Weight Loss
Calorie Deficit Calculator
To lose weight you need to eat below the calories you burn. This works out your daily burn, the deficit for the pace you choose, and roughly how long it will take to reach your goal.
How a calorie deficit works
A calorie deficit means eating fewer calories than your body uses, so it makes up the difference from stored energy and you lose weight. The size of the deficit sets the pace: roughly 7,700 calories per kilogram of body mass is the long-standing rule of thumb, so a 500-a-day deficit loses about half a kilo (1 lb) a week and 1,000 a day loses about a kilo (2 lb). Treat that as an approximation, not a promise — see below.
A worked example
A 30-year-old man, 180 cm and 85 kg, training moderately, burns about 2,837 calories a day (a Mifflin-St Jeor resting burn of 1,830 × 1.55). To lose 0.5 kg (about 1 lb) a week he eats roughly 550 below that — about 2,290 calories a day — and reaching a 75 kg goal takes around 20 weeks at that pace.
Why real-world loss runs slower than the straight line
The "7,700 calories per kilogram" rule assumes you keep burning the same number of calories throughout a diet. In reality your body burns less as you get lighter, so the deficit narrows over time and weight loss slows and levels off. Metabolic research (Kevin Hall and colleagues at the NIH) shows the true response is gradual rather than a straight line — which is why the simple rule tends to overestimate how much you will lose over longer timelines. Use the projection as a best-case straight-line estimate, and expect the real curve to flatten.
Picking a sustainable pace
Faster is not always better. A moderate deficit is easier to live with, protects your training and your muscle, and is less likely to stall. For most people a safe rate is about 1 to 2 pounds (0.5–1 kg) a week. Pair your target calories with enough protein to hold on to muscle while the fat comes off.
Where your numbers come from
Your maintenance figure is your TDEE, calculated from the Mifflin-St Jeor equation plus your activity level. The deficit is then sized to the weekly rate you select.
Common questions
How do I work out my calorie deficit?
First find your TDEE — the calories you burn in a day. Then eat below it. A deficit of about 500 calories a day loses roughly half a kilo (about 1 lb) a week; 1,000 a day loses about 1 kg (2 lb). This calculator does both steps for you.
How many calories should I eat a day to lose weight?
There is no universal number — it is your own TDEE minus a deficit. Find your TDEE first, subtract roughly 300–500 for a steady pace (up to about 1,000 for a faster one), and treat anything below your BMR as a flag to slow down. The calculator above works both out.
How big of a calorie deficit is safe?
For most people a safe, sustainable pace is about 1 to 2 pounds (roughly 0.5–1 kg) a week, which usually means a deficit of around 500 to 1,000 calories a day below maintenance. Larger deficits are not more effective long-term and are harder to sustain, so the calculator flags when your target drops below your resting burn (BMR).
How long will it take to reach my goal weight?
Divide the weight you want to lose by your weekly rate. Losing 10 kg at a steady 0.5 kg a week takes about 20 weeks; at 0.25 kg a week, about 40. This calculator shows an estimated finish date, assuming your deficit stays consistent — in practice progress slows as you get lighter, so treat the date as an estimate.
Is 1,200 calories a day a good target?
Only for some people. 1,200 is near the bottom of what is used in unsupervised weight loss, and for many adults it sits below resting burn (BMR). Diets under 800 calories a day are classed as very-low-calorie diets and are for medical supervision only. Set your target from your own TDEE instead — this calculator warns when a target falls below your BMR.
Is there an NHS calorie deficit calculator?
The NHS does not publish a TDEE or calorie-deficit calculator — its guidance works from fixed average intakes (around 2,500 kcal a day for men, 2,000 for women) with a recommended reduction for weight loss. A Mifflin-St Jeor calculation personalises that to your own weight, height, age, sex and activity.