A weight loss calculator estimates how long it takes to reach a goal weight at a given calorie intake, or the daily calories needed to reach it by a date. It works by comparing your intake with your TDEE and recalculating TDEE as you get lighter. A 35-year-old woman at 165 lb eating 1,900 kcal reaches about 151 lb in 24 weeks.
Key takeaways
- Weight loss slows over time because a lighter body burns fewer calories, so a straight-line projection from the first week overstates progress.
- The example woman (165 lb, TDEE 2,239 kcal) eating 1,900 kcal loses about 14 lb in 24 weeks, compared with about 16 lb predicted by the flat 3,500-kcal rule.
- Reaching a 145 lb goal at 1,900 kcal takes about 38 weeks, and 135 lb takes about 70 weeks.
- Target-date mode finds the required calories and warns you when the plan falls below safe minimums.
How this weight loss calculator works
Enter your sex, age, weight, height, activity level and goal weight, then pick a mode. In Daily calories mode you type an intake and the calculator returns the weeks and date at which you reach your goal, or “not reached in 5 years” if your intake never gets you there. In Target date mode you choose a date and the calculator finds the daily calories required.
Both modes draw a chart of your projected weight week by week with your goal as a dashed line, and show safety warnings similar to the calorie deficit calculator.
How does the projection work?
Each week the calculator recomputes your TDEE from your current weight using Mifflin–St Jeor (Mifflin et al., 1990) times your activity factor, then converts the gap between intake and TDEE into weight change at about 7,700 kcal per kg (3,500 per lb). Because the weight falls, so does TDEE, and the daily deficit shrinks. Hall and colleagues’ dynamic modelling and Thomas and colleagues’ correction of the 3,500-kcal rule support this approach (Hall et al., 2011; Thomas et al., 2014).
The calculation is a simplified relative of the NIDDK Body Weight Planner (NIDDK Body Weight Planner). It does not model changes in water, glycogen or the extra slowing some people see in expenditure (Rosenbaum & Leibel, 2010).
What does a weight loss projection look like?
For the example woman (35, 5′5″, 165 lb, moderately active, TDEE 2,239 kcal) eating 1,900 kcal a day:
| Week | Weight | Lost so far | Projected TDEE | Daily deficit |
|---|---|---|---|---|
| 0 | 165.0 lb | 0.0 lb | 2,239 kcal | 339 kcal |
| 4 | 162.3 lb | 2.7 lb | 2,220 kcal | 320 kcal |
| 8 | 159.8 lb | 5.2 lb | 2,202 kcal | 302 kcal |
| 12 | 157.5 lb | 7.5 lb | 2,186 kcal | 286 kcal |
| 16 | 155.2 lb | 9.8 lb | 2,170 kcal | 270 kcal |
| 20 | 153.1 lb | 11.9 lb | 2,155 kcal | 255 kcal |
| 24 | 151.1 lb | 13.9 lb | 2,141 kcal | 241 kcal |
Her deficit starts at 339 kcal and drops to 241 kcal by week 24, so the pace slows from about 0.7 lb a week to about 0.5 lb. The flat rule would predict about 16 lb lost over the same 24 weeks.
How do I set a good goal weight?
Start with a goal that is realistic and measurable. A first target of 5–10% of your current weight is easier to plan around than a distant number, and you can reset it when you arrive. The ideal weight calculator and the BMI calculator give common reference ranges, but neither accounts for muscle mass or personal health.
Body weight also naturally fluctuates from day to day and across weeks (Turicchi et al., 2020), so judge progress by a weekly or two-week average, not a single weigh-in.
What calories should I eat to lose weight by a date?
Choose target-date mode, set your date, and read the calories it returns. As a sanity check, divide your total weight to lose by the weeks available. Plans that need more than about 1–2 lb a week, intakes under 1,200 kcal (women) or 1,500 kcal (men), or deficits above 25% of TDEE are flagged as aggressive. A longer timeline is almost always the more sustainable choice. See how many calories to eat for more on setting an intake.
Why does the result not match my scale?
Your TDEE estimate has a margin of error, and your logged calories do too. After two to four weeks of consistent logging, compare your real weekly weight change with the projection. If it differs, the adaptive TDEE calculator works out your real maintenance from your own data, and you can feed that number back into your plan.
Frequently asked questions
How long will it take me to lose 20 pounds?
It depends on your calories relative to your TDEE. At a 500 kcal daily deficit the flat rule says 20 weeks; a dynamic model, which lowers TDEE as you lose, gives about 24 weeks for a 165 lb woman. Enter your numbers in the calculator for your own date.
What is a realistic goal weight?
A goal in the healthy BMI range of 18.5–24.9 is a common starting point, and a smaller first goal such as 5–10% of body weight is easier to plan around. See the ideal weight calculator and the BMI calculator for ranges.
How many calories do I need to eat to lose weight by a certain date?
Use target-date mode. It searches for the daily intake that reaches your goal on that date using the weekly projection, then checks it against safety warnings. If the plan needs a very large deficit, a later date is usually the better answer.
Why did my weight loss stall?
Normal causes are water retention hiding fat loss, a slightly smaller deficit as you get lighter, and drifting portion sizes. Give it two to four weeks of consistent logging before changing anything. See the weight loss plateau guide.
Is this calculator accurate?
It is a planning estimate based on the Mifflin–St Jeor equation and a dynamic energy balance model, not a prediction. Individual results vary with adherence, water weight and metabolic adaptation. Compare the projection with your real weekly average and adjust.
References
- Hall KD, Sacks G, Chandramohan D, et al. (2011). Quantification of the effect of energy imbalance on bodyweight. The Lancet 378(9793):826–837.
- Thomas DM, Gonzalez MC, Pereira AZ, Redman LM, Heymsfield SB (2014). Time to correctly predict the amount of weight loss with dieting. Journal of the Academy of Nutrition and Dietetics 114(6):857–861.
- Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO (1990). A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition 51(2):241–247.
- National Institute of Diabetes and Digestive and Kidney Diseases (2024). Body Weight Planner. NIDDK, U.S. National Institutes of Health.
- Turicchi J, O’Driscoll R, Horgan G, et al. (2020). Weekly, seasonal and holiday body weight fluctuation patterns among individuals engaged in a European multi-centre behavioural weight loss maintenance intervention. PLOS ONE 15(4):e0232152.
- Rosenbaum M, Leibel RL (2010). Adaptive thermogenesis in humans. International Journal of Obesity 34(Suppl 1):S47–S55.