How the deficit is worked out
A pound of body fat stores roughly 3,500 calories, or about 7,700 per kilogram. To lose a pound a week you need to eat about 3,500 calories less than you burn over that week, which is 500 a day.
daily deficit = weight to lose × 3,500 (per lb) ÷ days to your goal
Gaining weight works the same way in reverse: the calculator gives you a daily surplus instead.
What's a realistic pace?
A common guideline is to lose no more than about 1% of your body weight a week. Faster than that is harder to stick to and more of what you lose can be muscle. The calculator warns you when a goal date needs a faster pace than that, or when your daily calories would drop below 1,200.
Why think in weeks
Your weight doesn't track a daily target exactly, and days won't all go to plan. What matters is the deficit across the whole week, so a big Saturday can be balanced by a lighter Sunday and Monday. That's what the weekly tracker is built around.
Questions
How big should my calorie deficit be?
For most people, 300 to 500 calories a day is a sustainable deficit, losing roughly 0.5 to 1lb a week. Larger deficits work faster but are harder to keep up.
How long will it take to reach my goal weight?
Choose By pace in the calculator and it will give you a finish date for your goal at that weekly rate.
Is 1,200 calories a day too low?
Around 1,200 calories a day is often quoted as the lowest most adults should eat without medical supervision. The calculator warns you if your plan goes below it.
Why is the 3,500 calorie rule only an estimate?
As you lose weight you burn slightly fewer calories, and early weight loss includes water. Expect progress to slow a little over time and check your actual weight every few weeks.