Weight loss calculator
Tell us who you are, what you weigh, your target and how big a daily calorie deficit you plan. The calculator estimates your maintenance calories, how many weeks the plan takes and flags targets or deficits that are too aggressive.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Choose units, sex, age, height, current weight and activity level.
- Enter your target weight and the daily calorie deficit you plan to run (500 kcal a day is a common starting point).
- Read the daily calories, the weeks to your goal and the estimated date, and check the warnings.
- Maintenance calories = basal metabolic rate (Mifflin–St Jeor, 1990) × activity factor. BMR for men = 10 × kg + 6.25 × cm − 5 × age + 5; for women the constant is −161.
- Daily calories on the plan = maintenance − deficit.
- Days to goal = (current − target weight in kg) × 7,700 ÷ deficit; weeks = days ÷ 7.
- Average loss per week = deficit × 7 ÷ 7,700 kg.
- Target BMI = target weight ÷ height² (kg and metres).
Worked example
A 30-year-old woman, 70 kg, 175 cm, light activity: BMR = 700 + 1,093.75 − 150 − 161 = 1,482.75 kcal, maintenance = 1,482.75 × 1.375 = 2,039 kcal. With a deficit of 500 kcal she eats about 1,539 kcal. To reach 63 kg (7 kg less) takes 7 × 7,700 ÷ 500 = 107.8 days, about 15.4 weeks. Her target BMI is 63 ÷ 1.75² = 20.6.
Warnings and limits
- The tool warns when the target BMI is below 18.5, when daily calories fall below your BMR and when the deficit is above 1,000 kcal.
- It ignores metabolic adaptation, water shifts, muscle changes and the fact that the energy cost of a smaller body falls over time, so real progress is usually slower.
- It does not suit people under 18, pregnant or breastfeeding women, or anyone with an eating disorder or a medical condition.
- This is general information, not medical advice. Speak to a doctor or registered dietitian before starting a diet.
Frequently asked questions
How does the calculator work out the time?
It estimates your maintenance calories with the Mifflin–St Jeor equation times an activity factor, then assumes that about 7,700 kcal of deficit equals one kilogram lost (about 3,500 kcal per pound). Weeks = kilograms to lose × 7,700 ÷ (daily deficit × 7).
Why will it probably take longer in real life?
As you lose weight your body needs fewer calories, because there is less of you to move and the body also adapts by saving energy (metabolic adaptation). The calculator keeps your starting maintenance level, so it gives the best case; real plans slow down over time and need recalculating.
How fast is a safe rate of weight loss?
Health agencies such as the CDC describe about 1 to 2 pounds (0.5 to 1 kg) a week as a steady rate for most adults. A large weight-loss goal, a medical condition, or a very low calorie plan should be supervised by a doctor.
What does the BMI warning mean?
A BMI below 18.5 is classed as underweight by the WHO. If your target sits there, the plan would move you from a healthy range into a range linked with health risks, so the tool warns you.
Is the 7,700 kcal per kilogram rule exact?
No. It is a long-used approximation. Real changes include water and muscle as well as fat and vary between people, so use the result as a planning figure and adjust to what the scale shows over several weeks.