How the calculator works
To lose weight you need to take in less energy than you use. The gap is your calorie deficit. The calculator works it out in three steps:
- Your maintenance calories now. It estimates your basal metabolic rate with the Mifflin–St Jeor equation and multiplies by your activity level. This is your TDEE.
- The energy in the weight you want to lose. It uses the common estimate of about 7,700 kcal per kilogram of body weight (roughly 3,500 kcal per pound).
- A daily target that still works later in the plan. Your maintenance calories fall as you get lighter, so the calculator recalculates them every day of the plan and finds the one daily intake that reaches your goal on time.
This calculator: maintenance(w) = BMR(w) × activity, recalculated as w falls
Worked example
A 35-year-old woman, 165 cm and 80 kg, lightly active, wants to reach 72 kg in 16 weeks (112 days).
- Maintenance now: BMR = 10 × 80 + 6.25 × 165 − 5 × 35 − 161 = 1,495 kcal, × 1.375 = 2,056 kcal a day.
- Simple rule: 8 kg × 7,700 = 61,600 kcal ÷ 112 days = 550 kcal a day, so eat about 1,506 kcal.
- Recalculated plan: at 72 kg her maintenance is about 1,946 kcal, 110 kcal lower. To stay on schedule she needs about 1,449 kcal a day.
| Week | Weight | Maintenance | Daily deficit |
|---|---|---|---|
| 0 | 80.0 kg | 2,056 kcal | 607 kcal |
| 4 | 77.9 kg | 2,026 kcal | 577 kcal |
| 8 | 75.8 kg | 1,998 kcal | 549 kcal |
| 12 | 73.9 kg | 1,971 kcal | 522 kcal |
| 16 | 72.0 kg | 1,946 kcal | 497 kcal |
The deficit shrinks from about 607 to 497 kcal a day even though she eats the same amount, which is why progress slows near the end. The first week brings about 0.55 kg of loss; the last about 0.45 kg.
Why the 3,500-calorie rule falls short
The idea that a pound of fat holds about 3,500 kcal is a fair estimate for the first few weeks. The problem is treating it as a fixed exchange rate. Researchers at the US National Institutes of Health showed in 2011 (Hall and colleagues, The Lancet) that the rule overestimates weight loss over months, because energy needs fall as body weight falls and the body adapts. A 2013 commentary in the International Journal of Obesity reached the same conclusion: a steady 500 kcal daily deficit does not keep producing a pound a week.
This calculator handles the biggest part of that effect, the drop in maintenance calories, but it still leaves out metabolic adaptation and changes in water and muscle. Real results are often a little slower. For a more detailed model, try the NIH Body Weight Planner.
Safety checks
- Minimum intake. Common guidance, for example from Harvard Health Publishing, is not to eat below about 1,200 kcal a day for women or 1,500 kcal for men except under medical supervision. Below that it is hard to get enough protein, vitamins and minerals.
- Rate of loss. The CDC notes that people who lose weight gradually, about 1 to 2 lb (0.5 to 1 kg) a week, are more likely to keep it off. The calculator also warns when the first weeks would mean losing more than 1% of your body weight a week, a ceiling used in sports nutrition guidance to protect muscle.
- Goal weight. If your goal gives a BMI below 18.5, you will see a warning. Check your goal against your height with the BMI calculator.
If you are pregnant, breastfeeding, under 18, managing a medical condition or taking medicines that affect weight or appetite, talk to a doctor or registered dietitian before starting a deficit.
Tips for using your number
- Treat it as a starting point. Weigh yourself at the same time several times a week and compare weekly averages. After two to three weeks, adjust by 100–200 kcal if you are well ahead or behind.
- Keep protein up and stay active. Strength training and enough protein help you keep muscle while you lose fat.
- Plan the finish date. The projection shows calendar dates for each week; to count days to another event, use the date calculator. For maintenance and macros once you reach your goal, see the TDEE calculator.