ReckonBoxHow many calories should you eat?
A calorie calculator estimates how many calories to eat in a day to hold your weight, and how far to shift that number to lose or gain. It starts from your resting metabolism, the energy your body spends just staying alive, then scales it up for how much you move. The result is a maintenance figure, with lose and gain targets built around it.
The energy you burn is your TDEE; the number here is the mirror image, the energy you take in. Eat at maintenance and your weight holds. Eat under it and your body makes up the difference from fat stores. That single balance, calories in against calories out, drives every result on this page.
Choosing your formula
The resting-metabolism formula is the one assumption every calorie estimate rests on, so this tool puts the choice in your hands. Most calculators lock you into one equation and never name it. Three are worth knowing.
| Formula | Works from | Best when |
|---|---|---|
| Mifflin-St Jeor | Weight, height, age, sex | You do not know your body fat. This is the default. |
| Harris-Benedict | The same inputs, from the classic 1919 study revised in 1984 | You want the older equation for comparison |
| Katch-McArdle | Lean mass, using your body fat percentage | You know your body fat and carry more muscle than average |
A 2005 comparison in the Journal of the American Dietetic Association found Mifflin-St Jeor predicted resting energy within 10 percent for the most people, which is why it leads here. If you have a recent body fat reading, Katch-McArdle can edge it out, because two people at the same weight can carry very different amounts of muscle.
What a calorie deficit really does
A calorie deficit is eating fewer calories than your body spends, so it draws on stored fat to cover the gap. A pound of fat holds roughly 3,500 calories. Divide that by seven and you get the famous rule: cut 500 a day, lose about a pound a week.
That rule is right for the first few weeks and wrong after. Research led by Kevin Hall at the National Institutes of Health showed that as the weight comes off, resting metabolism falls with it, so a fixed 500 deficit returns less each month and the loss curve flattens. It's why month six never matches month one on the same diet. An honest target treats that early pound-a-week as the fast case, not the forever case, and plans to trim a little more over time to keep moving. The calculator shows the standard targets so you have a starting point; your own results over two to three weeks are the real guide.
A worked example
Picture a 28 year old woman, 165 cm and 62 kg, exercising a couple of days a week. Her Mifflin-St Jeor resting burn is 1,350 calories. Light activity multiplies that by 1.375, putting maintenance at about 1,856 calories a day. To lose around a pound a week she would eat near 1,356, a 500 deficit that still sits above the safe floor. To gain, she would push to about 2,356 and lean on strength work.
Switch her to Katch-McArdle with a 25 percent body fat reading and the resting figure shifts, because that equation works from lean mass and her muscle burns at a different rate than her scale weight implies. Same person, different lens, slightly different number. Both are close enough to act on and adjust.
The lowest you should safely go
Very low targets are where calorie math stops being harmless. Eating below about 1,200 calories a day for women, or 1,500 for men, makes it hard to get enough protein, vitamins, and minerals, and it tends to backfire as hunger and fatigue build. The fast-loss row can dip under those floors for smaller or older bodies. When it does, a smaller deficit held longer beats a bigger one you cannot sustain. Anyone who needs to eat that low for a medical reason should do it with a doctor or dietitian watching.
Why men and women get different numbers
Sex shifts the number because it shifts body composition. Men on average carry more lean muscle, which burns more at rest, so the Mifflin-St Jeor formula adds 5 to a man's resting figure and subtracts 161 from a woman's. That single constant is the whole difference. A 70 kg man and a 70 kg woman of the same age and height land a couple of hundred calories apart before activity is even counted, which is why a shared household calorie target rarely fits both people.
Frequently asked questions
How many calories should I eat a day? Most adult women hold their weight on roughly 1,800 to 2,400 calories a day and most men on 2,400 to 3,000, but your own number depends on your size, age, and how much you move. The calculator gives your figure, then the eating targets to lose or gain from it.
What is a calorie deficit? A calorie deficit is eating fewer calories than you burn, which forces your body to draw on stored energy. A deficit of about 500 calories a day loses roughly a pound of fat a week at first, though the loss slows as you get lighter.
Is 1,200 calories a day too low? For most adults, 1,200 calories is near the floor. Eating below about 1,200 for women or 1,500 for men risks missing nutrients and is not recommended without a doctor or dietitian supervising.
Which calorie formula is most accurate? Mifflin-St Jeor is the default because a 2005 review found it predicted resting metabolism most accurately for the general population. Katch-McArdle can beat it if you know your body fat percentage, since it works from lean mass.
Do I have to count calories to lose weight? No. Counting is one method; portion control, higher protein, and cutting liquid calories work without a tracker. The number here is a starting target, not a rule you must log against every day.
Sources
- Mifflin MD, St Jeor ST et al., 1990, American Journal of Clinical Nutrition
- Frankenfield D et al., 2005, comparison of predictive equations, Journal of the American Dietetic Association
- Hall KD et al., 2011, quantification of the effect of energy imbalance on bodyweight, The Lancet
- Dietary Guidelines for Americans, 2020 to 2025, estimated calorie needs