The 3,500-calorie rule is a simplification that overestimates fat loss, and water, glycogen and hormone changes make the scale unpredictable from day to day. Very large deficits risk muscle loss and nutritional problems. Weight loss is not linear, and plateaus are normal.
Medical conditions and medications change the picture.
Enter your current weight, your goal weight and the daily calorie deficit you plan to maintain, compared with your maintenance calories. A deficit of 250 to 500 calories a day is a moderate pace for most people, and 1,000 a day is the maximum many guidelines recommend. The tool uses the approximation that a pound of body fat holds about 3,500 calories.
Use your goal weight from a healthy range, not an arbitrary number.
Weight Loss Time: formula
Weight Loss Time: worked example
| Current weight (lb) | 180 |
|---|---|
| Goal weight (lb) | 160 |
| Daily calorie deficit | 500 |
| Weeks to goal | 20 |
| Total calorie deficit needed | 70,000 |
Common error: Doubling the deficit to hit the date sooner. Very large deficits cost muscle and are hard to hold, and the extra loss rarely arrives in proportion.
Weight Loss Time: changing Current weight (lb)
| Current weight (lb) | Weeks to goal | Total calorie deficit needed |
|---|---|---|
| 144 | 0 | 0 |
| 162 | 2 | 7,000 |
| 180 (example) | 20 | 70,000 |
| 198 | 38 | 133,000 |
| 216 | 56 | 196,000 |
Weight Loss Time: compare input ranges
| Input | Tested from | To | Weeks to goal: low input | Weeks to goal: high input | Swing |
|---|---|---|---|---|---|
| Current weight (lb) | 162 | 198 | 2 | 38 | 36 (180%) |
| Goal weight (lb) | 144 | 176 | 36 | 4 | 32 (160%) |
| Daily calorie deficit | 450 | 550 | 22.2 | 18.2 | 4 (20%) |
Weight Loss Time: individual input comparisons
| Input | Value | Weeks to goal | Total calorie deficit needed |
|---|---|---|---|
| Current weight (lb) | 144 | 0 | 0 |
| 162 | 2 | 7,000 | |
| 180 | 20 | 70,000 | |
| 198 | 38 | 133,000 | |
| 216 | 56 | 196,000 | |
| Goal weight (lb) | 128 | 52 | 182,000 |
| 144 | 36 | 126,000 | |
| 160 | 20 | 70,000 | |
| 176 | 4 | 14,000 | |
| 192 | 0 | 0 | |
| Daily calorie deficit | 400 | 25 | 70,000 |
| 450 | 22.2 | 70,000 | |
| 500 | 20 | 70,000 | |
| 550 | 18.2 | 70,000 | |
| 600 | 16.7 | 70,000 |
Weight Loss Time: combined input changes
| Current weight (lb) / Goal weight (lb) | 128 | 160 | 192 |
|---|---|---|---|
| 144 | 16 | 0 | 0 |
| 162 | 34 | 2 | 0 |
| 180 | 52 | 20 | 0 |
| 198 | 70 | 38 | 6 |
| 216 | 88 | 56 | 24 |
Weight Loss Time: which assumptions move the result?
Weight Loss Time: values and definitions
| Quantity and definition | Value used |
|---|---|
| Current weight (lb) — Your weight today, in pounds. | 180 |
| Goal weight (lb) — The weight you want to reach, in pounds. | 160 |
| Daily calorie deficit — How many calories below maintenance you eat or burn off each day. | 500 |
| Weeks to goal | 20 |
| Total calorie deficit needed | 70,000 |
Weight Loss Time: half and double Current weight (lb)
| Current weight (lb) | Weeks to goal |
|---|---|
| 90 | 0 |
| 180 | 20 |
| 360 | 200 |
Energy & Nutrition calculators: Carbohydrate Intake Calculator, Fat Intake Calculator, BMR Calculator, TDEE Calculator, Calorie Goal Calculator, Macronutrient Calculator.
The static 3,500-kcal-per-pound convention turns an assumed energy gap into an arithmetic equivalent, not a weight-loss forecast or earliest finish date. At 500 kcal/day, 20 lb corresponds to 140 days by that convention. Actual expenditure, body composition, intake and physiology change over time.
Recheck the deficit against your own maintenance calories every few weeks. A shrinking body needs a smaller deficit target to keep the same pace.
Find your maintenance on the TDEE calculator and a daily target on the calorie goal page. The BMI and healthy weight range pages help set a goal, and the protein and macronutrient calculators guide the diet. A clinician or dietitian can help with medical concerns.
Does the 3,500-kcal convention predict a completion date?
No. Twenty pounds at a constant assumed 500 kcal/day gap corresponds to 140 days arithmetically, but changing expenditure and physiology make that different from a personal weight-loss forecast.
Does a static result prescribe an intake?
No. It does not assess medical suitability, nutritional adequacy or an individual energy requirement. The model applies an entered energy-change assumption.
It does not determine a person's actual expenditure, an appropriate diet or a treatment plan.
How long will it take to lose 20 pounds?
A static energy model using an assumed 3,500 kcal per pound and 500 kcal/day deficit gives 20 × 3500/500 = 140 days, or 20 weeks. This is illustrative arithmetic rather than a personal forecast. Physiological adaptation, water changes, health and adherence can change the outcome.
Is the 3,500-calorie rule accurate?
It is a rough guide that overstates long-term loss. Your body adapts and burns less as you lose weight, so results lag behind the arithmetic.
Why does weight loss slow down over time?
Plateaus are common due to water shifts and a lower calorie burn. Recalculate your needs, check your tracking and be patient before making large cuts.