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16/8 Intermittent Fasting Calculator — Find Your Eating Window

Enter your last meal time and get your exact break-fast time and eating window. Supports 16/8, 18/6, and OMAD protocols. Free, instant, no signup.

Data verified · June 2026
Edited by Martín Rodríguez
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Edited by: Martín Rodríguez (editorial policy )

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How to use this calculator

Follow this tool’s steps, then review its formula, assumptions, and limits below.

Step by step

Enter your data

Tap the Calculate button

Check the result

Enter the time of your last meal and select your fasting protocol. The calculator tells you exactly when to break your fast and shows your full daily schedule split between fasting and eating. Works for 16/8, 18/6, and OMAD (20/4).

When to use this calculator

Find the exact time to break your fast each day on 16/8
Compare 16/8 vs 18/6 vs OMAD eating windows side by side
Plan meals around work, gym, and social schedules
Students and dietitians studying time-restricted eating protocols
Quick daily reference when starting a new fasting routine

What happens in your body at each hour of a fast

Approximate metabolic phases during a continuous fast (healthy adult, last meal at hour 0).

Hours fasted	Metabolic phase	What's happening
0–4 h	Fed / absorptive	Blood glucose and insulin rise; body burns the meal you just ate
4–8 h	Post-absorptive	Insulin drops; body draws on stored liver glycogen
8–12 h	Early fasting	Liver glycogen falling; fat-burning (lipolysis) increases
12–16 h	Glycogen depleted	Glycogen mostly used up; fat becomes the main fuel — 16/8 ends here
16–18 h	Fat-burning ramp	Ketone production begins to rise — 18/6 ends here
18–24 h	Ketosis building	Ketones become a significant fuel; autophagy signals increase — OMAD ends here
24–48 h	Established ketosis	Deeper ketosis and autophagy (extended fasting — do under medical guidance)

Timings are approximate and vary with prior diet, activity and individual metabolism. Based on general fasting physiology (post-absorptive state at ~4 h, glycogen depletion at ~12–16 h). Plain water, black coffee and unsweetened tea do not interrupt these phases.

How it works

How 16/8 Intermittent Fasting Works

The calculator adds the fasting hours to your last meal time and automatically adjusts if the result crosses midnight. The formula is:

Break-fast time = last meal time + fasting hours

Example: last meal at 8 PM (20:00) + 16 hours = 12:00 PM (noon). Your eating window would be 12 PM to 8 PM.

> What this tool does NOT calculate: calories, macros, estimated weight loss or compatibility with medical conditions. It only determines your time window.

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Common eating windows — quick reference table

Last meal (start of fast)	Protocol	Break-fast time	Eating window
6:00 PM (18:00)	16/8	10:00 AM	10 AM – 6 PM
7:00 PM (19:00)	16/8	11:00 AM	11 AM – 7 PM
8:00 PM (20:00)	16/8	12:00 PM	12 PM – 8 PM
9:00 PM (21:00)	16/8	1:00 PM	1 PM – 9 PM
10:00 PM (22:00)	16/8	2:00 PM	2 PM – 10 PM
8:00 PM (20:00)	18/6	2:00 PM	2 PM – 8 PM
8:00 PM (20:00)	OMAD 20/4	4:00 PM	4 PM – 8 PM

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Protocol comparison

Protocol	Fast	Window	Best for
16/8	16 hours	8 hours	Beginners, long-term maintenance
18/6	18 hours	6 hours	Intermediate, larger calorie deficit
OMAD (20/4)	20 hours	4 hours	Advanced, maximum simplicity

16/8 is the protocol with the most support in controlled human studies. A review published in the New England Journal of Medicine (2019, Mattson et al.) notes that intermittent fasting can improve insulin sensitivity, blood pressure and inflammation markers, though the effects depend on total calorie context and adherence.

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Why when you place the window matters, not just how long you fast

Not all 8-hour windows are physiologically equivalent. Research from the Salk Institute and time-restricted eating (TRE) studies suggests that earlier windows — those that end before 6–8 PM — tend to show better metabolic results than later ones, even with the same fasting duration. This relates to circadian rhythms: insulin sensitivity is naturally higher in the morning and declines toward night.

In practical terms: a 10 AM–6 PM window can have a different metabolic profile than a 2 PM–10 PM one, even though both are 16/8. For most people on standard work schedules, ending the window between 7–9 PM is a reasonable balance between the evidence and adherence.

