Fasting Calculators
16 free tools to plan a fast, time it, and check what you eat and drink around it. Every calculator page shows how it works out your result and the sources it was checked against.
Timing your fast
Estimate the hours in a fast when your body may change fuel. Each figure is a planning estimate, and its page shows how it is worked out.
- Autophagy calculator The hour your fast may reach autophagy, from your body fat, activity, prior diet and fasting experience.
- Metabolic switching calculator When your body may switch from stored glucose to fat, from your last meal, prior diet, activity and body fat.
- Ketosis calculator When light and nutritional ketosis may start, from your carb intake, prior diet, activity and exercise.
- Extended fasting calculator A fast of 24 hours or more: estimated fat loss, ketosis and autophagy hours, electrolyte ranges and a refeeding guide.
Planning your fasts
Pick a protocol, then set its schedule and calories.
- Fasting schedule generator Your daily eating and fasting windows from a protocol, 16:8 to one meal a day, and a first-meal time.
- Fasting protocol comparison Up to four protocols side by side on fasting hours, eating window and difficulty, with a pick for your goal.
- 5:2 diet calculator Fast-day and normal-day calories, your weekly deficit and the weight change it adds up to.
- OMAD calculator Calories and macros for one meal a day, from your weight, height, age, sex, activity and goal.
- PSMF calculator Calories and macros for a protein-sparing modified fast, from your lean mass, with its limits on body fat and age.
Eating and drinking around a fast
Check what you eat and drink, and set macros and electrolytes for your eating window.
- What breaks a fast Checks 237 drinks, foods, supplements, sweeteners and medicines against weight loss, autophagy and insulin control.
- IF macro calculator Training-day and rest-day calories and macros, with meal times inside your eating window.
- Fasting electrolyte calculator Daily sodium, potassium and magnesium reference intakes for your fast.
Tracking your progress
Time your fasts, follow them over weeks and score your routine.
- Fasting weight loss calculator How many weeks to your goal weight, with a daily calorie plan and a week-by-week roadmap.
- Fasting timer Times your fast against a target from 16 to 72 hours, with checkpoints and a history.
- Fasting progress tracker Logs your fasts and weight on a calendar, with a day streak, averages and weekly goals.
- Fasting habit score calculator Scores your fasting routine, lifestyle and exercise habits out of 100.
How to pick a fasting calculator
Start with the question you have. Each row names the tool that answers it.
| Your question | Start with |
|---|---|
| When do I eat each day? | Fasting schedule generator |
| Which fasting pattern should I try? | Fasting protocol comparison |
| Does this drink, food or supplement break my fast? | What breaks a fast |
| How much should I eat in my eating window? | IF macro calculator, or the OMAD, 5:2 diet and PSMF calculators for those plans |
| How long until I reach my goal weight? | Fasting weight loss calculator |
| I am planning a fast longer than a day | Extended fasting calculator, then the fasting electrolyte calculator |
| When might my body change fuel during a fast? | The metabolic switching, ketosis and autophagy calculators |
| How do I keep track? | The fasting timer during a fast, and the fasting progress tracker over the weeks |
Where the numbers come from
Each calculator page explains how it works out your result, step by step, and ends with the studies and reference pages it was checked against. Where a setting is the calculator’s own planning choice rather than a measured value, the page says so, and it shows the date we last checked it.
Fasting calculator questions
Are these calculators free?
Yes. Every calculator shows your result without an account.
Which calculator should I use first?
If you are new to fasting, start with the fasting schedule generator to set your daily windows. Add the others when you have a question they answer.
Are the results medical advice?
No. They are planning estimates. Ask a clinician before you fast if you take medicines, are pregnant or breastfeeding, are under 18, or have had an eating disorder.