Plan your fasting schedule. This free intermittent fasting calculator shows when your eating window opens and closes for 16:8, 18:6, 20:4 or OMAD — a timing planner, with no calorie or weight targets.
Enter when you finish your last meal. The calculator shows when your fasting window ends and your eating window opens and closes.
A timing guide for planning your eating window — it doesn’t recommend calories or weight goals. Stay hydrated during fasts, eat balanced meals in your window, and stop or seek advice if you feel unwell.
How the Intermittent Fasting Calculator Works
This tool is a timing planner, not a calorie or weight-loss calculator: it takes the time you finish your last meal and your chosen fasting method, then works out exactly when your next eating window opens and closes.
Formula: Eating Window Opens = Fast Start Time + Fasting Hours. Eating Window Closes = Eating Window Opens + Eating Hours, where Eating Hours = 24 − Fasting Hours.
- Method — your fasting schedule (16:8, 18:6, 20:4/Warrior, OMAD, 14:10, or a custom fasting-hours number)
- Fasting hours — used only when Method is set to Custom, entered directly
- Time you start fasting — the time of your last meal, which begins the fasting clock
Example Scenarios
| Method | Start Fasting (last meal) | Eating Window |
|---|---|---|
| 16:8 | 8:00 PM | 12:00 PM – 8:00 PM |
| 18:6 | 7:00 PM | 1:00 PM – 7:00 PM |
| 20:4 (Warrior) | 6:00 PM | 2:00 PM – 6:00 PM |
| OMAD | 5:00 PM | 4:00 PM – 5:00 PM |
| 14:10 | 9:00 PM | 11:00 AM – 9:00 PM |
| Custom (17h fast) | 10:00 PM | 3:00 PM – 10:00 PM |
Choosing a Schedule with the Intermittent Fasting Calculator
An intermittent fasting calculator gives exact clock times, but the best window is the one that fits your routine. Start with fixed points such as work hours, training and family meals, then place the window around them.
- Ease in. A shorter overnight fast of 12 or 14 hours is a gentle first step before trying 16:8 in the intermittent fasting calculator.
- Let sleep do the work. With a noon to 8 pm window, most of the 16-hour fast happens overnight.
Water, plain tea and black coffee are generally accepted during the fasting period. Enter your preferred first-meal time into the intermittent fasting calculator and it shows when the window closes, so you can plan the last meal instead of guessing.
Fasting is not suitable for everyone. Anyone who is pregnant or breastfeeding, under 18, has diabetes or takes medicine that affects blood sugar, or has a history of eating disorders should speak to a doctor before using an intermittent fasting calculator. The intermittent fasting article on Wikipedia summarises research beyond what an intermittent fasting calculator can show.
Intermittent Fasting Calculator FAQ
Is intermittent fasting safe for everyone?
No. It’s generally not advised during pregnancy or breastfeeding, for people with a history of disordered eating, or for some people with diabetes or on certain medications. This tool only plans timing — check with a doctor first if any of those apply to you.What’s the difference between 16:8 and 18:6?
The numbers describe hours of fasting versus eating. 16:8 means a 16-hour fast with an 8-hour eating window, while 18:6 stretches the fast to 18 hours and shrinks eating to 6 hours. Choosing between them is a personal preference based on how long a fast feels manageable.Does this calculator tell me how many calories to eat?
No, it only calculates the timing of your fasting and eating windows based on your last meal time and chosen method. It doesn’t set calorie or weight targets — pair it with a separate calorie calculator if you want that.What counts as “starting” my fast?
The calculator treats the time you enter as the moment you finish your last meal, since that’s when your body starts the fasting clock. Anything after that point (water, black coffee, etc., depending on your approach) happens during the fasting window.Can I use a custom fasting schedule that isn’t listed?
Yes, select Custom from the Method dropdown and enter any number of fasting hours you like, and the calculator will compute your eating window the same way it does for the preset methods.What should I do if I feel unwell while fasting?
Stop the fast and eat something. This calculator is a timing guide only, and feeling unwell, dizzy, or unusually fatigued is a signal to break the fast and, if it keeps happening, speak with a doctor about whether fasting is right for you.Related Calculators
Before starting a fasting routine, it’s worth reading the water fasting safety guide to understand who should be cautious, and checking your energy needs with the TDEE calculator so your eating window still supports your goals. If you prefer a live countdown instead of a one-time schedule, try the intermittent fasting window timer.