16:8 Intermittent Fasting Calculator

A 16:8 schedule doesn't change the arithmetic of weight loss — it just makes the arithmetic easier to hit. When you push all your meals into an eight-hour window, breakfast disappears, late-night grazing stops, and the number of eating occasions falls, so most people take in noticeably fewer calories without counting a thing. One controlled trial of an eight-hour eating window measured that spontaneous drop at about 341 calories a day (Gabel et al., 2018) — for many people that is somewhere around 15–20% of their intake. This calculator uses a 15–25% band for a 16:8 window; for our worked body — maintenance about 1,990 calories — a 20% cut is close to 400 calories a day, landing intake near 1,590. That gap between what you burn and what you eat is the entire engine of the loss; the fasting hours add nothing beyond the calories they help you skip. Enter your own maintenance and the calculator sizes the gap for you.

A 400-calorie daily deficit is about 2,800 calories a week, and since a kilo of fat stores roughly 7,700 calories, that works out to about 0.36 kg — near 0.8 lb — every week, or in the region of 1.5 kg (3.4 lb) across an average month. Those are steady, realistic numbers rather than the dramatic figures a scale shows in the first few days. Early on, a 16:8 start burns through stored glycogen and sheds the water bound to it, so week one often looks bigger than the fat-loss rate; that water returns the moment you eat normally again, which is why the honest projection separates it out. Confirm the maintenance figure the whole estimate rests on with the TDEE Calculator, then read your weekly range above.

Why the Scale Might Not Match

The single reason 16:8 stops working is quietly eating the deficit back. An eight-hour window makes it easy to fit a full day's calories — sometimes more — into two large meals, and if you do, the gap closes and the scale stalls. The projection above assumes you don't compensate, so treat the deficit figure as a reality check rather than a promise: log a few days honestly and compare what you actually eat against your maintenance. If the two are closer than you expected, that is the answer.

The window is only worth keeping if you can keep it, and two habits do most of the work. First, build meals around protein — roughly 1.6 grams per kilogram of bodyweight blunts hunger and protects muscle while you lose fat (Morton et al., 2018) — so the deficit doesn't leave you ravenous by evening. Second, don't force the window narrower than fits your life; if 16:8 feels hard, easing to 14:10 loses a little less each week but is far easier to hold, and a plan you follow beats a stricter one you abandon. Once you have a daily calorie number you're comfortable with, split it into protein, carbohydrate and fat with the Calorie Deficit Calculator to compare a steady every-day approach.