Alternate-Day Fasting Calculator (4:3)

A 4:3 pattern alternates ordinary eating days with low-calorie fast days across the week — roughly every other day — which works out to four normal days and three fast days. Like 5:2, the deficit is tallied on a weekly ledger rather than an everyday one, but with an extra fast day the weekly total falls further. For the worked body — maintenance about 1,990 calories — the sum is: four days at 1,990 is 7,960 calories, plus three fast days at 500 is 1,500, for a weekly total near 9,450. Divide by seven and the average daily intake is about 1,350 calories, roughly a 640-calorie daily deficit. Alternate-day fasting has been studied as an effective way to create that deficit (Varady et al., 2021), and the calculator above tallies it for your own numbers.

A 640-calorie average daily deficit is about 4,480 calories a week, which at roughly 7,700 calories per kilo of fat comes to about 0.58 kg — near 1.3 lb — every week for the worked body. That makes 4:3 one of the quicker whole-day patterns, a step up from 5:2's gentler pace. The speed comes entirely from the third fast day, so if the loss is faster than you want, dropping back to 5:2 eases it; if you want more, a lower fast-day allowance tightens it. Confirm the maintenance the estimate rests on with the BMR Calculator, then read your weekly range above.

Fast-Day Calories Matter Most Here

Because three of your seven days are fast days, the fast-day allowance is the biggest single lever on your result — more so than on 5:2. Nudging it from 500 to 400 calories noticeably deepens the weekly deficit; letting it drift to 700 softens it just as much. The four normal days matter too: if they creep above maintenance, they erode the deficit the fast days built. The tool lets you set your own fast-day figure so the projection matches the plan you'll actually run.

The choice between 4:3 and 5:2 is really a choice about how many hard days you can sustain. Three fast days lose faster — about 0.58 kg a week for the worked body against roughly 0.39 kg on 5:2 — but they are three days of genuine restriction rather than two, and that is a real weekly cost. Many people start on 5:2, get comfortable with fast days, and only then add the third. Neither is metabolically superior; the deficit does the work, and the better plan is the one you keep. Prefer a steady every-day deficit instead of fast days? Plan it with the Calorie Deficit Calculator.