What Your Body Burns Before You Do Anything
Basal metabolic rate is the energy your body spends staying alive at complete rest, breathing, circulating blood, maintaining temperature and running your brain. For most adults it accounts for around two thirds of daily energy use, which is why it is the number that anything else about calories has to be built on.
This calculator estimates your BMR from height, weight, age and sex, scales it by activity level to give a daily total, and separately shows the ideal-weight ranges that the common clinical formulas produce.
How to Use the Calculator
Accuracy depends almost entirely on the activity multiplier, which is the input people most often overestimate.
- Enter your height, weight, age and sex. These four values are what the standard equations need.
- Choose your activity level honestly. Most people who sit at a desk and exercise a few times a week are "lightly active", not "very active".
- Read the BMR figure, energy at complete rest, and the daily total, which is BMR multiplied by the activity factor.
- Compare the ideal-weight range with your current weight. Treat it as a broad band rather than a target, because it takes no account of build or muscle.
- To lose or gain weight, adjust from the daily total rather than from the BMR. Eating at your BMR is a substantial deficit for anyone who moves at all.
- Recalculate after any meaningful change in weight, since BMR scales with body mass and a figure from ten kilos ago will overestimate what you need now.
Which Formula, and How Much to Trust It
Mifflin-St Jeor is the current default, and studies generally find it closer than the older Harris-Benedict equation for modern populations. Katch-McArdle works differently: it is driven by lean body mass rather than total weight, so it needs a body fat percentage but handles very muscular or very lean people better than the others.
All of them are population averages, and individual metabolic rate varies by roughly plus or minus ten per cent around the estimate even when the inputs are correct. Treat the number as a starting point to test against reality: track your intake and weight for two or three weeks, and adjust based on what actually happens rather than on what the formula predicted.
Activity Multipliers
The factor applied to BMR to get daily energy expenditure. Choosing one level too high is the single largest source of error in these estimates.
| Level | Multiplier | What it actually means |
|---|---|---|
| Sedentary | 1.2 | Desk job, little or no deliberate exercise |
| Lightly active | 1.375 | Light exercise 1–3 days a week |
| Moderately active | 1.55 | Moderate exercise 3–5 days a week |
| Very active | 1.725 | Hard exercise 6–7 days a week |
| Extra active | 1.9 | Physical job, or hard training twice a day |
The gap between sedentary and moderately active is around 350 kcal a day for a typical adult, which is more than most people adjust their diet by. If your weight is not moving as the number predicts, an over-generous activity level is the first thing to check.
What Ideal Weight Formulas Do Not Know
The Devine, Robinson, Miller and Hamwi formulas all derive an ideal weight from height and sex alone. They were developed for clinical purposes such as drug dosing rather than as personal targets, and because they ignore frame size and muscle they classify many athletic people as overweight while missing low muscle mass entirely.
This calculator gives estimates for general interest and is not medical advice. If you are managing a health condition, pregnant, under eighteen, or planning a substantial change in weight, the number to act on is the one a doctor or registered dietitian gives you after looking at more than four inputs.