Lean — body fat percentage by the U.S. Navy method, plus BMR and TDEE

A tape measure and three numbers gets you closer to the truth than a bathroom scale ever will — body fat percentage, lean mass, and the calories you actually burn in a day.

Weight is the wrong metric to optimise

Two people of the same height and weight can have entirely different body compositions, and the scale reports them identically. Someone who trains and eats well for six months may see their weight barely move while their body composition changes substantially — and if the scale is the only instrument, that looks like failure.

Lean measures the thing that matters. Using the U.S. Navy circumference method it estimates body fat percentage from height, neck and waist — plus hips for women — and reports lean mass and fat mass separately. Then it works out your resting metabolic rate and daily calorie burn, which is the number you need before any nutrition plan makes sense.

What the body fat calculator measures

Enter the measurements once and every figure below comes from the same set.

• Body fat by the U.S. Navy method. The circumference method uses height, neck and waist, with hips added for women. It needs nothing but a soft tape measure, which is why the Navy uses it for tens of thousands of people rather than buying DEXA scanners.
• Lean mass and fat mass in kilograms. A percentage is abstract; kilograms of each are not. Seeing lean mass hold steady while fat mass falls is the clearest possible confirmation that a training block is working.
• BMR by Mifflin-St Jeor. Your resting metabolic rate — what you would burn lying in bed all day — calculated with the equation that has held up best in validation studies.
• TDEE across five activity levels. From Sedentary at 1.20× through Lightly active, Moderately active and Very active to Athlete at 1.9×, each with a description of what it actually means in days per week. Most people overestimate their activity level, and seeing the definitions side by side helps.
• Metric or imperial. Centimetres and kilograms, or inches and pounds, switched in Settings.
• Progress over time. Each calculation can be logged, and the Progress screen charts how body fat, lean mass and BMI have moved. This is where the app stops being a calculator and starts being useful.

Every formula has a citation

Fitness apps are full of numbers with no provenance. Lean does the opposite: a How this is calculated sheet inside the app names each formula and links its source — the U.S. Navy circumference method to the Naval Health Research Center work by Hodgdon and Beckett, and the BMR equation to the 1990 Mifflin-St Jeor paper in the American Journal of Clinical Nutrition. If a number looks wrong you can go and check the method.

The app is also honest about precision. Every surface labels these as estimates, because a tape-measure method has real error bars. What it does well is consistency: measured the same way each time, the trend is reliable even where the absolute figure is approximate.

Who it is for

Anyone in a cutting or bulking phase who needs to know whether the weight moving is fat or muscle. A scale cannot answer that; a monthly tape measurement can, and the progress chart makes the trend obvious after three or four data points.

It also serves people setting a calorie target for the first time. TDEE is the starting point for any deficit or surplus, and getting it from a cited equation with an honest activity multiplier beats the number a random website gives you. And it suits anyone who dislikes fitness apps that demand a subscription, a social feed and a daily streak before showing a single figure.

Accuracy and privacy

The Navy method is an estimate with a typical error of a few percentage points against gold-standard methods such as DEXA. Measure at the same time of day, in the same posture, with the tape snug but not compressing, and the trend will be trustworthy even if the absolute number is approximate.

Measurements and progress entries stay on the device. There is no account, no sync and no upload. The app is free and ad-supported, and it is informational rather than medical advice.

Frequently asked questions

What measurements do I need? Height, weight, neck and waist for men, plus hips for women. A soft tape measure is the only equipment needed.

How accurate is the U.S. Navy method? It is an estimate, typically within a few percentage points of laboratory methods such as DEXA. Measured the same way each time, the trend is reliable even when the absolute figure is approximate.

What is the difference between BMR and TDEE? BMR is what your body burns at complete rest. TDEE multiplies it by an activity factor — 1.20 sedentary up to 1.9 athlete — to estimate your real daily burn.

Does it track changes over time? Yes. Save a calculation and the Progress screen charts body fat, lean mass and BMI over time.

Which formulas does it use, and can I check them? The U.S. Navy circumference method and Mifflin-St Jeor. Both are named and linked to their published sources in the app's How this is calculated sheet.