Intermittent fasting calculator
Pick your fasting protocol and the time of your first meal, and we tell you exactly your eating window and your fasting window. No mental maths: just read the schedule and you know when to start and when to stop. Fasting works through the discipline of the schedule, but what decides the result is still what you eat inside that window.
The most popular and studied
Eating window · 8 h eating
12:00 – 20:00
Fasting window · 16 h fasting
20:00 – 12:00 (next day)
Fasting sets the WHEN; what you eat in the window sets the result. Renzy tracks your calories and protein with a photo.
Guidance only. Intermittent fasting is not for everyone (pregnancy, diabetes, eating disorders, some medications): consult a professional.
How is the fasting window calculated?
Intermittent fasting splits the day into two blocks: an eating window and a fasting window, and the protocol sets how many hours each lasts. In 16:8, the most popular, you fast 16 hours and eat in an 8-hour window; e.g. if your first meal is at 12:00, you eat until 20:00 and fast from 20:00 to 12:00 the next day. Other protocols adjust that ratio: 14:10 (gentler, to start), 18:6, 20:4 (advanced) and 23:1 or OMAD (one meal a day). This calculator just adds your protocol hours to your first meal time to give the exact schedule. Important: fasting organises the WHEN, not the HOW MUCH. Losing weight still depends on a calorie deficit; fasting just helps some people eat less by concentrating meals. And inside the window, prioritise protein and real food, do not offset with ultra-processed products.
FAQ
What is 16/8 fasting?
It is the most common intermittent-fasting protocol: you fast for 16 hours straight and concentrate all meals in an 8-hour window. Many do it naturally by skipping breakfast: first meal at 12:00-13:00 and last around 20:00-21:00. During the fast you only have water, coffee or unsweetened tea (no sugar or milk). It is popular because it fits social life (normal dinners) and is sustainable.
Does intermittent fasting help you lose weight?
It can help, but not by magic. Fasting does not burn fat by itself: it slims you if, by concentrating meals in fewer hours, you end up eating fewer total calories and creating a deficit. Many find it easier to control intake this way than counting every calorie. But if in the window you eat as much or more than you spend, you will not lose weight even fasting 16 hours. Fasting is an organising tool, not a shortcut that skips calories.
What can you have during the fast?
Calorie-free drinks: water (plain or sparkling), black coffee, tea and unsweetened infusions. These do not break the fast. What does break it is anything with calories: milk in coffee, juices, sugary drinks, broths or a snack. Zero-calorie sweeteners are a grey area; for most practical goals they are fine, but if you want the strictest fast, stick to plain water, coffee and tea.
Which fasting protocol should I choose?
If you are starting, 14:10 or 16:8 are the most sustainable and best-balanced; 16:8 is the standard for a reason. Longer protocols (18:6, 20:4 or OMAD, one meal a day) are not "better" nor slim you more by themselves: they just compress the window more, which helps some and leads others to binges or to missing their protein. Start gentle, see how you feel and how you eat in the window. Fasting is not for everyone (pregnancy, diabetes, history of eating disorders); consult a professional if in doubt.
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