What a calorie deficit actually is
A calorie deficit means you take in less energy than you spend. Your body covers the gap from stored energy, most of it fat, and you lose weight.
That is the whole mechanism. Energy in against energy out.
No food burns fat. Celery, green tea, cayenne, and lemon water do not create a deficit, and neither does eating after 8pm prevent one. Foods differ in how full they leave you and how much protein they carry, which changes how easy the deficit is to hold, but none of them change the arithmetic.
Step 1: Calculate your BMR
Your Basal Metabolic Rate is the energy you would burn lying in bed all day, doing nothing.
Use the Mifflin-St Jeor equation.
Men: (10 × kg) + (6.25 × cm) − (5 × age) + 5
Women: (10 × kg) + (6.25 × cm) − (5 × age) − 161
Mifflin-St Jeor was published in 1990 and is more accurate for most people than the older Harris-Benedict equation, which tends to overestimate. To get this figure without the arithmetic, use the BMR calculator.
Step 2: Calculate your TDEE
Your BMR only covers rest. Multiply it by an activity factor to get your Total Daily Energy Expenditure, which is the number of calories that keeps your weight exactly where it is.
| Activity level | Multiplier | What it really means |
|---|---|---|
| Sedentary | × 1.2 | Desk job, no deliberate exercise. Most office workers. |
| Light | × 1.375 | Desk job plus one to three light sessions, or a job on your feet. |
| Moderate | × 1.55 | Three to five real training sessions a week. Most regular gym-goers. |
| Very active | × 1.725 | Six or seven hard sessions a week. Serious amateur athletes. |
| Extra active | × 1.9 | Hard daily training on top of physical work. Rare. Be honest here. |
You will probably pick one level too high. Most people do. Three gym sessions a week is moderate, not very active, and one step up this ladder is worth roughly 300 calories a day.
If you are unsure, take the lower option. Adding calories back once the scale moves is easy. Discovering after two months that you have been eating at maintenance is not. The TDEE calculator and the maintenance calorie calculator both return this figure directly.
Step 3: Subtract your deficit
Take your TDEE and subtract the deficit you want. What is left is what you eat.
A kilogram of body fat stores roughly 7,700 calories, so the size of your deficit sets your rate of loss.
250 a day loses about 0.23 kg (0.5 lb) a week.
500 a day loses about 0.45 kg (1 lb) a week.
750 a day loses about 0.68 kg (1.5 lb) a week.
Past roughly 750 to 1,000 calories a day, the deficit starts working against you. Adherence collapses because the hunger is hard to hold. You lose more lean tissue alongside the fat, which lowers your BMR and makes the next kilogram harder than the last. Training quality drops, and the movement you do without thinking about it quietly falls.
There is also a floor. If your target lands below 1,200 calories as a woman or 1,500 as a man, the deficit is too aggressive for your size. Pick a slower rate.
Calculate your calorie deficit
You now know the method. Put your own numbers in and it runs all three steps for you.
A 0.5 kg per week loss needs a 550 calorie daily deficit.
Your daily calorie target
2,190kcal
Step 1 · BMR
1,768
Mifflin-St Jeor
Step 2 · TDEE
2,740
BMR × 1.55
A worked example
Daniel is 38, 180 cm, and 92 kg. He trains four times a week, so he is moderately active. He wants to lose 0.5 kg a week.
Step 1, his BMR. (10 × 92) + (6.25 × 180) − (5 × 38) + 5 = 920 + 1,125 − 190 + 5 = 1,860 calories.
Step 2, his TDEE. 1,860 × 1.55 = 2,883 calories. That is his maintenance level.
Step 3, his deficit. Losing 0.5 kg a week needs 3,850 calories a week, which is 550 a day. So 2,883 − 550 = 2,333 calories a day.
That is Daniel's target. It sits well above the 1,500 floor for men, so it is safe to run. If he wants to reach 84 kg, he has 8 kg to lose, which at 0.5 kg a week takes about 16 weeks.
He should recalculate at around 87 kg, because by then his TDEE will have fallen and his 550-calorie deficit will have quietly shrunk.
How big should your calorie deficit be?
For most people, 500 calories a day. It is fast enough to see progress and slow enough to live with.
| Daily deficit | Weekly loss | Sustainable? | Who it suits |
|---|---|---|---|
| 250 kcal | 0.23 kg (0.5 lb) | Very high | Lighter or less active people, and anyone who has failed a faster diet before |
| 500 kcal | 0.45 kg (1 lb) | High | Most people. The default choice |
| 750 kcal | 0.68 kg (1.5 lb) | Moderate | Heavier or very active people with a high TDEE |
| 1,000 kcal | 0.91 kg (2 lb) | Low | Rarely appropriate. Only with a high TDEE and ideally medical supervision |
Pick the largest deficit you can genuinely hold, not the largest one the arithmetic allows. A 250-calorie deficit held for six months beats a 1,000-calorie deficit abandoned in three weeks, and it is not close.
