Six free tools
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.
Every tool sits on its own page, next to the arithmetic it runs and the questions people ask about it.
Six calculators, one page each. Nothing asks for an account and nothing is emailed to you.
Kilograms or pounds, centimetres or feet. The switch in the header changes every field at once.
Each page prints the equation it used, a worked example, and the point where the estimate stops holding.
What each formula is doing, where it was derived, and the situations where it gives the wrong answer.
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.
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.
How Big Should a Calorie Deficit Be?
Compare calorie-deficit examples and learn why fixed calorie rules cannot promise a weekly weight-loss result.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Four equations do most of the work across these tools, and each has a known range where it stops being reliable.
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.
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.
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.
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.
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.