18/6 Intermittent Fasting — Schedule and Results

18/6 is 18 hours of fasting and a 6-hour eating window — two hours tighter than 16/8 on each side. It is the usual next step for someone already comfortable with the standard schedule. Enter your last meal time to see the hours.

The schedule

• Last meal: 21:00
• Fast ends, 18 hours later: 15:00 the next day — the window opens
• Window closes, 6 hours later: 21:00 — the fast restarts

Six hours generally holds two meals, sometimes with something small in between. The block can sit anywhere on the clock; a 12:00-to-18:00 window is the same protocol with an earlier dinner.

18/6 against 16/8

The difference is two hours of fasting and two fewer hours to eat in. What that buys is the question, and the answer is less than the extra effort suggests.

Weight change follows the calorie deficit. The trials that separated meal timing from calorie restriction found that once restriction was accounted for, the independent contribution of the timing was limited — that is StatPearls’ summary of the current evidence, and it does not distinguish between a 16-hour and an 18-hour fast because no trial has shown a reason to.

So a shorter window helps only insofar as it makes you eat less without trying. For some people it does. For others it produces a hungrier, larger evening meal that cancels the difference. Compare against the 16/8 schedule before assuming tighter is better.

The protein problem in six hours

This is the concrete cost of the shorter window, and it is worth planning around:

• Two meals have to carry a full day of protein. That is a real target, and it is where short windows most often fall short.
• Lead with protein when you break the fast, for satiety and for lean mass in a deficit.
• Do not compensate. Eighteen hours of waiting followed by an oversized meal leaves the daily total where it started.

The intermittent fasting macro calculator will split the day’s targets across the meals your window holds.

What 18 hours does not do

It does not put you in ketosis. StatPearls chapters place glycogen depletion at roughly 24 hours in one and 42 in another — either way, an 18-hour fast ends before the switch anyone is waiting for. Treat 18/6 as a way to organise eating, not as a metabolic intervention.

If the window becomes comfortable and you want to go shorter still, the next step is 20/4 — a genuinely advanced schedule with its own drawbacks. The protocol overview and the contraindications are on the intermittent fasting calculator page.

This content is informational and does not replace evaluation by a physician or dietitian.