Intermittent Fasting Calculator
Find your eating and fasting windows for 16:8, 18:6, 20:4, or 14:10 intermittent fasting schedules.
This is a scheduling estimate only, not medical advice. Consult a doctor before starting intermittent fasting, especially if you have a health condition, are pregnant, or have a history of disordered eating.
How does intermittent fasting scheduling work?
Intermittent fasting splits your day into an "eating window," when you consume all your calories, and a "fasting window," when you eat nothing (though water, black coffee, and plain tea are typically allowed). The protocol name reflects the split — for example, 16:8 means fasting for 16 hours and eating within an 8-hour window each day. This calculator takes your chosen protocol and the time of your first meal, then calculates exactly when your eating window closes and when your next fasting-to-eating transition begins.
What's the difference between 16:8, 18:6, 20:4, and 14:10?
These are simply different ratios of fasting hours to eating hours in a 24-hour day. 14:10 is a gentler starting protocol popular with beginners, 16:8 is the most widely practiced and studied schedule (often called the Leangains method), 18:6 is a moderate step up for people who've adapted to 16:8, and 20:4 (sometimes called the "Warrior Diet" approach) is a more restrictive schedule with a single main eating period. Longer fasting windows mean less time to eat, so meal planning and getting adequate nutrition becomes more important as the eating window shrinks.
How do I pick my first meal time?
Most people set their first meal to match their natural hunger cues and daily schedule — for example, someone who skips breakfast and has 16:8 might set their first meal at noon and last meal by 8 PM. This calculator automatically works out your eating window end time and your fasting window based on whatever first-meal time you choose, so you can adapt the schedule around work, family meals, or workout timing rather than forcing your life around a fixed clock.
Frequently Asked Questions
What is the most popular intermittent fasting schedule?
16:8 is the most widely practiced intermittent fasting protocol, involving a 16-hour fasting window and an 8-hour eating window each day. It's often considered a good balance between effectiveness and sustainability for beginners.
What can I drink during the fasting window?
Water, black coffee, and plain unsweetened tea are generally considered acceptable during a fasting window since they contain negligible calories. Adding sugar, cream, or milk introduces calories that technically break a strict fast.
Which intermittent fasting protocol should I start with?
14:10 or 16:8 are commonly recommended starting points because they're easier to sustain than more restrictive schedules like 20:4. Many people gradually extend their fasting window over time as they adapt.
Does intermittent fasting timing matter for weight loss?
The main driver of weight loss is still overall calorie balance, but a defined eating window can make it easier for many people to naturally eat less by limiting snacking opportunities and simplifying meal planning.
Can I change my first meal time every day?
Yes, the calculator recalculates your eating and fasting windows for whatever first meal time you enter, so you can adjust daily based on your schedule, workouts, or social plans.
Is 20:4 fasting safe for beginners?
20:4 is a more advanced, restrictive protocol with only a 4-hour eating window. It's generally recommended to build up to it gradually from a less restrictive schedule like 16:8 rather than starting there directly, and to check with a doctor first.
Who should avoid intermittent fasting?
People who are pregnant or breastfeeding, have a history of disordered eating, have diabetes or blood sugar regulation issues, or are underweight should talk to a doctor before trying intermittent fasting, since prolonged fasting windows can pose risks for these groups.
What happens if I eat outside my calculated eating window?
Nothing dangerous happens immediately — intermittent fasting is a flexible lifestyle choice, not a medical requirement. Occasionally eating outside your window simply means you didn't complete that day's fasting protocol; most people just resume their normal schedule the next day.