How do I calculate my calorie deficit?

Log your normal eating for two to three weeks while weighing yourself most mornings. If your weekly average weight is flat, that intake is your maintenance. Subtract 15 to 25 per cent for a deficit. This is more reliable than any calculator because it is measured from you.

Is a 500 calorie deficit right for everyone?

No. Five hundred calories is about 17 per cent below maintenance for a large man and 33 per cent for a small woman, which are very different diets. A percentage of your own maintenance scales sensibly; a fixed number does not.

Does 3,500 calories equal one pound of fat?

As a rough conversion for fat tissue it is reasonable; as a prediction of weight loss over months it overstates the result, because expenditure falls as body mass falls. Actual losses are typically smaller than the rule predicts over long periods.

How often should I recalculate my deficit?

Every four to six weeks, or after about 5 per cent of bodyweight is lost. A smaller body costs less to maintain, so an intake that was a deficit at the start can become maintenance by the end without you changing anything.

How to calculate your calorie deficit properly

Calculators give you a starting guess. Your own two weeks of data give you the answer. The method, the maths, and why the 3,500-calorie rule overstates things.

By Gajendra Singh RathorePublished October 2, 20265 min read

The short answer

Measure your maintenance from two to three weeks of logging, then subtract 15 to 25 per cent.
The 3,500 kcal per pound rule overstates long-run loss because expenditure falls as you get lighter.
A percentage deficit scales sensibly across body sizes; a fixed 500 kcal does not.
Recalculate every four to six weeks as your weight falls, or the deficit shrinks without you noticing.

Every calorie calculator asks for your age, sex, height, weight and an activity multiplier, then produces a confident number. That number is an estimate from population averages, and your real expenditure can sit a few hundred calories either side of it.

Here is how to get a figure that is actually about you.

Step one: measure your maintenance

Log everything you eat for two to three weeks. Change nothing. Weigh yourself most mornings under the same conditions.

At the end, take the average daily intake and the change in your weekly weight average.

If the weight is flat, that intake is your maintenance.

If it fell by an average of 0.3 kg per week, you were already in a deficit of roughly 330 kcal a day, so your maintenance is your intake plus 330.

If it rose by 0.3 kg per week, subtract 330.

That number is measured rather than predicted. The procedure in more detail is in maintenance calories, and how to find yours.

Step two: subtract a percentage, not a fixed number

Conservative: 10 to 15 per cent below. Slow, very sustainable, minimal muscle loss. Good for people who are already lean or who have a long time.

Standard: 15 to 25 per cent below. The usual recommendation. Produces roughly 0.5 to 1 per cent of bodyweight per week.

Aggressive: 25 to 30 per cent below. Faster, more muscle lost, harder to adhere to, and appropriate mainly for people with a lot of fat to lose and medical supervision.

Why a percentage rather than a fixed 500 kcal: 500 is 17 per cent below maintenance for a 100 kg man and 33 per cent for a 50 kg woman. The same subtraction produces completely different diets, and the second one is unnecessarily harsh.

Step three: convert to an expected rate

A deficit of 500 kcal a day is 3,500 a week. At roughly 7,700 kcal per kilogram of fat tissue, that predicts about 0.45 kg a week.

The prediction will overstate reality over months. The old "3,500 calories per pound" rule assumes expenditure stays constant, and it does not: a lighter body costs less to move and to maintain, and people unconsciously move less when eating less.

Expect the first weeks to match the prediction reasonably well and later weeks to fall short. That is normal, not a metabolic problem.

Step four: verify against reality

Hold the target for two to three weeks, then compare your weekly weight average with what the deficit predicted.

Matching? Continue.

Losing slower than predicted? The log is almost certainly reading low. Check cooking oil, weekend days, portion drift and the small unlogged extras before touching the target — the list is in nine calorie tracking mistakes that quietly add up.

Losing faster than predicted? Eat a little more. Faster is not better; it costs muscle and adherence.

This verification step is the one people skip, and it is the entire point of the exercise.

Step five: recalculate as you go

Every four to six weeks, or after losing about 5 per cent of your bodyweight, redo the arithmetic.

A 90 kg person's maintenance is not an 82 kg person's maintenance. If you hold the original intake all the way down, the deficit shrinks as you go and eventually disappears, which looks exactly like a plateau and is not one — see what a weight loss plateau usually turns out to be.

The floor

There is a lower bound worth respecting. Below roughly 1,200 kcal for women and 1,500 for men, it becomes difficult to meet protein and micronutrient needs, and adherence collapses.

If your calculated target falls below that, the answer is to increase expenditure — more steps, more training — rather than to cut further. That is the practical argument for the step count in steps, NEAT, and the calories you burn standing up.

What the deficit does not tell you

It tells you the direction and roughly the rate. It says nothing about what you lose.

Fat versus muscle is decided by protein intake and resistance training, not by the size of the deficit — although a very large deficit tilts the ratio the wrong way. The two together are the subject of losing fat while keeping the muscle you built.

The short version

Measure your maintenance over two or three weeks. Take 20 per cent off. Hold it for three weeks. Compare the weight trend with the prediction. Adjust the target, not your expectations, and recalculate every month or so.

That loop is the whole of it, and it is the same loop described from the food-logging side in how to track calories.

Questions people ask

How do I calculate my calorie deficit?: Log your normal eating for two to three weeks while weighing yourself most mornings. If your weekly average weight is flat, that intake is your maintenance. Subtract 15 to 25 per cent for a deficit. This is more reliable than any calculator because it is measured from you.
Is a 500 calorie deficit right for everyone?: No. Five hundred calories is about 17 per cent below maintenance for a large man and 33 per cent for a small woman, which are very different diets. A percentage of your own maintenance scales sensibly; a fixed number does not.
Does 3,500 calories equal one pound of fat?: As a rough conversion for fat tissue it is reasonable; as a prediction of weight loss over months it overstates the result, because expenditure falls as body mass falls. Actual losses are typically smaller than the rule predicts over long periods.
How often should I recalculate my deficit?: Every four to six weeks, or after about 5 per cent of bodyweight is lost. A smaller body costs less to maintain, so an intake that was a deficit at the start can become maintenance by the end without you changing anything.

Sources

1.Body Weight Planner — NIDDK, National Institutes of Health
2.Weight management — NIDDK, National Institutes of Health

How to lose fat without quitting in three weeks
A deficit you barely notice, protein high enough to keep muscle, and a weekly trend to steer by. The whole method, and the parts people get wrong.
Maintenance calories, and how to find yours
The intake that holds your weight steady, measured from three weeks of your own data rather than predicted. Why it is a band, and why it matters most.
How to calculate your TDEE, and what it can't say
Four components, three equations, and one better method. Why every calculated TDEE is a hypothesis and how to replace it with your own measurement.
How fast should you lose weight? The honest range
0.5 to 1 per cent of bodyweight per week for most people. Why faster costs muscle, why slower is often better, and how the right rate changes as you get leaner.