About the Weight Loss Calculator
This weight loss calculator projects how your weight will change if you eat a fixed number of calories each day, and when you will reach your goal. Unlike a simple “3,500 calories per pound” estimate, it recalculates your maintenance calories every day as your weight falls, so the projection shows the realistic slowdown most dieters experience.
Enter your sex, age, height, current and goal weight, activity level and planned daily intake. You get a goal date, the number of weeks it takes, your first-week and average weekly loss, and the calories you will need to maintain your new weight once you get there. The chart compares the adaptive projection with the straight-line rule of thumb.
Maintenance calories use the Mifflin-St Jeor equation multiplied by your activity factor, and each 7,700 kcal of cumulative deficit is counted as one kilogram (about 3,500 kcal per pound). It is a planning estimate: water shifts, changes in activity and metabolic adaptation mean real results vary, so compare it against your own weekly averages.
How to use the weight loss calculator
- 1Enter your sex, age and height.
- 2Enter your current weight and your goal weight.
- 3Choose your usual activity level.
- 4Enter the daily calories you plan to eat and your start date.
- 5Read your goal date and use the chart to see the expected weekly trend.
Formula and method
The projection is a day-by-day simulation. Each day your maintenance calories are recalculated from your current weight with the Mifflin-St Jeor equation and your activity factor. The difference between that and your planned intake is the day’s deficit, and every 7,700 kcal of deficit removes one kilogram (about 3,500 kcal per pound). Because maintenance falls as you get lighter, the daily deficit shrinks and weight loss gradually slows.
The simulation stops the day you reach your goal, or after five years. If your intake equals the maintenance calories of a weight above your goal, weight levels off there — the calculator reports that plateau. Age is held constant and activity is assumed not to change.
- w
- Body weight in kg on a given day
- TDEE(w)
- Maintenance calories at weight w
- intake
- Planned daily calorie intake
Worked examples
40-year-old man, 5 ft 10 in, 200 lb → 175 lb on 2,000 kcal/day, lightly active
His starting maintenance is about 2,507 kcal, so 2,000 kcal/day is a 507 kcal deficit and he loses about 1 lb in the first week. As his weight drops the deficit narrows, so reaching 175 lb takes about 29.4 weeks (206 days, an average of 0.85 lb a week) instead of the ~24.6 weeks the simple rule predicts — landing on 30 July 2026.
35-year-old woman, 165 cm, 80 kg → 70 kg on 1,500 kcal/day, moderately active
Her maintenance at 80 kg is about 2,318 kcal, an 818 kcal daily deficit at 1,500 kcal. Recalculating as she loses weight, she loses about 0.74 kg in the first week and reaches 70 kg after 105 days (15 weeks, an average of 0.67 kg a week), on 20 April 2026, when her maintenance has fallen to about 2,163 kcal.
Intake too high to reach the goal
At 2,300 kcal/day the deficit starts at only about 207 kcal, and his maintenance calories would equal his intake at roughly 167 lb, so the projection levels off there and 150 lb is never reached — the weeks figure shows “Not reached” and the goal date and average weekly loss are hidden. Lowering intake or raising activity is needed.
Frequently asked questions
How long will it take to lose 20 pounds?+
At a steady 500 kcal daily deficit, roughly 20–24 weeks — about 1 lb a week at first, slowing a little as your maintenance calories fall. A 750–1,000 kcal deficit shortens that, but is harder to sustain and increases muscle loss.
Why does weight loss slow down over time?+
A lighter body burns fewer calories at rest and during movement, so the same intake creates a smaller deficit. Some people also move less unconsciously while dieting. This calculator models the first effect directly by recomputing maintenance every day.
How much weight can I safely lose per week?+
A common guideline is 0.5–1% of body weight, or about 1–2 lb (0.5–1 kg) per week. Faster loss is possible early on, largely from water and glycogen, but is not usually sustainable.
Why did I lose more than predicted in the first week?+
When you cut calories and carbohydrates, your body releases stored glycogen and the water bound to it, which can drop 1–4 lb (0.5–2 kg) quickly. That early drop is not fat, so judge progress from week two onwards.
What should I eat once I reach my goal weight?+
Increase gradually to the maintenance figure shown for your goal weight, which is lower than your maintenance today. Adding 100–200 kcal per week while watching the scale helps avoid regain.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.