Intermittent Fasting Calculator (16:8, 18:6, 20:4, OMAD)
Choose a fasting schedule and your first meal time; get your eating window and the calories to eat inside it.
How it’s calculated
Maintenance calories come from Mifflin–St Jeor × activity, adjusted slightly by the protocol; 5:2 is a weekly average.
16:8 = TDEE × 1.0 · 18:6 = × 0.95 · 20:4 = × 0.90. 5:2 = (5 normal days + 2 × 500 kcal) ÷ 7.
Source: TDEE via Mifflin–St Jeor (1990); protocol factors are a practical framework.
Fasting schedules compared
16:8 — fast 16 h, eat within 8 h (for example 12:00–20:00); the easiest to keep. 18:6 and 20:4 shorten the window further. OMAD is one meal a day. Alternate-day and 5:2 patterns restrict calories on specific days instead of hours. None is proven superior; the best schedule is the one that fits your work, training and family meals.
How many calories to eat on intermittent fasting
Fasting changes when you eat, not how much you need. The calculator sets calories from your TDEE and goal exactly as the calorie calculator does, then shows them inside the eating window. A short window makes overeating harder for many people — that, not “fasting magic”, is the mechanism behind most of the weight-loss results.
Expected weight loss on IF
In trials, time-restricted eating produces weight loss similar to ordinary calorie restriction when calories are matched — typically 3–8% of body weight over 8–12 weeks with a consistent deficit. See our evidence review of intermittent fasting for the details.
Intermittent fasting for women
Most studies include women and show similar results; some women report cycle changes or poor sleep with long fasts, in which case a 12–14-hour overnight fast is a gentler option. Pregnant or breastfeeding women, teenagers and anyone with a history of disordered eating should not fast without medical advice.
Sources
- de Cabo R, Mattson MP. Effects of intermittent fasting on health, aging, and disease. N Engl J Med 2019 (PubMed 31881139) — https://pubmed.ncbi.nlm.nih.gov/31881139/
- Harvard T.H. Chan Nutrition Source, Diet review: intermittent fasting — https://nutritionsource.hsph.harvard.edu/healthy-weight/diet-reviews/intermittent-fasting/
- Mifflin-St Jeor equation — https://pubmed.ncbi.nlm.nih.gov/2305711/
This calculator is educational and not medical advice. If you have a health condition, are pregnant, or are under 18, ask a clinician before changing your diet.
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