Health

BMI, calories and body metrics

These calculators turn a few measurements into health indicators in common use: BMI, body-fat percentage using the US Navy tape method, estimated daily calorie needs, and a pregnancy due date.

They are screening and planning aids, not diagnoses. BMI in particular ignores muscle mass and body composition, and the body-fat formula is an estimate from tape measurements rather than a measurement of body fat itself. For anything that matters to your health, take the figure to a qualified professional instead of acting on it directly.

Each tool answers a different question, and it helps to know which. BMI compares weight with height and says nothing about how much of that weight is muscle, so a muscular person can read as overweight while carrying little fat. The body-fat calculator estimates a percentage from tape measurements using the US Navy method, which typically lands within about three to five percentage points of a measured result rather than measuring fat directly. The calorie calculator applies an activity multiplier to an energy-requirement equation, which makes it a planning figure rather than a prescription. And a due date is a 40-week estimate counted from your last period: only around four to five per cent of babies arrive on it. Where a number here matters, follow the trend across several readings and take it to a professional.

Which calculator to use

A general weight-for-height check

Use the BMI Calculator, in metric or imperial units. Read it as a screening band rather than a diagnosis, and read the trend over months rather than a single reading.

BMI Calculator

Estimating body composition

Use the Body Fat Calculator. It needs a tape measure and applies the US Navy circumference method, so the accuracy of the answer depends mostly on the accuracy of the measurements — take each one twice.

Body Fat Calculator

How much to eat

Use the Calorie Calculator for an estimated daily energy requirement from age, sex, size and activity level. It answers 'roughly how much', which is what planning a diet needs; it is not a prescription for any medical condition.

Calorie Calculator

Pregnancy dates

Use the Pregnancy Due Date Calculator for an estimated due date and the current week of pregnancy from the first day of your last period. Your antenatal clinic's dating scan is the more accurate measure — the tool gives context for the dates you have been given, not a replacement for them.

Pregnancy Due Date Calculator

Common mistakes here

Reading BMI as a measure of body fat. It uses height and weight only, so it cannot tell muscle from fat; athletes and people with a lot of muscle routinely read as overweight.
Comparing a body-fat estimate with a clinical measurement as though they were the same number. The tape method estimates; a DEXA scan or hydrostatic weighing measures. Expect a few percentage points of difference.
Treating a calorie estimate as a fixed target. The equation predicts an average requirement, and activity multipliers are broad; use the result as a starting point and adjust from what actually happens over weeks.
Taking a due date literally. It is a 40-week estimate from the last period, and only a small minority of births happen on it — a window either side is normal.
Using these tools for a child, during pregnancy, or alongside a medical condition without professional input. The BMI categories and the calorie equations are defined for adults and do not transfer.

Worked examples

BMI in metric units
A 70 kg adult who is 1.75 m tall: BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9, which falls in the normal-weight band. The same arithmetic is what the calculator runs — there is no hidden adjustment.
Why the due date is an estimate
Counting 40 weeks (280 days) from the first day of the last period gives the estimated due date. Because ovulation timing varies between cycles and only a small proportion of births land on the estimated day, antenatal care works to a window around it rather than to the date itself.

Maintained by David M at Delta Tech Solutions · Figures last checked 26 September 2026 · Next check due 26 March 2027