Calvin
How many calories should I eat a day?
Most adult women need roughly 1,600 to 2,400 calories a day and most adult men 2,000 to 3,000, according to the Dietary Guidelines for Americans. A personal estimate starts with the Mifflin-St Jeor equation and an activity multiplier — but only your own weight trend confirms the number.
How do you calculate your own calorie needs?
Two steps. First, resting metabolic rate from the Mifflin-St Jeor equation: 10 × weight in kilograms + 6.25 × height in centimetres − 5 × age, then +5 for men or −161 for women. Second, multiply by an activity factor between 1.2 and 1.9.
Worked through for a 35-year-old woman, 165 cm and 70 kg: (10 × 70) + (6.25 × 165) − (5 × 35) − 161 = 700 + 1,031 − 175 − 161 ≈ 1,395 calories at rest. Lightly active, at ×1.375, that is about 1,918 calories a day to maintain weight.
| Activity level | Multiplier | What it means in practice |
|---|---|---|
| Sedentary | 1.2 | Desk job, little deliberate exercise |
| Lightly active | 1.375 | Light exercise 1–3 days a week |
| Moderately active | 1.55 | Moderate exercise 3–5 days a week |
| Very active | 1.725 | Hard exercise 6–7 days a week |
| Extra active | 1.9 | Physical job, or twice-daily training |
The multiplier is where most of the error enters. It is one number standing in for an entire week of movement, and people routinely place themselves a category too high.
How accurate is the Mifflin-St Jeor equation?
It is the most reliable of the common equations and still an estimate. A systematic review found it predicted resting metabolic rate within 10% more often than Harris-Benedict, Owen or the WHO equation. Its own validation study reported an R² of 0.71 across 498 adults, so roughly a third of the variation is unexplained.
Published in 1990 from measurements on 251 men and 247 women aged 19 to 78, the equation is a good population average. What it cannot know is your particular body composition, thyroid function, or how much you fidget. Two people matching on every input can genuinely differ by a few hundred calories a day.
So treat the output as a starting point with a plausible error of ±200–300 calories, not a prescription.
How big a calorie deficit should you actually run?
Modest and sustainable beats aggressive. A deficit of roughly 10 to 20% below maintenance is the usual guidance, and even that slows as your body adapts. Anyone considering a large deficit, or eating below the Dietary Guidelines ranges, should talk to a doctor or a registered dietitian first.
- A smaller deficit is not a slower one, past a point. Aggressive restriction tends to end in abandonment, which nets out worse than a deficit you can hold for months.
- Expect the rate to taper. Expenditure falls as weight falls, so the same intake buys less change over time.
- Protein and resistance training matter for what you lose, not just how much. That is a topic for a dietitian rather than a tracking app.
Why does the calculated number stop working after a few weeks?
Because it was an average to begin with, and because your body adapts. Energy expenditure falls as weight comes down, which is why the NIH Body Weight Planner models the decline rather than assuming a fixed 3,500 calories per pound. Checking the number against your weight trend is the correction.
This is the argument for an adaptive calorie target: run the formula once to get started, then let a smoothed weight trend tell you what your maintenance calories really are. The formula guesses; the scale, read over weeks rather than days, measures.
Common questions
Is 1,200 calories a day enough?
It is below the Dietary Guidelines range for adult women, which starts at about 1,600 calories a day. Very low intakes are hard to meet nutrient needs on and are usually medically supervised. This is a question for a doctor or a registered dietitian, not for a calculator.
How many calories do I need to lose weight?
Fewer than you spend, by a margin you can hold. A deficit of roughly 10 to 20% below your maintenance calories is the usual guidance. The harder part is that maintenance is not a fixed number — it falls as body weight falls, so the same intake becomes a smaller deficit over time.
Which calorie calculator is the most accurate?
Mifflin-St Jeor, among the common ones. A systematic review found it predicted resting metabolic rate within 10% of measured values more often than Harris-Benedict, Owen or the WHO equation. All of them are population averages, so none is accurate for a specific individual.
Do I need to eat back my exercise calories?
Only if your target was set without any activity included. Wearable and app estimates of exercise burn carry large errors, so adding them back tends to erase a deficit. A target derived from your actual weight trend already accounts for how much you normally move.
Sources
- Dietary Guidelines for Americans, 2020–2025 (USDA / HHS). Estimated calorie needs by age, sex and activity level.
- Frankenfield, Roth-Yousey & Compher, "Comparison of Predictive Equations for Resting Metabolic Rate", J Am Diet Assoc. The systematic review that found Mifflin-St Jeor the most reliable.
- NIH Body Weight Planner (NIDDK). Models the decline in expenditure that the 3,500-calorie rule ignores.
- Mifflin-St Jeor equation (Medscape reference). The equation as used clinically.
Calvin checks the number against the scale
Start from the formula, then let three weeks of weigh-ins correct it. Free, with no account and no scan limit.
Coming soon to the App Store, for iPhone and Apple Watch. See what Calvin does.
This page is general information, not medical advice, and Calvin is not a medical device. Calorie targets are individual: talk to a doctor or a registered dietitian before making significant changes to what you eat, especially if you are pregnant, under 18, managing a medical condition, or have a history of disordered eating.