FitnessCalculator

11 free tools

Free Health & Fitness Calculators

Body composition, daily energy and hydration. Each tool sits on its own page next to the formula it runs, a worked example you can follow by hand, and the reference charts that go with it.

Calculate your BMIWork out your calories
  • Metric and imperial
  • Every formula written out
  • Runs in your browser
  • No sign-up, no tracking of your measurements
The tools

Pick the one you need

Every tool sits on its own page, next to the arithmetic it runs and the questions people ask about it.

BMI Calculator for Adults Weight against height, scored on the adult bands, with the healthy weight range for your height. Open calculator Calorie Calculator How much to eat to lose, hold or gain weight, with the deficit and surplus worked out for you. Open calculator TDEE Calculator What you burn across a whole day, from resting energy expenditure and the activity band you pick. Open calculator BMR Calculator Your resting burn through all three standard formulas at once, so you can see how far apart they land. Open calculator Macro Calculator Daily grams of protein, carbohydrate and fat for your calories, with the split you choose. Open calculator Protein Calculator Grams of protein a day from your weight and your training, printed as a range because the research is one. Open calculator Navy Body Fat Calculator Body fat percentage from neck, waist and hip, using the United States Navy tape method. Open calculator Ideal Weight Calculator Four classic formulas side by side, checked against the healthy BMI band for your height. Open calculator Water Intake Calculator by Weight A daily fluid target from body weight, with an allowance added for the training you actually do. Open calculator Army Body Fat Calculator The waist-to-height standard the Army adopted in July 2026, with the largest waist that still passes at your height. Open calculator ACFT Calculator: Army Fitness Test Score Calculator Points for all five AFT events against the official scales, on either the general or the combat standard. Open calculator
1

Open a tool

Six calculators, one page each. Nothing asks for an account and nothing is emailed to you.

2

Enter your numbers

Kilograms or pounds, centimetres or feet. The switch in the header changes every field at once.

3

Read the working

Each page prints the equation it used, a worked example, and the point where the estimate stops holding.

Background reading

Read the guides

What each formula is doing, where it was derived, and the situations where it gives the wrong answer.

BMI scale showing the underweight, healthy, overweight and obese bands with cutoffs marked at 18.5, 25 and 30 What BMI Actually Measures, and Where It Falls Apart Learn how adult BMI is calculated, where its categories come from, and why it cannot diagnose health or distinguish muscle from fat. Bar chart comparing a 1,780 kcal resting BMR against a 2,759 kcal TDEE, with the 979 kcal activity gap bracketed BMR vs TDEE: Which Number Should You Actually Eat To? Understand estimated resting energy, daily expenditure and calorie scenarios without treating BMR as a personal minimum intake. Chart of four calorie deficit sizes from 250 to 1,000 kcal per day and the weekly weight loss each one predicts How Big Should a Calorie Deficit Be? Compare calorie-deficit examples and learn why fixed calorie rules cannot promise a weekly weight-loss result. Column chart of the five TDEE activity multipliers, from 1.2 for a desk job to 1.9 for a physical job plus training Picking an Activity Multiplier Without Fooling Yourself See how activity multipliers change TDEE, why step counts are only context, and how to avoid counting exercise twice. Diagram of a torso marking the three US Navy tape measurement sites at the neck, waist and hip The Navy Body Fat Method: How to Measure It and How Wrong It Can Be Measure for the traditional Navy circumference equations, understand their units, and avoid treating body-fat estimates as a diagnosis. Weight scale for a 180 cm man showing Devine at 75 kg, Robinson at 72.7 kg and the healthy BMI band from 59.9 to 80.7 kg Why Ideal Weight Formulas Disagree With Each Other Compare Devine, Robinson, Miller and Hamwi formulas, including their different outputs and why none defines a personal ideal weight. Stacked bar splitting daily fluid into 50 percent plain water, 30 percent other drinks and 20 percent food, beside the 3.7 L and 2.7 L targets How Much Water You Actually Need Per Day Understand total water from food and drinks, the assumptions in the water calculator, and why there is no universal bottle target. Line chart of seven days of scale weight swinging 1.6 kg around a flat weekly trend line Why the Scale Jumps Two Kilos Overnight Learn how water, food and measurement conditions affect scale weight, and when an unexplained change needs medical advice. Bar chart of calorie surplus sizes by training year, from 17.5 percent above maintenance in year one down to 7.5 percent for advanced lifters How Big Should a Calorie Surplus Be? Compare small calorie-surplus scenarios and understand why extra food cannot predict a specific amount of muscle gain. Curve showing the muscle benefit of protein rising then plateauing at 1.62 grams per kilogram, with the 0.8 gram RDA marked far below it How Much Protein You Actually Need Per Day Put protein research ranges in context, calculate an example intake, and understand why study averages are not personal prescriptions. Bar chart of calorie tracking errors: 20 percent for food labels, 27 percent for wrist trackers and 47 percent for self-reported intake Why Your Calorie Count Is Wrong, and by How Much Understand serving sizes, food logs and watch estimates without assuming a fixed error percentage applies to your calorie count. Line chart of body weight falling from 95 kg then flattening into a plateau, beside the four-step order for working through the causes Why Weight Loss Stalls, and What to Change First Review a weight-loss plateau without automatically cutting calories, blaming tracking or assuming metabolism is permanently damaged.
The arithmetic

What the numbers rest on

Four equations do most of the work across these tools, and each has a known range where it stops being reliable.

BMI

Quetelet, 1832

Weight in kilograms divided by height in metres squared. Adolphe Quetelet built it to describe populations and said in the same work that it did not apply to individuals, which is still the fairest summary of what it can and cannot do.

Energy

Mifflin-St Jeor, 1990

Predicts resting energy expenditure from weight, height, age and sex. Multiplying that figure by an activity factor gives total daily energy expenditure. The resting half lands within about 10 percent for most adults. The activity factor is the loose half, because you are grading your own week.

Body fat

US Navy circumference method

Reads neck, waist and, for women, hip against height. Ideal weight compares the Devine, Robinson, Miller and Hamwi formulas with the healthy BMI band. All four take height and sex and nothing else, so they return a reference figure rather than a target.

Hydration

Body weight plus exercise

No equation here measures you. Each one predicts the average of a study population, and two weeks of your own data will beat any of them.

Scope and limits

These calculators are general estimates for healthy adults aged 18 and over. They are not diagnostic tools, they are not medical advice, and they are not built for children, pregnancy, or athletes at the edges of the range. Speak to a qualified professional before acting on any of them. If a figure on this site is wrong, tell us and we will correct it and date the correction.