Eating a little less than you use
A calorie deficit means eating fewer calories than your body uses in a day. Over weeks and months, a steady deficit is what leads to weight loss. The skill is choosing one that is big enough to make progress and small enough to live with, so you can keep it up.
This calorie deficit calculator estimates your maintenance calories, takes off the deficit you choose and gives you a daily target. It never shows a target under 1,000 calories a day and flags any target below your resting energy use. A second mode works out the percentage of your weight you have lost so far.
How to use it
- Pick metric or US units, and fill in your sex, age, height and weight.
- Pick the activity level that matches a normal week.
- Choose a daily deficit. 500 calories a day is a common middle choice.
- Add a goal weight if you want to see what share of your weight it means.
- Read the daily target. Switch to Weight lost so far to check your progress as a percentage.
How the target is worked out
BMR = 10 × kg + 6.25 × cm − 5 × age, then + 5 (men) or − 161 (women)
maintenance (TDEE) = BMR × activity factor
daily target = TDEE − deficit
weight loss percentage = (starting − current) ÷ starting × 100The resting figure uses the Mifflin-St Jeor equation. The calculator stops at 1,000 calories a day, the same floor the NIH Body Weight Planner uses. That planner also models how the body adapts as weight comes off, which is why a simple rule like "a fixed number of calories per pound" over-promises over longer periods. Use it if you want a timeline to a goal date.
Worked example
A 35 year old woman, 165 cm and 68 kg, exercises moderately and wants to reach 62 kg. She picks a 500 calorie daily deficit.
- Maintenance: BMR 1,375 × 1.55 = about 2,132 calories
- Daily target: 2,132 − 500 = 1,632 calories
- Over a week, the deficit adds up to 3,500 calories
- Her goal is 6 kg, 8.8% of her current weight
A target of 1,632 calories a day sits above her BMR, which is a sensible place to be. Later, if she has gone from 68 kg to 64 kg, Weight lost so far shows 5.88% of her starting weight.
Choosing a deficit
| Daily deficit | Per week | Feels like |
|---|---|---|
| 250 | 1,750 | Small changes: a smaller portion, one less snack |
| 500 | 3,500 | A common, steady approach |
| 750 | 5,250 | Noticeably harder; plan meals carefully |
| 1,000 | 7,000 | Large; only with medical advice for most people |
Smaller deficits are easier to keep, and keeping it is what counts. Losing around 5% to 10% of body weight is a goal health bodies often mention because it can bring real benefits for people above a healthy weight.
Common mistakes
- Picking too big a deficit. Very low intakes are hard to sustain and can leave you short of nutrients.
- Over-rating activity. If progress stalls, try the next activity level down rather than eating less again.
- Expecting a straight line. Weight moves up and down from day to day with water and food. Watch the trend over weeks.
- Forgetting protein. Enough protein helps keep muscle while losing weight; the macro calculator can help plan it.
Frequently asked questions
What is a calorie deficit?
It is the gap between the calories your body uses in a day and the calories you eat. If you use 2,100 and eat 1,600, your deficit is 500 calories. Kept up over time, a deficit leads to weight loss. This calculator estimates what you use and sets a target below it.
How big should my calorie deficit be?
Many people start with 250 to 500 calories a day, which is noticeable but manageable. Bigger deficits are harder to keep and are best discussed with a doctor. The calculator never shows a target under 1,000 calories a day and flags any target below your resting energy use.
How do I calculate my weight loss percentage?
Take your current weight from your starting weight, divide by your starting weight and multiply by 100. Going from 90 to 81 kg is 9 ÷ 90 × 100 = 10%. Choose Weight lost so far above; it works in any unit, as long as both weights use the same one.
How long will it take to reach my goal?
It varies, because the body adapts as weight comes off, so loss usually slows over time. Simple rules that convert a fixed number of calories into a fixed weight tend to over-promise. For a realistic timeline, the NIH Body Weight Planner models these changes; use the target here to plan your days.
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