What's the difference between 16:8 and 18:6?

Two hours, and where they land. 18:6 tightens the eating window from eight hours to six and extends the fast from sixteen to eighteen — which is roughly where ketone production starts to climb for most people. 18:6 spends more time in the fat-burning and early-ketosis range than 16:8 does.

Why switch from 16:8 to 18:6?

Usually because 16:8 has stopped producing change, or because you want more time in ketosis without committing to OMAD. 18:6 is the natural middle step: a six-hour window is still comfortable for two meals.

When does ketosis start on an 18-hour fast?

For most people on a normal diet, ketone production starts climbing around 16–18 hours, so an 18:6 fast tends to end right in that zone — earlier if you are low-carb or train fasted. It is an estimate, not a blood reading. See the phase timeline for detail.

Is 18:6 in the free tier?

18:6 is a Pro protocol ($3.99/month), along with OMAD, 24h, custom windows, phase tracking, and a year of history. The 16:8 timer is always free.

18:6 intermittent fasting: the tighter window for when 16:8 plateaus

18:6 is 16:8’s tighter sibling: an 18-hour fast and a 6-hour eating window. It is the step most people take when 16:8 has become routine and stopped moving the needle, or when they want more time in the fat-burning and early-ketosis range without going all the way to one meal a day.

How to run an 18:6 fast

Set a 6-hour eating window — for example 13:00–19:00.
Start the timer at your last meal. FastHQ counts from the exact second.
Fast for 18 hours. Water, black coffee, and plain tea only.
Open the window, stop the timer, and log it to the second.

A six-hour window is still wide enough for two real meals, which is why 18:6 is sustainable for daily use where OMAD often isn’t.

What 18:6 changes metabolically

The extra two hours matter because of where they land on the fasted-state phase timeline. A 16-hour fast ends just as ketone production begins; an 18-hour fast pushes into it. So 18:6:

spends more time in gluconeogenesis (12–18h), where fat-burning is already high, and
reaches the start of ketosis (~18h) for most people on a normal diet.

If you are already keto-adapted or train fasted, you will reach ketosis sooner; after a big carb meal, later. The timer models the average — a blood ketone meter tells you your number.

Who 18:6 is for

18:6 suits people who have a stable 16:8 habit and want a bit more, without the social friction of a single daily meal. It is a clean middle ground.

It is not for everyone. As with any fasting protocol, check with a doctor first if you are pregnant or breastfeeding, diabetic or on blood-sugar medication, underweight, or have a history of disordered eating.

Common mistakes

Jumping straight from skipped breakfast to 18:6. Build the habit on 16:8 first; the last two hours are the hardest.
Cramming the window. Six hours, two meals — not a six-hour buffet.
Expecting deep ketosis. 18:6 reaches the start of ketosis, not deep ketosis. For more time there, see OMAD and 24-hour fasts.

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