BMR, TDEE and Calories: The Numbers Behind Every Diet

Calorie targets do not come from nowhere. They come from two numbers — BMR and TDEE — plus the size of the gap you choose to eat in. Once you can see the arithmetic, a lot of diet advice stops looking mysterious.

Definitions that finally stick

Most confusion starts with abbreviations. Here are the three that matter most:

• BMR (basal metabolic rate): calories burned at complete rest just staying alive — breathing, circulating blood, maintaining temperature. Not walking, not digesting a meal, not fidgeting.
• TDEE (total daily energy expenditure): BMR multiplied by an activity multiplier. This is your actual daily burn once movement and digestion are included.
• Deficit: eating below TDEE, so the body has to pull stored energy from somewhere else.

A surplus is the opposite — eating above TDEE. Maintenance means eating roughly at TDEE. Every calorie goal is just one of these three positions.

Why three BMR formulas exist

Three formulas show up constantly, and they exist because each one weighs your inputs differently. All three estimate the same resting burn.

Mifflin-St Jeor

The newest of the three and generally the most accurate for most people. It uses weight, height, age, and sex. This is the one most modern calculators default to.

Harris-Benedict

The 1919 original, revised in 1984. It also uses weight, height, age, and sex, but it tends to read higher than Mifflin-St Jeor for the same person.

Katch-McArdle

This one uses lean body mass instead of total body weight, which means it needs a body-fat estimate first. If you know your body-fat percentage reasonably well, it can be useful; if you guess that number, the guess becomes part of the result.

Body-fat percentage is where many people stall, and it is also why some people reach for a BMI figure instead. Our BMI Calculator covers that neighbouring number, though BMI does not change any of the three BMR formulas below.

Worked example: one man, three BMRs

The same person can get three slightly different answers. Here is a 30-year-old man, 80 kg, 178 cm, with a body-fat estimate of 20% where the formula needs it.

That spread — 1,752 to 1,844 — is about 92 kcal per day. None of them is “wrong.” They are three estimates of the same resting burn.

From the Mifflin-St Jeor figure, the TDEE works out to about 2,121 kcal/day if sedentary and about 2,740 kcal/day if moderately active. A 500 kcal deficit on that moderate figure means eating about 2,240 kcal/day.

You can run all three formulas side by side on your own numbers with our BMR Calculator, which also converts each result to TDEE.

Activity multipliers: the honest weak spot

TDEE is built from BMR plus activity, and the multiplier is where most error sneaks in. The common levels are:

• Sedentary: ×1.2
• Light activity: ×1.375
• Moderate activity: ×1.55
• Very active: ×1.725

These are broad buckets, and two people can both choose “moderately active” while living very differently. One may train four times a week and stand all day; another may take three long walks and a yoga class. The calculator cannot see that distinction — it only knows the bucket you picked.

When in doubt, choose the lower activity level. That keeps a deficit more honest and leaves a little room for life.

Deficits in practice: the 3,500 kcal rule

The classic rule of thumb is that about 3,500 kcal equals one pound of body fat. In reverse, a steady 500 kcal/day deficit — 500 × 7 days — totals about 3,500 kcal per week, or roughly a pound a week.

That arithmetic is clean, which is why it appears everywhere. Real bodies are not spreadsheets. Water weight, digestion, measurement error, and changes in non-exercise movement all shift the result week to week. Treat the 3,500 kcal rule as a compass, not a contract.

If you already have a TDEE estimate, our Calorie Calculator turns it into daily targets for losing or gaining at a chosen rate, with protein, carbohydrate, and fat splits built in.

Why every online calculator disagrees slightly

Two sites can return different BMRs for the same body because they may use different formulas and round differently. One might use Mifflin-St Jeor, another Harris-Benedict, another Katch-McArdle. Even two sites using the same formula may round the intermediate steps differently.

The range is the information. A spread of 50 to 100 kcal is not a sign that one tool is broken — it is a reminder that you are working with an estimate. Pick one formula, stay consistent, and adjust based on what your body and the scale actually do over a few weeks.