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What breaks a fast (and what doesn't)

Does NOT break the fast:

Water (still or sparkling)

Black coffee with no sugar or dairy

Unsweetened tea

Calorie-free electrolyte water

Breaks the fast:

Any food with calories

Milk, cream or oil in coffee (bulletproof coffee breaks the metabolic fast, even though some use it in keto protocols with different goals)

Juices, even fresh-pressed ones

Supplements with calories or amino acids (BCAAs, for example, trigger an insulin response)

Debated in the literature:

Diet sodas / artificial sweeteners: some observational studies link their consumption to an insulin response, though the causal evidence in humans is still inconsistent.

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Common mistakes when using the calculator

Confusing last meal time with bedtime. The fast begins when you finish eating, not when you go to bed.

Not counting nighttime snacks. If you have dinner at 8 PM but snack on something at 10 PM, your fast starts at 10 PM, not 8 PM.

Assuming fasting guarantees a calorie deficit. 16/8 shrinks the window for eating, but it does not control how much you eat within it. Without a calorie deficit there is no fat loss.

Changing the window every day. Irregular timing reduces the circadian benefits of the protocol. Consistent timing is part of the mechanism, not just the fasting duration.

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Important considerations

Intermittent fasting is not right for everyone. It is best to consult a healthcare professional before starting if you have a history of eating disorders, type 1 diabetes, are pregnant or breastfeeding, or take medication that requires food intake. This calculator is a time-planning tool, not medical or nutritional advice.

Worked Example

Last meal: 8:00 PM (20:00), protocol: 16/8

16 hours after 20:00 = 12:00 (noon) next day

Eating window: 12:00 PM – 8:00 PM (8 hours)

Break fast at 12:00 PM — eating window is 12:00 PM to 8:00 PM

Frequently asked questions

If I eat my last meal at 8 PM, when do I break my fast on 16/8?

At 12:00 PM (noon) the next day. 8 PM + 16 hours = 12 PM. Your eating window is then 12 PM to 8 PM — exactly 8 hours.

If I eat my last meal at 7 PM, when do I break my fast?

At 11:00 AM the next day (7 PM + 16 hours = 11 AM). Your eating window shifts to 11 AM – 7 PM.

What is the difference between 16/8 and 18/6 fasting?

16/8 means 16 hours fasting with an 8-hour eating window; 18/6 means 18 hours fasting with a 6-hour eating window. With the same 8 PM last meal, 16/8 lets you eat from 12 PM, while 18/6 only lets you eat from 2 PM.

Does sleeping count toward fasting hours?

Yes. Sleep is the easiest part of your fast. A typical 7–8 hour sleep covers most of your 16-hour window, leaving only 8–9 waking hours where you are not eating.

Can I drink coffee during the fasting window?

Plain black coffee (no milk, cream, or sugar) is generally considered fast-safe. It contains negligible calories and does not spike insulin in most people. Bulletproof coffee with fat does break the fast.

How long does it take to adapt to 16/8 fasting?

Most people adapt within 1–2 weeks. The first 3–5 days may include hunger pangs, especially in the morning. After adaptation, many people report reduced hunger during the fasting window.

Is 16/8 intermittent fasting safe for women?

Research shows mixed results. Some women do well on 16/8; others report hormonal disruption (irregular cycles, sleep issues) and do better with a gentler 14/10 approach. Start conservatively and adjust based on your body's response.

Can I work out while fasting on 16/8?

Light to moderate cardio during the fasting window is generally fine and may enhance fat oxidation. Heavy resistance training is best scheduled close to or within your eating window so you can refuel with protein immediately after.

Does 16/8 fasting actually work for weight loss?

Systematic reviews (e.g., Cioffi et al. 2018, Harris et al. 2018) find that 16/8 produces modest weight loss similar to continuous caloric restriction when total calorie intake is controlled. The main advantage is simplicity — fewer meals to plan.

Sources & references

Methodology & trust

Editorial

Cooking calculator with its formula verified automatically against Cioffi et al. (2018) — Intermittent vs. Continuous Energy Restriction on Weight Loss — Nutrients, per our editorial policy and methodology.

Updates

Updated: June 2026. Parameters are verified periodically against the cited sources.

Privacy

Calculations run 100% in your browser. We do not store or transmit your data.

Limitations

Indicative results. For critical decisions, consult a professional.

📌 How to cite this calculator

Rodríguez, M. (2026). 16/8 Intermittent Fasting Calculator — Find Your Eating Window. Hacé Cuentas. https://hacecuentas.com/en/intermittent-fasting-16-8-calculator

Content licensed under CC-BY 4.0 — reuse it citing the source with a link to Hacé Cuentas.

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