools for anyone under 18, and treat any result as a conversation to have with a GP.
Put your numbers in and see the whole story
Two fields to begin. Your BMI result then hands your figures to the waist, body fat and calorie tools with nothing to type twice.