Free health calculators

Why the calculators run in your browser

BMI, TDEE, body fat percentage, and the rest are formulas. They are simple enough to evaluate directly in JavaScript on the device, and our servers are not involved at any point. You enter the numbers into a form on the page, the script runs the formula, and the result appears. Close the tab and the inputs are gone from memory.

Results are estimates from population-level formulas (WHO BMI categories, Mifflin-St Jeor BMR, Hodgdon-Beckett body fat, Tanaka max heart rate, and so on, all cited on the individual tool pages). They are not a substitute for clinical assessment. Discuss anything that matters with a healthcare provider who can interpret the numbers in context.

How the health calculators work

Each calculator implements a published formula directly in JavaScript. The BMI calculator divides weight in kilograms by height in metres squared and looks up the WHO category for the result. The TDEE calculator applies the Mifflin-St Jeor basal metabolic rate equation and multiplies by the activity factor you choose. The body fat calculator uses the US Navy circumference method (Hodgdon & Beckett, 1984), which estimates body fat from neck, waist, and (for women) hip measurements together with height.

The one rep max calculator combines the Epley and Brzycki formulas. The heart rate zone calculator uses the Tanaka equation (208 minus 0.7 times age) for maximum heart rate, then computes five training zones as percentages of that value, with the Karvonen formula available if you supply your resting heart rate.

The lean body mass calculator applies the Boer (1984) equations to weight and height, with separate versions for men and women. The ideal weight calculator runs the Hamwi and Devine rules next to the weight range that gives a BMI of 18.5 to 24.9 at your height. The due date calculator adds 280 days to the first day of the last period (Naegele's rule), or 266 days to a conception date.

All math is done in standard JavaScript with no library beyond the language itself. The formulas are simple enough that there is nothing to outsource to a server.

In-browser health calculators vs the alternatives

The three options answer different questions. Use the in-browser calculators for a quick number worked out in your browser tab. Use a spreadsheet, or a fitness app with cloud sync, to track changes over months. For medical advice, talk to a doctor or another qualified health professional.

When to use these tools

• Setting weight or nutrition goals? Combine the BMI calculator with the TDEE and calorie target estimator to plan a realistic surplus or deficit.
• Curious about body composition rather than just total weight? Estimate body fat percentage with tape measurements, or compute lean body mass using the Boer formula.
• Programming gym sessions? Predict your strength ceiling with the one rep max calculator and dial in cardio intensity through the heart rate zone calculator.
• Looking for a healthy weight range? The ideal weight calculator compares Hamwi, Devine, and BMI-range estimates for your height.
• Expecting? Map out a timeline with the pregnancy due date calculator, which shows gestational age and trimester.

Frequently asked questions

Are these health calculators free?

Yes. Open any calculator, enter your numbers, and the result appears immediately. Every calculator on the site is free to use.

Is my personal data (weight, age, body fat) sent to a server?

Every calculator runs as JavaScript inside your browser. All processing is local: the numbers you enter are used only for the calculation in that browser tab.

How accurate are these calculators?

Each calculator uses a published formula: BMI follows the WHO classification, TDEE uses Mifflin-St Jeor (1990), body fat uses the US Navy circumference method, lean body mass uses Boer (1984), ideal weight uses Hamwi, Devine, and the BMI range, one rep max uses Epley and Brzycki, heart rate zones use the Tanaka formula (2001), and the due date uses Naegele's rule. Clinicians and coaches use the same formulas, but the results are estimates, not diagnoses.

Should I use these results to make medical decisions?

Treat them as a starting point for a conversation with a doctor or a qualified health professional, not as the basis for a medical decision. A BMI in the overweight range, an unexpected body fat percentage, or any other surprising result should be discussed with them. The calculators give you a starting number, not advice.

Why does BMI sometimes feel wrong?

BMI is a ratio of weight to height squared. It doesn't account for muscle mass, bone density, or body composition. A very muscular person can register as overweight by BMI while having low body fat. For a more nuanced view, combine BMI with the body fat calculator.

Do I need to enter metric units, or does imperial work too?

Most calculators take either. BMI and lean body mass switch between metric (kg, cm) and imperial (lb, ft + in), ideal weight takes height in cm or ft + in and shows results in kg and lb, the body fat calculator takes cm or inches, and the one-rep max takes kg or lbs. The calorie calculator is metric only (kg and cm), so convert pounds (x 0.4536) and inches (x 2.54) first. Heart rate zones and the due date calculator have no unit choice.

Do these calculators work offline?

Once a calculator's page has loaded, yes, in that tab. Every calculation runs in the page, so you can disconnect and keep using the calculator until you close or reload the tab or open another page. Opening or reloading a page needs a connection, because the site installs no service worker and the browser checks each page with the server as it opens. Heading to a gym with bad Wi-Fi? Open the calculator before you go.

Why doesn't the site offer to save my history?

The calculators keep your inputs only in the open tab, so each result is a one-off calculation. To follow results over time, copy the numbers into a spreadsheet or a tracking app you choose.