If a steady daily number is hard to stick to, the zig-zag calorie cycling calculator varies your intake across the week while hitting the same average.
Why your deficit stops working (and how to fix it)
Your deficit shrinks as you lose weight, and it does it without telling you.
A lighter body costs less to run. When Daniel reaches 84 kg, his BMR falls to about 1,780 and his TDEE to roughly 2,759. He is still eating 2,333, so his deficit is now 426 calories, not 550. His weekly loss slows from 0.5 kg to about 0.39 kg.
Nothing has broken and your metabolism has not stopped. The gap closed while you were not looking.
The fix is to recalculate every four to six weeks, or every time you lose about 5 kg (10 lb). It takes thirty seconds. This single habit is the difference between a plan that keeps working and one that stalls at week ten and gets abandoned at week twelve.
How to actually hit your deficit
Knowing your number is the easy part. Hitting it is where diets are won.
Count what you drink. Juice, lattes, beer, and smoothies are the most commonly missed calories, and they are easy to repeat daily without noticing.
Weigh your food for two weeks. Not forever. Long enough to calibrate your eye, because almost everyone underestimates portions, and the error is usually larger than the deficit itself.
Prioritise protein. Aim for 1.6 to 2.2 grams per kilogram of body weight. Protein protects lean mass while you are in a deficit and keeps you fuller on fewer calories. Set the split with the macro calculator.
Do not eat back your tracker calories. Watches and gym machines overstate exercise burn, and your TDEE already includes your activity level, so eating them back counts the same calories twice. If you want a realistic figure for a specific session, the calories burned calculator gives a more honest one.
Common mistakes when calculating a calorie deficit
- Overestimating your activity level. The single biggest error, and worth about 300 calories a day. Three sessions a week is moderate, not very active.
- Confusing a 1,200-calorie deficit with a 1,200-calorie intake. They are not the same thing. A deficit that size is only plausible with a very high TDEE.
- Setting the deficit once and never revisiting it. Your TDEE falls as your weight does. A number calculated at 92 kg is wrong at 84 kg.
- Subtracting the deficit from your BMR instead of your TDEE. This produces a target hundreds of calories too low and is genuinely unsafe. The deficit comes off maintenance, not off rest.
- Chasing the biggest deficit the maths allows. The best deficit is the one you will still be running in three months.
Frequently asked questions
Calculate your BMR with the Mifflin-St Jeor equation, multiply it by your activity factor to get your TDEE, then subtract the deficit you want. A 500-calorie daily deficit produces about 0.45 kg (1 lb) of loss per week. For example, a TDEE of 2,883 minus 550 gives a daily target of 2,333 calories.
Your weight-loss target is your maintenance calories minus a deficit. Find maintenance (your TDEE), then subtract 250 for slow loss, 500 for standard loss, or 750 for faster loss. Never take the result below 1,200 calories for women or 1,500 for men.
500 calories a day suits most people. It produces roughly 0.45 kg (1 lb) of loss per week, which is fast enough to see progress and slow enough to sustain. If you are lighter or less active, 250 is often the more realistic choice.
About 1,100 calories a day. One kilogram of body fat stores roughly 7,700 calories, so losing a kilogram a week means a 7,700-calorie weekly deficit, or 1,100 a day. That is aggressive, hard to sustain, and drops many people below a safe intake, so check the calculator before choosing it.
Yes. The Mifflin-St Jeor equation used here needs only your sex, age, height, and weight. Body fat is optional and only matters if you want to use the Katch-McArdle equation instead, which works from lean mass.
The two usual causes are that the deficit is smaller than you think, or that it shrank as you lost weight. Untracked liquid calories and eating back exercise calories both quietly close the gap. A lighter body also has a lower TDEE, so a deficit set at your starting weight is smaller at your current one. Recalculate every four to six weeks.
Until you reach your goal weight, with breaks at maintenance if you need them. There is no fixed limit, but adherence falls the longer and harder you diet. Most people do better with a moderate deficit held for months than an aggressive one abandoned in weeks.
A 1,200-calorie deficit is different from a 1,200-calorie intake, and the distinction matters. A deficit that large is only plausible for someone with a very high TDEE, and for most people it drives intake below a safe level. Calora will not recommend a target below 1,200 calories for women or 1,500 for men. If your calculated deficit pushes you under those figures, choose a slower rate of loss.
Ready to run your own numbers? The calorie deficit calculator projects your loss week by week, and the main calorie calculator gives you daily calories and macros in one pass.
Sources
- Mifflin MD, St Jeor ST, et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990;51(2):241–7.
- U.S. Departments of Agriculture and Health and Human Services. Dietary Guidelines for Americans.
- Mayo Clinic. Counting calories: Get back to weight-loss basics.
These results are estimates for general educational purposes only and are not medical or nutritional advice. Individual needs vary. Consult a qualified healthcare professional or registered dietitian before making significant changes to your diet, especially if you are pregnant, breastfeeding, under 18, or have a medical condition.