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How many calories
should you eat?
Pick the goal first. The numbers follow from it, including the protein target and the one figure no ordinary calorie calculator will show you.
Check the numbers
Where these numbers come from
Every equation, named
The contested position in this corner of the internet is not the arithmetic, everybody has the same arithmetic. It is whether anyone tells you where it came from and how wrong it can be.
Mifflin-St Jeor
Mifflin MD, St Jeor ST, et al. Am J Clin Nutr 1990;51(2):241–7
Derived from 498 healthy adults aged 19–78. A 2005 systematic review found it predicted measured resting rate to within 10% for more people than Harris-Benedict, Owen or WHO/FAO/UNU. Accuracy falls away above a BMI of about 35 and in older adults.
Harris-Benedict, revised
Roza & Shizgal, 1984 revision of Harris & Benedict, 1919
The revision, not the 1919 original, whose coefficients are known to overestimate. Included because it is the default on most other calculators, so you can compare against them without concluding one of the two is broken.
Katch-McArdle
BMR = 370 + 21.6 × lean body mass (kg)
Ignores sex, age and height and works from lean mass, which is why it suits a lifting audience, two people of the same height and weight with different lean mass genuinely differ. It is only as good as the body-fat figure you feed it, which is why we ask for one rather than guessing.
Protein
ISSN Position Stand (JISSN 2017) · Morton et al., Br J Sports Med 2018
1.4–2.0 g/kg/day is sufficient for most people training; the meta-analysis puts the plateau for training gains at about 1.62 g/kg. In a deficit the ISSN puts trained lifters at 2.3–3.1 g/kg to hold onto lean mass, which is why our cutting number goes up, not down.
Energy availability
IOC consensus statement on Relative Energy Deficiency in Sport, 2018, updated 2023
Below 30 kcal per kg of fat-free mass per day is the threshold where negative health outcomes begin to appear; around 45 is suggested as optimal for athletes. Stated limit: the 30 figure originates in laboratory studies of non-athletic women, and its universal application is questioned in the literature. We use it as a warning line, never as a diagnosis.
The activity multiplier
No primary validation found, searched 31 July 2026
1.2 / 1.375 / 1.55 / 1.725 / 1.9 are conventionally attributed to Harris-Benedict, and the trail runs into training-course material and calculators citing each other. It moves your answer more than any other input on this page. We show it because leaving it out would be worse, and we tell you it is a bracket rather than a measurement.
Questions
- Which equation should I pick?
- Mifflin-St Jeor unless you know your body fat percentage, in which case Katch-McArdle is structurally better for a lifter because it works from lean mass rather than from height and age. Harris-Benedict is here because it is what most other calculators use, so you can compare like for like, it reads a little higher.
- Why is your protein number higher than other calculators?
- Because most of them set protein as a share of calories, which quietly cuts it exactly when you are dieting and need it most. This sets it per kilogram of bodyweight: 1.6 g/kg normally, which is where the Morton 2018 meta-analysis found the dose-response plateaus, and 2.3 g/kg in a deficit, the lower end of the ISSN's hypocaloric range for trained lifters.
- What is energy availability and why does nothing else show it?
- It is what is left to run your body on after training has taken its share, measured per kilogram of lean mass. It is the measure sports medicine actually uses, and a calorie target that looks fine on paper can sit under the threshold once you are training six days a week. Generic calculators do not show it because they are not built for people who train.
- How accurate is any of this?
- Less accurate than the number of digits suggests. Mifflin-St Jeor lands within 10% of measured resting rate for more people than the alternatives, but that still means one person in several is out by more, and it degrades at higher BMIs. The activity multiplier is a bigger source of error than the equation. Use it as a starting point and correct it against two or three weeks of real weigh-ins.
- Do you store any of this?
- No. Every calculation runs in your browser and nothing is sent anywhere. There is no account, no email box and no cookie behind this page.
This is general information, not advice. It is a calculator, and it does not know your medical history. If you are pregnant, under 18, managing a health condition, recovering from an eating disorder, or being treated by a clinician, work from what they tell you rather than from this page.
A target is a guess until you check it
The honest way to find your real maintenance is to eat a known amount and watch the scale for a few weeks. Spottr stores your weigh-ins and your training, so it can tell you what actually happened rather than what an equation predicted.