Intermittent Fasting Calculator
Free intermittent fasting calculator. Pick a protocol (16:8, 18:6, 20:4, OMAD, or custom) and your first meal time to instantly see your eating window, fasting window, and tomorrow's schedule.
Intermittent Fasting Calculator
Fasting protocol
Eating window: 8 hours (24 minus fasting hours).
First meal time
Local clock time. The calculator does not adjust for timezones or DST.
Right now (optional)
Leave empty to skip the live status panel. Fill in to see how much time is left in your current window.
16:8, a 16-hour fast and an 8-hour eating window. The most popular beginner protocol.
Live status
Estimates only. Intermittent fasting affects people differently. Talk to a registered dietitian or your doctor before starting, especially if you are pregnant, nursing, under 18, taking medication tied to meal timing, or managing a medical condition.
How to use this intermittent fasting calculator
Pick a protocol (16:8, 18:6, 20:4, OMAD, or a custom split), set your first meal time, and the calculator returns your eating window, fasting window, and tomorrow's schedule. If you want, drop in the current time to see whether you are eating or fasting right now and how much of that window is left.
Worked example: pick 16:8 and set your first meal to 12:00. You get a noon to 8:00 PM eating window, an 8:00 PM to 12:00 PM next-day fast, and a "stop eating by 8:00 PM" cutoff. About twelve of those sixteen fasting hours land while you sleep, which is why 16:8 is the easiest protocol to start with.
Choosing your protocol: 16:8 vs 18:6 vs 20:4 vs OMAD
16:8 is the standard. Sixteen hours fasting, eight hours eating, and most of the fast happens overnight. Almost everyone who tries IF starts here. 18:6 tightens the eating window to six hours, which usually means cutting either breakfast or a late dinner. It works well once 16:8 feels like second nature.
20:4 (the Warrior protocol) leaves a four-hour eating window. Most people struggle to hit protein and micronutrient targets in that span, especially during a hard training block. OMAD (one meal a day) compresses eating into a single 60-minute window. Skip it as a starting point; treat it as an occasional choice for experienced fasters, not a default.
Tips for sticking to your eating window
Anchor the window to existing meals. If breakfast was never a meal you cared about, 16:8 is basically free. If late dinners are non-negotiable, shift your window later instead of fighting your schedule.
Hydrate aggressively during the fast. Water, black coffee, and plain tea handle most morning cravings. The water intake calculator gives you a daily target.
Front-load protein in your first meal. A protein-heavy break-fast steadies appetite for the rest of the eating window. The protein intake calculator sets the daily number.
Plan around social meals, not against them. Shift your window for a known late dinner instead of skipping it. Consistency over weeks beats a perfect week followed by three abandoned ones.
Track adherence. A calendar with a daily check beats memory. Five out of seven hits per week is the realistic target.
Pairing intermittent fasting with a 75-day challenge
IF pairs well with the structured discipline of a 75-day challenge. A typical 75 Tough or 75 Soft round already covers workouts, water, reading, and photos. Adding a daily 16:8 window as a personal rule (not an official task) gives you appetite control and natural calorie discipline without adding another thing to log. Pair the calculator with the macro calculator for what to eat inside the window, and the calorie deficit calculator if your goal is fat loss.
If you want a free way to log daily IF adherence alongside the rest of your challenge, Reset75 handles checklists, photos, and stats with no account required. Use the "Follow Diet" task or a custom task labeled "16:8" and check it off each day the window holds. Twelve weeks of steady hits will move the needle more than any single perfect day, which is what a 75-day challenge is built for.
Frequently Asked Questions
Common questions about intermittent fasting calculator
What is the 16:8 intermittent fasting method?
A 16-hour fast paired with an 8-hour eating window. Most people anchor it to a noon-to-8pm window so the bulk of the fast happens while they sleep. It is the most common starting protocol because most of the discipline takes care of itself once you skip breakfast and stop eating after dinner.
What's the best time to start your fasting window?
Pick an eating window that overlaps your sleep. The popular noon-to-8pm window puts about 12 of your 16 fasting hours in bed. Shift earlier (10am to 6pm) if your social meals are at lunch, later (1pm to 9pm) if dinners run long. Consistency matters more than the exact clock time.
Can you drink coffee, tea, or water while fasting?
Plain water, black coffee, and unsweetened tea are widely accepted as not breaking a fast. Adding cream, sugar, or sweeteners is debated and depends on whether your goal is calorie control or a stricter metabolic fast. Bone broth and electrolyte drinks fall in the same gray zone.
16:8 vs 18:6 vs 20:4 vs OMAD, which protocol should I choose?
Start at 16:8 for two to four weeks. Step up to 18:6 only if your eating window feels easy and you can still hit your protein target. 20:4 (Warrior) and OMAD compress eating too much for most people and are not a good starting point, especially during a high-training block.
How long until you see results from intermittent fasting?
Appetite and energy usually settle within two to four weeks as your body adapts to delayed meals. Visible body composition changes take longer, typically four to twelve weeks, and only if your eating window includes a modest calorie deficit. Run the calorie deficit calculator to set a target.
Is OMAD (one meal a day) safe?
OMAD may suit experienced fasters who can hit protein and micronutrient targets in a single meal. It is not appropriate during pregnancy or nursing, with a history of disordered eating, type 1 diabetes, or in adolescents. Talk to a clinician before starting, especially if you take medication tied to meal timing.
Does intermittent fasting break if I eat at the wrong time?
The window is a guideline, not a switch. Missing it by 15 to 30 minutes does not undo your progress. What matters is the weekly average: hit your window five or six days out of seven and you'll see results. Use a tracker so the misses don't quietly turn into a habit.
Can I combine intermittent fasting with a 75-day challenge?
Yes. IF stacks well on top of a 75-day or 75 Soft challenge as a personal rule rather than an official task. Use the Reset75 app's "Follow Diet" task or a custom task to mark daily adherence next to your workouts, water, reading, and progress photos.