Intermittent Fasting Calculator — Your Hours

Intermittent fasting splits the day into a fasting window and an eating window, and working out the hours is clock arithmetic: add the protocol’s fasting hours to your last meal. Pick your protocol and time below, then read what the trials found — it is less dramatic than the marketing.

The protocols, and what separates them

Nothing distinguishes the schedules except how many hours the fast runs. Fast and window always add to 24.

Protocol	Fast	Eating window	Typical user
16/8	16 h	8 h	beginners; the most common
18/6	18 h	6 h	people already comfortable with 16/8
20/4 (Warrior Diet)	20 h	4 h	advanced
24 h (Eat Stop Eat)	24 h	one meal that day	advanced, once or twice a week

In 16/8 a good half of the fast happens while you are asleep, which is the whole reason it is the default recommendation rather than any metabolic property.

Worked example

The calculation the tool runs, done by hand on 16/8:

• Last meal (dinner): 20:00
• Fast ends, 16 hours later: 12:00 the next day — the window opens
• Window closes, 8 hours later: 20:00 — the fast restarts

Shift the whole block earlier or later and the arithmetic is identical. To work only on the moment the fast ends, use the when to break my fast page; to plan the hours you eat in, the eating window calculator.

What the evidence actually supports

This is where most fasting pages overreach, so here is the careful version. StatPearls’ current review of dietary approaches to obesity treatment states plainly that few randomized controlled trials provide evidence of weight loss with intermittent fasting, and that the rationale rests on plain caloric restriction rather than on anything unique to the clock.

Two trials are worth knowing by name:

• TREAT (2020) found that 16:8 time-restricted eating produced no significantly greater weight loss than unrestricted eating over 12 weeks in adults with overweight or obesity.
• A 12-month randomized trial found that calorie restriction combined with time-restricted eating produced no significantly greater weight loss than calorie restriction alone.

StatPearls’ conclusion: once caloric restriction is controlled for, the independent contribution of meal timing to weight loss is limited. What fasting may offer instead is adherence — for some people a rule about hours is easier to follow than portion control at every meal — and the review notes it may be more sustainable over time.

There is one population where the signal is stronger. A 2023 randomized trial found that an 8-hour eating window, without calorie counting, reduced body weight and HbA1c significantly more than control at six months in adults with obesity and type 2 diabetes. That result is specific to that group, and anyone with diabetes on glucose-lowering medication should read the contraindications first.

What breaks a fast

The working definition is calories. Water, plain coffee and unsweetened tea are the conventional allowances, and they help with hunger and hydration. Juice, soda, milk, coffee with sugar or milk, and any solid food end the fast.

Fasting is not for everyone

Intermittent fasting is not advised during pregnancy or breastfeeding, for children and adolescents, for anyone with a history of disordered eating, or for people taking insulin or sulfonylureas, where skipping meals carries a real hypoglycemia risk. StatPearls also notes that data on underweight, geriatric and pediatric patients is still lacking. The full list, with the reasoning behind each entry, is on who should not do intermittent fasting.

For the weight-loss question specifically, see intermittent fasting for weight loss; for the schedule most people start with, the 16/8 calculator. If you want to know how the fasting hours interact with your daily energy needs, the TDEE calculator gives you the number the window has to fit inside.

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