Body and fitness
Metabolism, nutrition targets, heart and training — the numbers that describe how much energy your body uses, what it is made of, and how hard you are working. Every formula named, every error band shown.
Four numbers, and the order they go in
This wing is one chain, not fifteen unrelated calculators. It starts with energy: a resting metabolic rate from a published equation, multiplied by an activity factor to give TDEE, which is the same quantity as maintenance calories under a different name. A calorie deficit only means anything as a gap from that figure, which is why the deficit tool comes second and not first.
From the energy figure the nutrition targets follow. The AMDR splits energy into protein, carbohydrate and fat as percentages, and the Atwater factors of 4, 4 and 9 kcal per gram convert those percentages into grams. Protein is the exception worth knowing: its published ranges are set per kilogram of body weight, so a protein target does not depend on the energy estimate being right.
Then composition, which asks what the mass is rather than how much of it there is, and finally the heart and training numbers, which describe effort. Each stage inherits the uncertainty of the one before it, which is why every result on this site is rendered with a band.
Where the estimates are weakest
The honest answer is: at the activity multiplier, by a wide margin. The ladder from 1.2 to 1.9 that every calculator uses has no primary source. Moving one rung changes the daily figure by roughly 200 to 300 kcal, which is larger than the disagreement between the three published resting equations. The FAO/WHO/UNU physical activity level bands are the documented counterpart, and two rungs of the conventional ladder fall below the lowest band those bodies document for adults.
Composition is the next weakest link. The three lean-mass equations - Boer, James and Hume - span 4.93 kg at 80 kg and 180 cm, which is 6.2 points of implied body fat from identical inputs. Body-fat estimates from the tape method and from BMI each carry about 4 percentage points of stated error, so two of them differing by 3 points are not actually disagreeing.
Heart rate is more precise-looking and no more precise. Age-predicted maxima carry a standard error of 7 to 11 beats per minute, so zone edges printed to the beat are false precision - and the five-zone chart's edges were never published by anyone in the first place.
The parts that are solid
Not everything here is an estimate stacked on an estimate. The Atwater factors, the AMDR percentages and the protein ranges are published figures with named owners: the Institute of Medicine, the ACSM joint statement, the ISSN position stand, the PROT-AGE group. Converting a percentage of energy into grams is exact arithmetic once you have the energy figure.
The unit conversions are exact by definition - a pound is 0.45359237 kg, an inch is 2.54 cm - and this site rounds them with a helper that handles the floating-point cases rather than with a naive round, because that was the difference between agreeing and disagreeing with a published table.
And waist-to-height ratio needs no regression at all: it is one measurement divided by another, with NICE's own groupings for the result. Where a number can be measured rather than predicted, this site prefers the measurement and says which is which.
What this wing deliberately does not do
There is no BMI landing page and no calorie-calculator head-term page. BMI appears only where a published equation takes it as an input, such as the Deurenberg body-fat estimate and the IOM pregnancy weight-gain lookup, and it is labelled as an input rather than a result.
There are no weight projections stated as outcomes. The weight tools present the Wishnofsky rule as the arithmetic it is, alongside Hall's finding that the energy density of the weight lost is not a constant, and they return ranges and durations rather than dates - a date reads as a promise.
Sleep and stress numbers are not here at all. They belong to a sister site by a standing portfolio rule, and pointing you there is a placement decision rather than a courtesy.
Where to start, depending on what you have
If you have a weight, a height, an age and nothing else, start at the BMR calculator. It runs three published equations side by side and shows the spread between them, which is a more useful first impression than any single figure. From there the TDEE calculator adds the activity layer and tells you where your chosen multiplier sits against the FAO/WHO/UNU bands.
If you have a tape measure, the body fat calculator is the better entry point: the circumference method uses measurements rather than a regression on your weight, and the waist-to-height ratio it also returns needs no equation at all. If you have a lean-mass figure from a scan, the Katch-McArdle route on the BMR page becomes the strongest option rather than the weakest.
If you have a watch, start at the heart end. The max heart rate calculator shows how far apart the three published equations are at your age, and the heart rate zone calculator builds both the percent-of-maximum and Karvonen tables from whichever you choose. Neither will match your watch exactly, and the pages explain why rather than pretending they should.
Whatever you start with, the numbers connect: each tool ends with a strip explaining which figure naturally comes next and why. That chain is the thesis of this site rather than a navigation convenience.
