Estimates only, not medical advice. These are population formulas, and any one person can be well above or below them. If you are pregnant or breastfeeding, under 18, have diabetes, kidney disease, an eating disorder or another medical condition, or take medicine that affects weight, talk to your doctor or a registered dietitian before changing how you eat.
How the deficit and date are worked out
First the calculator finds your maintenance calories (TDEE): your resting burn from the Mifflin-St Jeor equation times your activity factor. Then it takes off 500 calories a day for every pound a week you want to lose, using the rule of thumb that a pound of body weight is about 3,500 calories. The goal date is the weight to lose divided by the weekly rate.
Maintenance = Mifflin-St Jeor BMR × activity factor
Daily target = maintenance − 500 × pounds per week
Weeks to goal = (weight now − goal weight) ÷ pounds per week
Two safety rules are built in. The rate is capped at 2 lb a week, the top of the range the CDC describes as gradual. And we don’t show targets under 1,200 (women) or 1,500 (men), the low ends of the 2013 AHA/ACC/TOS ranges. The guideline reserves diets under 800 calories a day for medically supervised programs. When the floor applies, the calculator works out the slower rate you would really get and moves the date back. If your maintenance is already at or under the floor, it says so instead of giving a date.
Worked example: 40-year-old man, 6 ft, 220 lb, aiming for 190 lb
220 lb is 99.79 kg and 6 ft is 182.88 cm. His BMR is 10 × 99.79 + 6.25 × 182.88 − 5 × 40 + 5 = 997.9 + 1,143.0 − 200 + 5 = 1,946 calories. He has a desk job and does little exercise, so his maintenance is 1,946 × 1.2 = 2,335 calories. At 1.5 lb a week he cuts 750 calories a day and eats about 1,585 calories, comfortably above the 1,500 floor for men. He has 30 lb to lose, so 30 ÷ 1.5 = 20 weeks. Starting on October 12, 2026, that lands on March 1, 2027. His BMI would go from 29.8 to 25.8.
At 190 lb his maintenance would be about 2,172 calories, 163 less than today. If he kept eating 1,585, his deficit by then would have shrunk to about 590 a day, so the last few pounds would come off more slowly than the straight-line date suggests. Re-running the calculator every 10 lb keeps the target honest. When you open the page, the start date is today, so your date for this example will differ.
Limits
- The 3,500-calories-per-pound rule overstates long-term loss because your body adapts. The NIH Body Weight Planner accounts for this and is worth a look for goals more than a few months away.
- Water, salt and hormones can move the scale by several pounds from day to day. Judge progress by the trend over 2 to 3 weeks.
- If the goal weight would give a BMI under 18.5, the calculator flags it. A lower weight is not always healthier.
- For adults 18 and over only, and not during pregnancy or breastfeeding.
For maintenance calories with three formulas side by side, see the TDEE calculator. To make sure you keep muscle while losing fat, the protein calculator gives a daily target.
Sources checked October 2026.
Questions people ask
What is a calorie deficit?
It is the gap between the calories you burn in a day (your TDEE) and the calories you eat. If you burn 2,300 and eat 1,800, your deficit is 500 calories a day. Over time, a steady deficit makes your body draw on stored energy, and you lose weight.
How big a calorie deficit is safe?
For most adults, 500 to 1,000 calories a day, which gives about 1 to 2 lb a week. The CDC notes that people who lose weight at that gradual pace are more likely to keep it off. This calculator caps the rate at 2 lb a week. We don’t show targets under 1,200 (women) or 1,500 (men), the low ends of the 2013 AHA/ACC/TOS ranges. The guideline reserves diets under 800 calories a day for medically supervised programs. If your goal needs more, it shows the floor and a later date.
How long will it take to lose 20 pounds?
At 1 lb a week (a 500-calorie daily deficit), about 20 weeks, or roughly 5 months. At 2 lb a week, about 10 weeks. The real pace usually slows as you get lighter because you burn fewer calories, so treat the date as the earliest likely date, not a promise.
Why is my goal date later than I expected?
Two things can push it back. If your chosen rate would take you under the 1,200 or 1,500 calorie floor, the calculator uses the floor, which means a smaller deficit and slower loss. And if you are sedentary, your maintenance calories are lower, so there is less room to cut. Adding activity raises your TDEE and lets the same intake give a bigger deficit.
Is it better to eat less or exercise more?
Both work, and they work best together. The 2013 AHA/ACC/TOS guideline recommends a reduced-calorie diet as part of a program that also includes more physical activity. Exercise adds to your deficit and helps keep muscle while you lose fat. If you start exercising more, re-run the calculator with your new activity level.
Should I recalculate as I lose weight?
Yes. Every 10 lb or so, enter your new weight. Your maintenance calories drop as you get lighter, so the same intake gives a smaller deficit. The NIH Body Weight Planner models that slowdown in detail if you want a month-by-month view.
Can I use this if I am pregnant or breastfeeding?
No. Pregnancy and breastfeeding raise calorie needs, and dieting during them should only happen with your doctor or a registered dietitian. The same goes for anyone under 18 or with a medical condition such as diabetes.