Intermittent Fasting Calculator

What time-restricted eating is

Intermittent fasting covers several patterns, but the one most people mean is time-restricted eating: keeping all of your food inside a fixed daily window. A 16:8 protocol means a 16-hour fast followed by an 8-hour eating window, which usually looks like skipping breakfast and eating between noon and 8 p.m. A 14:10 is a gentler version, and an 18:6 or 20:4 is stricter.

The important point is that time-restricted eating is a way of organising intake, not a metabolic trick. Controlled studies comparing it with ordinary calorie restriction at the same total intake find similar weight loss and similar changes in insulin sensitivity. What it reliably does is reduce the number of eating occasions, which for many people reduces total intake without counting anything.

How the calculator works

Enter the time of your last meal and choose a fasting length. The calculator adds the fasting hours to that time, rolls past midnight where necessary, and prints when the fast ends, what the eating window looks like, and how many fasting hours that adds up to each week. It also repeats your daily protein target, because that is the number most likely to be missed when the eating window shrinks.

The protein figure comes from your body weight and activity level: 1.6 to 2.0 g per kilogram for most people who train. Fitting 150 g of protein into six or eight hours is doable - two protein-led meals plus a shake - but it takes planning rather than hope.

Reading your result

The first row names the protocol, the second marks the start of the fast and the third its end - including the next-day rollover when the fast crosses midnight. The fourth row shows the eating window as two clock times, which is the part people actually need: a 16:8 with a last meal at 8 p.m. means eating between noon and 8 p.m. the following day.

The weekly total is a useful reality check. A 16-hour daily fast is 112 hours a week, which sounds extreme until you notice that most of it is sleep. A 20:4 protocol is 140 hours, which is where adherence usually breaks down for anyone with a job or a family.

Does timing matter for weight loss?

Not directly. Energy balance still decides whether weight goes up or down, and studies that hold calories and protein equal find no meaningful difference in fat loss between a 16:8 schedule and three ordinary meals. The benefit of time restriction is behavioural: fewer eating occasions, less grazing, and a clear rule that removes evening snacking.

There is a small effect that works in your favour if you train in the fasted window, because training with low glycogen tends to increase fat oxidation during the session. That is a short-term shift in fuel use, not extra fat loss over the week, and it comes at a cost in training quality for anything high-intensity.

Who should not fast

Time-restricted eating is unsuitable for some groups regardless of its popularity. Anyone with a history of disordered eating should be cautious, because fixed rules around food can reinforce restriction. People with type 1 diabetes, pregnant or breastfeeding women, and anyone taking medication that must be taken with food need clinical advice first.

• Pregnancy and breastfeeding
• Type 1 diabetes or insulin-treated type 2 diabetes
• A history of eating disorders
• Underweight, or recovering from illness or surgery
• Medication that must be taken with food

Making a fasting window work

Start shorter than you think you need. A 14:10 held comfortably for two weeks beats a 20:4 abandoned after three days. Keep water, black coffee and plain tea during the fast, since they do not break it and help with appetite; anything with calories does, including milk and most supplements.

Front-load protein inside the eating window and keep a source at every meal. Training usually fits better at the end of the fast or early in the eating window, so the post-session meal can be a real one rather than a rushed snack.

• Open the window with 30 to 40 g of protein rather than carbohydrate alone.
• Keep a protein source in every meal inside the window.
• Drink water during the fast; thirst is often mistaken for hunger.
• Break the fast deliberately, not with whatever is nearest.

Frequently asked questions