Last verified against the revised Harris-Benedict equation (Roza and Shizgal, 1984) and the 3,500-calories-per-pound rule (Wishnofsky, 1958).
Fasting Results Timeline
See how long it will take to reach your weight loss goals with intermittent fasting
Related Calculators
Medical Disclaimer: This calculator provides estimates for educational purposes only and does not constitute medical advice. Intermittent fasting is not appropriate for everyone. Do not fast if you are pregnant or breastfeeding, under 18 years of age, have a history of eating disorders, or have been diagnosed with Type 1 diabetes. If you take medications (especially insulin or blood pressure medication), have any chronic health conditions, or are recovering from surgery, consult your physician before beginning any fasting protocol. If you experience chest pain, fainting, persistent dizziness, or heart palpitations during a fast, stop immediately and seek medical attention.
How your timeline is worked out
The calculator works in four steps. Every step is an estimate, so treat the goal date as a planning target, not a promise.
1. Calories at rest (BMR)
Your weight, height, age and sex go into the revised Harris-Benedict equation, which estimates the calories your body burns at complete rest.
Men: 88.362 + (13.397 x weight in kg) + (4.799 x height in cm) – (5.677 x age)
Women: 447.593 + (9.247 x weight in kg) + (3.098 x height in cm) – (4.330 x age)
Source: Roza and Shizgal, American Journal of Clinical Nutrition, 1984.
2. Calories in a normal day (TDEE)
The calculator multiplies your BMR by a factor for the activity level you pick. These are the standard planning factors used with Harris-Benedict, not measurements of you, so pick the level that matches your usual week.
| Activity level | Factor |
|---|---|
| Sedentary | 1.2 |
| Light | 1.375 |
| Moderate | 1.55 |
| Very active | 1.725 |
3. The deficit your protocol creates
The calculator assumes each fasting protocol cuts a fixed share of your TDEE, then scales that share by the consistency you expect. On 16:8 at 80% consistency, the daily deficit is 20% x 80% = 16% of your TDEE.
| Protocol | Assumed deficit |
|---|---|
| 16:8 | 20% of TDEE |
| 18:6 | 25% of TDEE |
| 20:4 | 30% of TDEE |
| OMAD | 35% of TDEE |
| 5:2 | 28% of TDEE |
| Alternate-day fasting | 35% of TDEE |
These shares are the calculator’s planning assumption, not a measured effect of each protocol. Your real deficit depends on what you eat in your eating window.
4. Weekly loss and weeks to goal
The weekly deficit is divided by 3,500 calories, the traditional estimate of the energy in a pound of body fat. Weeks to goal is the weight you want to lose divided by that weekly loss, rounded up to a whole week.
The 3,500-calorie rule overstates loss over several months, because you burn fewer calories as you get lighter. Recalculate with your new weight every few weeks, and expect the later weeks to go slower than the first projection.
Sources: Wishnofsky, American Journal of Clinical Nutrition, 1958; Hall and colleagues, The Lancet, 2011.
Worked example: the default inputs
A 35-year-old man, 5 ft 8 in and 180 lb, moderately active, aiming for 160 lb on 16:8 at 80% consistency. These are the numbers the calculator shows before you change anything.
| Step | Result |
|---|---|
| BMR | 1,812 calories a day |
| TDEE (x 1.55) | 2,809 calories a day |
| Daily deficit (16% of TDEE) | 449 calories |
| Target intake | 2,360 calories a day |
| Weekly loss (3,146 / 3,500) | 0.9 lb |
| Weeks to lose 20 lb | 23 weeks |
Fasting weight loss questions
How long until I see results from intermittent fasting?
The scale often drops more in the first week, mostly from water and stored carbohydrate rather than fat. After that, expect roughly the weekly loss the calculator shows, which is 0.9 lb a week at the default inputs.
Will I lose muscle while fasting?
Some muscle loss can happen in any calorie deficit. Eat enough protein in your eating window and keep up strength training to protect lean mass.
Which fasting protocol is best for weight loss?
The one you can keep up. In a one-year trial, alternate-day fasting and ordinary daily calorie cutting led to similar weight loss when the overall calorie cut was about the same. Longer fasts such as OMAD cut more calories for some people but are harder to stick with.
Why did my weight loss stall after a few weeks?
As you lose weight you burn fewer calories each day, because you carry less mass, so the same eating pattern creates a smaller deficit. Recalculate with your current weight, and check whether portions or snacks have crept up.
Sources
- The Harris Benedict equation reevaluated: resting energy requirements and the body cell massRoza AM, Shizgal HM. American Journal of Clinical Nutrition, 1984; 40: 168-182.
- Caloric equivalents of gained or lost weightWishnofsky M. American Journal of Clinical Nutrition, 1958; 6: 542-546.
- Quantification of the effect of energy imbalance on bodyweightHall KD, Sacks G, Chandramohan D, et al. The Lancet, 2011; 378: 826-837.
- Effect of alternate-day fasting on weight loss, weight maintenance, and cardioprotection among metabolically healthy obese adultsTrepanowski JF, Kroeger CM, Barnosky A, et al. JAMA Internal Medicine, 2017; 177: 930-938.
More fasting tools
- IF macro calculatorTurn your calorie target into protein, carbohydrate, fat and meal times.
- Fasting schedule generatorPick a protocol and a start time to get your daily eating and fasting windows.
- OMAD calculatorCalories and macros for one meal a day.
- All fasting calculatorsEvery fasting calculator in one list.
