Type the same age, height and weight into two different calorie calculators and you'll often get answers that differ by 100 or 200 calories a day. That can be confusing, especially if you're trying to plan meals around a target. The difference usually isn't a mistake. It comes from which formula each tool uses and how it treats your activity level. Once you understand the parts of the estimate, the numbers make much more sense.
What BMR actually measures
Basal metabolic rate (BMR) is the energy your body uses at complete rest to keep you alive: breathing, circulating blood, maintaining body temperature, and running your organs and brain. It's measured, in a lab, after a night's sleep and a period of fasting, lying still in a comfortable room. For most people, BMR makes up the largest share of daily energy use, typically well over half.
A closely related figure, resting metabolic rate (RMR), is measured under slightly less strict conditions and comes out a little higher. Many calculators use the terms loosely, so don't worry if you see both.
The main formulas
Because lab measurement isn't practical for most people, calculators estimate BMR from body size, age and sex. The best-known equations are:
- Harris–Benedict (1919, revised 1984): one of the oldest, still widely used.
- Mifflin–St Jeor (1990): generally considered more accurate for modern adults and used by many dietitians.
- Katch–McArdle: uses lean body mass instead of total weight, so it needs a body-fat percentage.
The Mifflin–St Jeor equation looks like this, with weight in kilograms, height in centimetres and age in years:
- Men:
BMR = 10 × weight + 6.25 × height − 5 × age + 5 - Women:
BMR = 10 × weight + 6.25 × height − 5 × age − 161
For a 35-year-old woman who is 165 cm tall and weighs 68 kg:
10 × 68 + 6.25 × 165 − 5 × 35 − 161 = 680 + 1,031.25 − 175 − 161 ≈ 1,375 kcal/day
Harris–Benedict would give a somewhat different figure for the same person, which is the first reason calculators disagree.
From BMR to TDEE
Nobody spends the whole day lying still, so BMR alone underestimates your needs. Total daily energy expenditure (TDEE) adds everything else:
- Thermic effect of food: energy used to digest and process meals, roughly 10% of intake.
- Exercise: planned workouts, sport and training.
- NEAT (non-exercise activity thermogenesis): walking around, standing, fidgeting, housework and everyday movement. It varies more between people than most of us realise.
Calculators usually roll all of this into a single activity multiplier:
| Activity level | Typical multiplier | |---|---| | Sedentary (desk job, little exercise) | 1.2 | | Lightly active (exercise 1–3 days/week) | 1.375 | | Moderately active (3–5 days/week) | 1.55 | | Very active (6–7 days/week) | 1.725 | | Extremely active (physical job plus training) | 1.9 |
Using our example, a lightly active TDEE would be 1,375 × 1.375 ≈ 1,890 kcal/day.
Why estimates differ
- Different equations. Mifflin–St Jeor and Harris–Benedict can differ by 5–10% for the same person.
- Different multipliers. Some tools use slightly different values or more activity categories.
- Body composition. Two people with the same weight but different muscle mass have different BMRs. Standard formulas can't see that, which is why Katch–McArdle exists.
- Self-reported activity. Most people overestimate how active they are, which inflates TDEE.
- Rounding and units. Converting pounds and inches can introduce small differences.
Using the numbers sensibly
Treat any calorie estimate as a starting point, not a prescription. A practical approach:
- Calculate your BMR and TDEE with one consistent method.
- Eat around that level for two or three weeks while tracking your weight trend (use a weekly average; daily readings bounce around).
- Adjust gradually by 100–200 kcal/day if your weight moves in a direction you didn't intend.
If you want a quick, transparent figure to start from, the BMR Calculator - Basal Metabolic Rate on Quick Calculators shows the formula it applies so you can check the working yourself.
When to get professional advice
Online estimates are designed for healthy adults. If you're pregnant or breastfeeding, under 18, managing a medical condition such as diabetes or thyroid disease, or have a history of disordered eating, speak to a doctor or registered dietitian rather than relying on a calculator.
Summary
BMR is what you burn at rest; TDEE adds food, movement and exercise. Calculators differ because they use different equations and multipliers, and none can see your exact body composition. Choose one method, use it consistently, and let your real-world results guide small adjustments.