18:6 Intermittent Fasting Calculator

An 18:6 schedule trims two hours off the popular 16:8 window, and that small change tends to shave a little more off your daily intake, because a six-hour window leaves less room for a third meal or casual grazing. Reviews of intermittent fasting put the reduction in intake at 10–30% across protocols, with shorter windows at the higher end (Varady et al., 2021); this calculator estimates a 20–30% cut for 18:6, a little more than 16:8. For the worked body — maintenance about 1,990 calories — a 25% cut is close to 500 calories a day, landing intake near 1,490. The mechanism is identical to every other fasting window: the shorter eating period helps you eat less, and that deficit is what drives the loss. Enter your own maintenance and the calculator sizes the gap for you.

A 500-calorie daily deficit is about 3,500 calories a week, which at roughly 7,700 calories per kilo of fat comes to about 0.45 kg — near 1.0 lb — every week for the worked body. At that pace, a 5 kg goal is roughly eleven weeks away. Those are steady numbers; the scale will usually move faster in the first week from water and glycogen and then settle to this fat-loss rate. Confirm the maintenance the whole estimate rests on with the TDEE Calculator, then read your weekly range above.

Is the Extra Two Hours Worth It?

Moving from 16:8 to 18:6 buys you a slightly larger deficit, but only if the shorter window doesn't change your behaviour for the worse. Some people compress the same food into six hours and lose a little faster; others find a tight window nudges them toward two very large meals that quietly add the calories back, or that leaves them short on protein. The two extra fasting hours are worth taking only when they cut intake without those side effects. If 18:6 feels forced, 16:8 loses only marginally less and is easier to hold.

A six-hour window rewards planning. Anchor two solid, protein-forward meals inside it rather than snacking your way through, keep the fasting hours filled with water, black coffee or plain tea, and judge progress on the one- to two-week trend rather than any single morning. If you have a target weight in mind, add it in the calculator above to see your projected finish date, and pair the plan with the Goal Weight Calculator to map the milestones along the way.