In-depth guides
How TDEE is calculated, in full
The complete derivation of total daily energy expenditure: the Mifflin-St Jeor equation term by term, the two equations that compete with it, the activity ladder that has no primary source, the published activity bands that do, and the error band that belongs around the result.
What heart rate zones actually mean
Heart rate zones are percentages of an estimated ceiling, drawn at edges nobody published. This guide separates the five-zone convention from ACSM's published intensity classification, sets Karvonen against percent-of-maximum, and quantifies how much each choice moves the boundaries.
Calculators for this topic
Your TDEE is the calories you burn in a day: a resting metabolic rate from the Mifflin-St Jeor equation, multiplied by an activity factor. This calculator shows both, brackets the answer with the 10 percent error the equations carry, and names where your chosen activity factor sits against the FAO/WHO/UNU activity bands.
BMR is the energy your body uses at complete rest. This calculator runs the Mifflin-St Jeor equation, the 1984 revision of Harris-Benedict, and Katch-McArdle when you have a lean-mass figure, then shows the gap between them, because that gap is bigger than most calculators admit.
Maintenance calories are the same quantity as TDEE, under a different name: the energy that leaves your weight where it is. This calculator estimates it from the Mifflin-St Jeor equation and an activity factor, shows the range that estimate really covers, and explains why the scale is the only real check.
A calorie deficit is the gap between the energy you use and the energy you eat. This calculator works out that gap from your estimated maintenance figure, converts it using the 3,500 kcal per pound rule, and shows the correction Hall published in 2008 for why that rule overstates things.
This projects weight change from a daily calorie deficit as a range, not a single figure and not a date. The high end is the straight-line 3,500 kcal per pound arithmetic; the low end applies the 2011 finding by Hall and colleagues that about half of an eventual change arrives in the first year.
This turns a daily calorie figure into grams of protein, carbohydrate and fat using the Atwater factors of 4, 4 and 9 kcal per gram. Every preset sits inside the Acceptable Macronutrient Distribution Range the IOM published, and any custom split is checked against those percentages rather than scored.
Four organisations have published protein ranges: the RDA of 0.8 g per kg from the IOM, 1.2 to 2.0 for athletes in the 2016 ACSM joint statement, 1.4 to 2.0 from the ISSN, and 1.0 to 1.2 for healthy over-65s from PROT-AGE. This shows each of them in grams for your weight.
The eight-glasses rule is about 1.9 litres of drinking water and nobody published it. The IOM set an Adequate Intake for total water of 3.7 litres a day for men and 2.7 for women, food included, and this calculator shows that figure beside the two conventions people quote instead.
A maximum heart rate calculator estimates the fastest your heart beats at all-out effort. This one runs three published equations at once: Fox 220 minus age, Tanaka 208 minus 0.7 times age, and Gulati 206 minus 0.88 times age for women. It shows the gap between them and states that 220 minus age was never a validated regression.
A heart rate zone calculator turns your age into five training bands: 50 to 60 percent of maximum through 90 to 100 percent. Enter a resting heart rate and it also computes Karvonen zones from heart rate reserve, which sit higher, and prints the intensity classification ACSM actually published beside the five zone convention.
A lean body mass calculator estimates how much of your weight is not fat, from your height, weight and sex. It runs the Boer, James and Hume equations together rather than picking one, because at 80 kilograms and 180 centimetres they land 4.93 kilograms apart, and that spread is the honest answer.
A body fat calculator estimates what share of your weight is fat. This one runs the Hodgdon and Beckett tape method from your neck, waist and hip measurements and the Deurenberg BMI equation, shows each with the error band its own paper reports, and adds your waist to height ratio with the NICE bands.
A one rep max calculator predicts the heaviest single you could lift from a set you actually completed. It runs Epley and Brzycki together, shows where they agree and where they diverge, gives the percentage table for every rep count, and widens the stated uncertainty above ten reps where the published validation stops.
A running pace calculator converts a finish time and a distance into pace per kilometre and per mile, with even splits for every marker including the fractional last one. It also projects equivalent times at 5K, 10K, half marathon and marathon using the Riegel equation, shown as a spread rather than a promise.
A calories burned calculator multiplies an activity's MET value by your body weight and duration, using the equation kcal per minute equals MET times 3.5 times kilograms divided by 200. Every MET value comes from the 2024 Adult Compendium with its activity code, and the tool shows the net figure alongside the gross one.
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