How much do you actually burn?
Your basal metabolism, your everyday movement and your training are three different numbers. This calculator keeps them apart, so you get an energy need that matches the week you actually live.
Four numbers make up your day.
Total daily energy expenditure is not one number but four, and they behave very differently. Separating them is what makes a calculator useful instead of just quick.
Basal metabolism
What your body uses to keep you alive at complete rest: heartbeat, breathing, temperature, repair. It is the largest single part of the day, usually 60 to 70 % of the total, and it scales with how much lean mass you carry.
Everyday movement
Everything you do that is not training: walking, standing, stairs, fidgeting, carrying groceries. This is the part that varies most between two people of the same size, and the part that quietly shrinks when you eat less.
Training
Planned sessions. Usually a smaller share than people expect: an hour of hard strength training is worth a few hundred calories, not a thousand. It is what training does to your body over months that matters, not the hour itself.
Digestion
Breaking food down costs energy: roughly 10 % of what you eat, and more when the meal is high in protein. Standard activity factors already include this, which is why the calculator leaves it switched off by default.
What is under the hood.
No hidden constants. These are the equations the calculator uses, in the form they were published.
Mifflin-St Jeor
Men: 10 × kg + 6.25 × cm − 5 × age + 5
Women: 10 × kg + 6.25 × cm − 5 × age − 161
The default. In the largest independent comparison it predicted resting metabolism within 10 % of measured values more often than any other equation, in both normal weight and obese adults.
Katch-McArdle
370 + 21.6 × lean body mass in kg
Ignores sex, height and age and works only from lean mass, which is what actually burns the energy. Better than Mifflin if body fat has been measured, and clearly better at the extremes: very muscular or very heavy.
Harris-Benedict
Men: 88.362 + 13.397 × kg + 4.799 × cm − 5.677 × age
Women: 447.593 + 9.247 × kg + 3.098 × cm − 4.330 × age
The original from 1919, in the 1984 revision by Roza and Shizgal. Included because it is still the most widely quoted, but it tends to overestimate slightly compared with Mifflin.
Cunningham
500 + 22 × lean body mass in kg
Also built on lean mass, and developed on a leaner and more athletic sample than Katch-McArdle. It usually lands a little higher, which fits trained people better.
Training
(MET − 1) × 3.5 × kg ÷ 200 × minutes
Energy cost per minute from the Compendium of Physical Activities, minus the resting energy you would have used anyway. Without that subtraction, training gets counted twice: once in your basal metabolism and once in the session.
Deficit and surplus
7 700 kcal ≈ 1 kg body mass
A simplification, and a useful one for a few months at a time. Over longer periods your body adapts: you weigh less, move less and burn less, so the calculator is a starting point to adjust from, not a promise.
Common questions.
How accurate is this?
For most people the estimate lands within about 10 % of measured energy expenditure, which is as good as a prediction equation gets without a metabolic chamber. Treat the number as a starting point: weigh yourself over two to three weeks, and if the trend is not moving the way you planned, adjust by 100 to 200 kcal.
Why is the training number lower than my watch says?
Two reasons. The calculator subtracts the energy you would have burned sitting still during that hour, because it is already counted in your basal metabolism. Watches usually report the gross figure. Wrist based heart rate also overestimates the cost of strength training, where the heart rate is high but the actual work is low.
Should I eat back the calories I burn training?
They are already in the number. The calculator spreads your weekly training across seven days, so the target it gives you is the same every day, including rest days. That is easier to follow than a figure that moves around, and the weekly total is what decides the outcome.
Why so much protein?
In a deficit, protein is what protects muscle while you lose fat, and it keeps you fuller per calorie than fat or carbohydrate. Around 1.6 g per kg of body weight covers most people who train; in a deficit, or if you are lean and want to stay that way, more is better.
Is this medical advice?
No. It is an estimate built on population equations for healthy adults. If you are pregnant or breastfeeding, under 18, have a metabolic or thyroid condition, or have a history of disordered eating, talk to a dietitian or a doctor instead of following a calculator.
Where the numbers come from.
- Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition. 1990;51(2):241-247. PubMed
- Frankenfield D, Roth-Yousey L, Compher C. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. Journal of the American Dietetic Association. 2005;105(5):775-789. PubMed
- Roza AM, Shizgal HM. The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass. American Journal of Clinical Nutrition. 1984;40(1):168-182. PubMed
- Cunningham JJ. Body composition as a determinant of energy expenditure: a synthetic review and a proposed general prediction equation. American Journal of Clinical Nutrition. 1991;54(6):963-969. PubMed
- Ainsworth BE, Haskell WL, Herrmann SD, et al. 2011 Compendium of Physical Activities: a second update of codes and MET values. Medicine & Science in Sports & Exercise. 2011;43(8):1575-1581. PubMed
- FAO, WHO, UNU. Human energy requirements. Report of a joint expert consultation. Rome, 2004. Physical activity level values. fao.org
- Morton RW, Murphy KT, McKellar SR, et al. A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training induced gains in muscle mass and strength. British Journal of Sports Medicine. 2018;52(6):376-384. PubMed
- Helms ER, Aragon AA, Fitschen PJ. Evidence based recommendations for natural bodybuilding contest preparation: nutrition and supplementation. Journal of the International Society of Sports Nutrition. 2014;11:20. PubMed
- Trexler ET, Smith-Ryan AE, Norton LE. Metabolic adaptation to weight loss: implications for the athlete. Journal of the International Society of Sports Nutrition. 2014;11:7. PubMed
- Hall KD, Sacks G, Chandramohan D, et al. Quantification of the effect of energy imbalance on bodyweight. The Lancet. 2011;378(9793):826-837. PubMed
- Nordic Nutrition Recommendations 2023. Nordic Council of Ministers, Copenhagen, 2023. Reference values for fibre, fat and protein. norden.org
- EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific opinion on dietary reference values for water. EFSA Journal. 2010;8(3):1459. efsa.europa.eu
Frog Fitness is a training platform, not a healthcare provider. The calculator gives an estimate for healthy adults and does not replace advice from a dietitian or a doctor.
A number is a start. A habit is the point.
Log your food and your training in the same app, and watch the target adjust to a body that changes.