Calculate daily calorie deficit, days of a water fast, or estimated fat loss using the 3,500-calorie-per-pound rule for water fasting.
Water Fast Weight Loss Formula
The calculator uses the common estimate that about 3,500 Calories equals 1 pound of body fat. It estimates fat loss from your calorie deficit during a water fast, not total scale weight change. Scale weight can also change from water, glycogen, sodium, and food volume.
FL = (DCD * Days) / 3500
DCD = (FL * 3500) / Days
Days = (FL * 3500) / DCD
- FL = estimated fat loss in pounds
- DCD = estimated daily calorie deficit in Calories per day
- Days = number of days of water fasting
- 3500 = estimated Calories per pound of body fat
If you enter daily calorie deficit and fasting days, the calculator estimates fat loss. If you enter fat loss and fasting days, it estimates the daily calorie deficit needed. If you enter fat loss and daily calorie deficit, it estimates the number of fasting days required.
Estimated Fat Loss by Daily Deficit and Fast Length
Use this table as a quick check for common inputs. Values are estimated fat loss in pounds.
| Daily Calorie Deficit | 1 Day | 3 Days | 5 Days | 7 Days |
|---|---|---|---|---|
| 1,500 Calories/day | 0.43 lb | 1.29 lb | 2.14 lb | 3.00 lb |
| 2,000 Calories/day | 0.57 lb | 1.71 lb | 2.86 lb | 4.00 lb |
| 2,500 Calories/day | 0.71 lb | 2.14 lb | 3.57 lb | 5.00 lb |
| 3,000 Calories/day | 0.86 lb | 2.57 lb | 4.29 lb | 6.00 lb |
Scale Weight Versus Estimated Fat Loss
| Weight Change Type | What It Means | Included in This Calculator? |
|---|---|---|
| Fat loss | Energy deficit converted using 3,500 Calories per pound | Yes |
| Water weight | Fluid shifts from glycogen, sodium, hydration, and hormones | No |
| Food volume | Less food in the digestive tract during a fast | No |
| Lean mass change | Changes in muscle, organ tissue, or other non-fat mass | No |
Example Problems
Example 1: Estimate fat loss from a 3-day water fast
You estimate a daily calorie deficit of 2,200 Calories/day and fast for 3 days.
FL = (2200 * 3) / 3500
FL = 6600 / 3500 = 1.89 lb
The estimated fat loss is 1.89 lb.
Example 2: Find the fasting days needed for 2 lb of estimated fat loss
You want to estimate how many days would be needed for 2 lb of fat loss with a 2,500 Calories/day deficit.
Days = (2 * 3500) / 2500
Days = 7000 / 2500 = 2.80 days
The estimated fasting length is 2.80 days.
FAQ
Does this calculator predict total weight lost during a water fast?
No. It estimates fat loss from a calorie deficit. Your scale weight may drop more than this during a water fast because of water loss, lower glycogen stores, less sodium, and less food in your digestive tract. Some of that weight can return after eating and rehydrating.
How should I estimate daily calorie deficit during a water fast?
For this calculator, daily calorie deficit is usually close to your estimated daily energy expenditure, since a water fast has little to no calorie intake. For example, if you normally burn about 2,300 Calories per day and consume 0 Calories, your estimated daily deficit is about 2,300 Calories/day.
Is the 3,500 Calories per pound rule exact?
No. It is a simple estimate. Actual fat loss can vary because metabolism changes, activity changes, water balance shifts, and body composition differs between people. Use the result as an approximation, not an exact prediction.
