Calorie deficit calculator
Pick a pace and see what it asks you to eat, and where it leads. The line bends, because losing weight changes how much you burn.
Why the line bends
A deficit is the gap between what you eat and what you need, and what you need falls as you get lighter. Eat the same amount every day and the gap shrinks a little each week, so the loss slows on its own.
The simple version skips this. It takes one deficit, divides by 7,700 kcal a kilo (Wishnofsky, 1958) and draws a straight line that never slows. Hall and colleagues showed in The Lancet in 2011 that the body's response to a lasting change in what you eat is slow, taking about a year to get halfway, and published a model that accounts for it.
This page does the simplest version of that: it works out your need again each week at your new weight, using the Mifflin St Jeor equation and your activity level. The model in that paper does more than this, so treat the line as an estimate.
Why the choices stop where they do
The NHS suggests aiming for 0.5 to 1 kg a week, so the choices stop at 1 kg. Separately, no pace may take your daily intake under 1,200 kcal for a woman or 1,500 kcal for a man. Harvard Health gives those as the figures intake should not fall below without a health professional involved. If your floor comes before the pace you picked, the calculator slows to the fastest pace that stays above it and says so.
Where the line stops
The projection ends at the bottom of the healthy BMI range for your height, a BMI of 18.5, and this page does not draw a route past it. If you are already under that range, it shows no losing option at all.
Want to know what you burn without picking a pace? The TDEE calculator shows that number on its own.
Common questions
How many calories should I eat to lose weight?
Fewer than you burn, and how many fewer sets the pace. A gap of about 550 kcal a day comes to roughly half a kilo a week by the usual arithmetic. The calculator works out what you burn from your height, weight, age, sex and activity, then takes off the gap for the pace you pick.
Why does weight loss slow down?
Because a smaller body needs less energy. Eat the same amount every day and the gap between what you eat and what you need shrinks as you get lighter, so each week takes off a little less than the one before. Hall and colleagues found the body takes about a year to get halfway to where a lasting change in eating leads.
Is 7,700 kcal really a kilo of fat?
It is the figure everyone uses, from Wishnofsky in 1958 (3,500 kcal a pound), and it is a rough one. What actually comes off is a mix of fat, water and lean tissue, and the mix changes as you go. This page uses it because it is the standard figure, and names it so you know.
Why will it not show a faster pace?
Two limits. The choices stop at 1 kg a week, the top of the range the NHS suggests. And no pace is allowed to take your daily intake under 1,200 kcal for a woman or 1,500 for a man, which Harvard Health gives as the floors for dieting without a health professional involved. If you are small, older or not very active, the second limit is the one you will reach first.
Sources
Every figure above comes from one of these. Check them.
- Hall KD, Sacks G, Chandramohan D et al. (2011). Quantification of the effect of energy imbalance on bodyweight. The Lancet 378(9793), 826-837
- Wishnofsky M (1958). Caloric equivalents of gained or lost weight. American Journal of Clinical Nutrition 6(5), 542-546
- Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO (1990). A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition 51(2)
- World Health Organization. Body mass index classification for adults, the 18.5 lower boundary of the healthy range
- NHS: Tips to help you lose weight, including the 0.5 to 1 kg a week guideline
- Harvard Health Publishing: Calorie counting made easy, on the 1,200 and 1,500 kcal floors
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This is not medical advice. Body Numbers is a calculator. It does not diagnose, treat or monitor any condition, and no number it produces should replace a conversation with a doctor or a dietitian. If a result worries you, or if you are pregnant, unwell, or under eighteen, speak to a health professional.
