We estimate maintenance calories with the Mifflin-St Jeor equation times an activity factor, then subtract what you plan to eat to get your daily deficit. Every 7,700 kcal of deficit is roughly 1 kg (3,500 per lb), so dividing the weight to lose by the weekly loss gives the weeks and the date.
How it's calculated
Four steps:
- Maintenance = Mifflin-St Jeor (the same as our daily calorie calculator) × activity factor.
- Daily deficit = maintenance − the calories you plan to eat.
- Weekly loss (kg) ≈ deficit × 7 ÷ 7,700. In pounds we use 3,500 kcal per lb.
- Weeks = (current weight − target) ÷ weekly loss, and the date counts forward from today.
The 7,700 kcal rule and its limits
The popular rule says 1 kg of body weight is worth about 7,700 kcal (3,500 per lb). It is a decent estimate for the first weeks only. As you lose weight your body needs fewer calories, so the deficit shrinks, and part of what you lose is water and lean mass, not just fat. That is why the rule overestimates long-term loss, as the model by Hall and colleagues (2011), behind the NIH Body Weight Planner, shows. Treat our date as the earliest realistic one and redo the calculation with your new weight about once a month.
Example
A 30-year-old man, 172 cm (5 ft 8 in), 84 kg (185 lb), lightly active, aiming for 74 kg (163 lb) and eating 1,900 kcal. BMR = 840 + 1,075 − 150 + 5 = 1,770; maintenance = 1,770 × 1.375 ≈ 2,434. Deficit = 2,434 − 1,900 = 534 kcal a day. Weekly loss = 534 × 7 ÷ 7,700 ≈ 0.49 kg (1.07 lb with the 3,500 rule). He needs 84 − 74 = 10 kg, so 10 ÷ 0.49 ≈ 21 weeks (about 5 months).
Limits
We do not accept less than 1,500 kcal for men or 1,200 for women, and we warn you when the expected loss is over 1 kg (about 2 lb) a week. The result is an estimate for the average adult. Pregnancy, breastfeeding, diabetes, kidney or heart disease, a history of eating disorders and being under 18 all call for a doctor or dietitian before any diet.
General information only, not a substitute for advice from your doctor or a registered dietitian.
