Calorie Deficit Calculator
A kilogram of body fat holds roughly 7,700 calories. Everything on this page follows from that one number — and from the fact that a straight-line projection is optimistic.
Time to target
17.7weeks
- Eat each day
- 2,100 cal
- To lose
- 8.0 kg
- Weekly loss
- 0.45 kg
- Weeks
- 17.7
- Total deficit
- 61,600 cal
- A kilogram of body fat holds roughly 7,700 calories, which is where the timeline comes from. It is a straight line, and real loss is not: the body adapts to a deficit and a lighter body burns less, so the last few kilograms take longer than this suggests.
- Weight also moves day to day with water, food in transit and hormones, by more than a week of fat loss weighs. Weigh weekly and watch the trend, not the number.
About the calorie deficit calculator
Losing weight means spending more energy than you take in, and the deficit is the gap. At 500 calories a day, that is 3,500 a week, which is about 0.45kg of fat — the familiar "a pound a week". Losing 8kg at that rate takes around 18 weeks.
The projection is a straight line and real weight loss is not, in two ways that both run against you. A lighter body burns less, so the same food becomes a smaller deficit as you go — the last few kilograms genuinely take longer than the first. And the body adapts to a sustained deficit by spending less on movement you do not notice, which shrinks the gap further.
The other thing to expect is that the scale will not cooperate week to week. Body weight moves with water, food in transit, glycogen and hormones by more than a week of fat loss weighs. Weighing daily and reading the weekly average is far more informative than weighing weekly, because it separates the signal from the noise.
A larger deficit is not proportionally better. Aggressive deficits cost more lean tissue, are harder to sustain, and are usually slower in the end because they get abandoned. Around 0.5 to 1% of body weight a week is the range most guidance settles on.
What it works out
- What to eat each day at a given deficit
- Weekly loss and the total time to a target
- The total calorie deficit the whole thing needs
The formula
Days = (Weight to lose × 7,700) ÷ Daily deficit
Eight kilograms at 7,700 calories each is 61,600 calories to find. At 500 a day that is 124 days, or 17.7 weeks. Eating 2,100 against a maintenance of 2,600 is what produces it.
The 7,700 figure — 3,500 calories a pound — is the energy density of stored body fat. It is an approximation, and it assumes what you lose is fat, which is never entirely true. Some of any loss is water and some is lean tissue, and the proportions depend on the deficit size, protein intake and whether you are training.
Two things make the line optimistic. A lighter body has a lower maintenance, so a fixed intake produces a shrinking deficit as you go. And the body reduces spontaneous movement under a sustained deficit, which is real and unconscious and can be a couple of hundred calories.
In practice that means recalculating every five kilograms or so, and expecting the last part to take longer than the arithmetic says.
- Weight to lose
- Current less target.
- 7,700
- Calories in a kilogram of body fat. 3,500 to the pound.
- Daily deficit
- Maintenance less what you eat. The whole mechanism.
- Maintenance
- What you burn in a day. It falls as you get lighter.
A worked example
Going from 80kg to 72kg on a 500 calorie daily deficit, from a maintenance of 2,600.
That works out to 17.7 weeks.
- Eat each day
- 2,100 cal
- To lose
- 8.0 kg
- Weekly loss
- 0.45 kg
- Weeks
- 17.7
- Total deficit
- 61,600 cal
Questions
How many calories should I cut to lose a pound a week?
500 a day, which is 3,500 a week — the calories in roughly a pound of fat. In metric that is about 0.45kg a week.
How long does it take to lose 8kg?
About 18 weeks at a 500 calorie daily deficit. Expect it to run slightly longer, because a lighter body burns less and the deficit shrinks as you go.
Is a bigger deficit better?
Not proportionally. Aggressive deficits cost more lean tissue, are harder to hold, and often end in abandonment — which makes them slower overall. Half to one per cent of body weight a week is the usual recommendation.
Why has my weight stopped moving?
Usually one of three things: maintenance has fallen with your weight, intake has crept up unmeasured, or water is masking fat loss. Genuine plateaus of more than two or three weeks at a real deficit are rare.
How low can I safely go?
Below about 1,200 calories a day it becomes difficult to get enough protein and micronutrients, and very low intakes are not something to attempt without medical supervision. A large deficit is not the same as a fast result.
Why does my weight jump around day to day?
Water, food still in transit, glycogen and hormones move body weight by more than a week of fat loss weighs. Weigh daily and read the weekly average — the trend is the signal and any single reading is noise.
Does exercise count towards the deficit?
Yes, but less than people expect, and the estimates are generous. It is easier to create a deficit by eating a few hundred calories less than by burning a few hundred more. Exercise earns its place for other reasons.
Will I lose muscle as well as fat?
Some, unavoidably. Adequate protein and resistance training minimise it and an aggressive deficit maximises it, which is the main practical argument for going slower.