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.
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.
What the result tells you
Your daily calorie target for a chosen fasting window. The window sets when you eat; calories still decide weight change.
How to use it
- Hit your daily calorie and protein targets within the eating window.
- Keep the window's timing consistent so hunger adapts.
- Ease in — start with a shorter fast and extend it if it suits you.
Recommendations
- Build eating-window meals around protein and fibre for fullness.
- Water, black coffee, and tea are fine during the fast.
- Fasting isn't magic — it mainly helps by making a calorie deficit easier.
Health & wellbeing
Fasting suits some people, not all. Stop if it harms your sleep, mood, or performance, or triggers overeating. It's not advised in pregnancy, for under-18s, or in eating-disorder recovery.
Troubleshooting
Ravenous in the window? Shorten the fast or add protein and volume. Light-headed? Break the fast and reassess.
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.
Common questions
Can I drink coffee while fasting?
Black coffee, tea and water are fine; anything with calories ends the fast.
Will fasting slow my metabolism?
Not within normal schedules; metabolic rate follows body weight and lean mass, not meal timing.
Should I work out fasted?
You can; for hard sessions most people perform better with a meal 1–3 hours before.