What is a calorie deficit?
You are in a calorie deficit when you eat less than you burn. Your body covers the gap from stored energy, mostly fat. How much you burn in a day is your TDEE, which you can also work out with our TDEE calculator.
How the calculator works
It estimates your BMR with the Mifflin-St Jeor equation, multiplies it by your activity level to get maintenance calories, then subtracts the deficit you choose.
Target = TDEE − daily deficit
Weekly loss (kg) = daily deficit × 7 ÷ 7,700
Floor: 1,200 kcal (women), 1,500 kcal (men)
For the goal date we don't just divide. Every simulated week your weight drops a little, your TDEE is recalculated and the deficit gets slightly smaller. That's why the estimate is a few weeks longer than the simple math suggests, and closer to what usually happens.
Why the scale doesn't follow the plan
Water, salt and digestion can move your weight by a kilo or more from one day to the next. Look at the weekly trend, not single weigh-ins. If the trend is flat after three weeks, cut another 100 to 200 kcal or add a few thousand steps a day.
Most diets fail at the logging part, not the math. myCalCoach on Telegram lets you log a meal with a photo or a voice note and tells you how much of your budget is left.
Common questions
How big should my calorie deficit be?
For most people 300 to 500 kcal a day is a good start. That works out to roughly 0.3 to 0.5 kg (0.6 to 1 lb) a week. Bigger deficits work faster on paper but are harder to stick to and cost more muscle.
How many calories do I need to burn to lose 1 kg?
About 7,700 kcal, which is the energy stored in a kilogram of body fat. For 1 lb it is about 3,500 kcal. Real results vary because early weight loss includes water.
Why does weight loss slow down over time?
A lighter body burns fewer calories, so the same intake gives a smaller deficit each week. The goal-date estimate on this page accounts for that by recalculating your TDEE as your weight drops.
Is it safe to eat less than 1,200 calories?
Not without medical supervision. The calculator never suggests less than 1,200 kcal for women or 1,500 kcal for men. If your target hits that floor, choose a smaller deficit and move a bit more instead.
Other free calculators
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):241-247.
- Hall KD, et al. Quantification of the effect of energy imbalance on bodyweight. Lancet. 2011;378(9793):826-837.
- Morton RW, et al. Protein supplementation and resistance training-induced gains in muscle mass and strength. Br J Sports Med. 2018;52(6):376-384.
This calculator gives general estimates and is not medical advice. If you are pregnant, under 18, have a medical condition or a history of disordered eating, talk to a doctor before changing your diet.