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How many calories should I eat?
Enter your data into three calculators and you get three different numbers. That is not a bug – each uses a different formula and, above all, a different activity estimate. The good news: the exact starting number matters less than adjusting it to reality after a few weeks.
In short
- A calculator first estimates your resting metabolism, then multiplies it by an activity factor.
- Most of the difference comes from the activity factor, not the formula itself.
- Even the best equation is off by more than 10 % for some people.
- Treat the number as a starting point; adjust after 3–4 weeks based on your weight trend.
How daily calories are calculated
Almost every calculator works in two steps:
- Resting metabolic rate – how much energy your body burns at rest. The most common choice is the Mifflin-St Jeor equation, which came out as the most accurate of the common equations in a comparison study.
- Multiplying by an activity factor – roughly 1.2 for a sedentary life up to 1.9 for heavy physical work plus training.
Women: 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161
| Activity | Factor |
|---|---|
| Sedentary (desk job, little movement) | 1.2 |
| Lightly active (exercise 1–3× a week) | 1.375 |
| Moderately active (3–5× a week) | 1.55 |
| Very active (6–7× a week) | 1.725 |
| Extremely active (physical job + training) | 1.9 |
A worked example
Woman, 35, 165 cm, 75 kg, office job and two walks a week:
1,445 × 1.375 = about 1,990 kcal (maintenance)
400 kcal deficit → about 1,590 kcal a day to lose weight
Man, 40, 180 cm, 90 kg, moderately active: 1,830 kcal × 1.55 = about 2,840 kcal for maintenance.
Why calculators differ
- Different formula: older calculators use the Harris-Benedict equation, others account for body fat. The difference is usually tens to low hundreds of kcal.
- Different activity factor: the gap between 'lightly' and 'moderately active' at 1,800 kcal is about 300 kcal. Most people overestimate their activity.
- Different deficit: some calculators subtract 500 kcal, others 20 %, others work from a target pace.
- Individual differences: even Mifflin-St Jeor predicted resting expenditure within 10 % for only about 70–80 % of people in the study. For the rest the error was larger.
How to adjust to reality
The most accurate 'calculator' is your own diary. Here is how:
- Pick a starting number and stick to it honestly for 3–4 weeks – weekends included.
- Weigh yourself under the same conditions and compare weekly averages, not single days.
- If the average does not move and your logging is complete, take off 100–200 kcal. If it drops faster than is healthy, add some.
- Expect weight loss to slow over time – as weight drops, so does expenditure. The '3,500 kcal = 1 lb' rule only holds at the start.
What to watch out for
- Very large deficits are hard to sustain and increase the risk of losing muscle. Very low calorie diets belong under medical supervision.
- If you are pregnant, breastfeeding, have diabetes, a thyroid condition or take medication that affects weight, talk to your doctor.
- A calculator knows nothing about your sleep, stress or health. It is an estimate, not a prescription.
Frequently asked questions
How many calories should I eat to lose weight?
Estimate your maintenance intake (resting metabolism × activity factor) and subtract roughly 300–500 kcal. After 3–4 weeks adjust the number based on your weight trend.
Why does every calculator show a different number?
Calculators use different formulas, different activity factors and different ways of calculating the deficit. The activity estimate causes the biggest difference.
Which equation is the most accurate?
Among the common equations, Mifflin-St Jeor performs best in comparison studies, but it is still off by more than 10 % for some people. That is why checking it against your own data matters.
Sources
- Mifflin et al. (1990), Am J Clin Nutr – new predictive equation for resting energy expenditure
- Frankenfield et al. (2005), J Am Diet Assoc – comparison of predictive equations for resting metabolic rate
- EFSA (2013) – Scientific Opinion on Dietary Reference Values for energy
- Hall et al. (2011), Lancet – quantifying the effect of energy imbalance
- NIH/NIDDK – Body Weight Planner
This guide is general information for healthy adults, not medical advice. If you are pregnant, breastfeeding, have or have had an eating disorder, or need a therapeutic diet, talk to a doctor or registered dietitian.