Intermittent Fasting Calculator

How the Intermittent Fasting Calculator Works

The calculator turns a fasting schedule into a day you can follow: when to eat, how much to eat at each meal, and what to expect on the scale.

1. Your eating window. Pick a schedule (12:12, 14:10, 16:8, 18:6, 20:4, OMAD or your own fast length), then set either the time of your first meal or the time you want to stop eating. The window lasts 12 hours on 12:12, 10 on 14:10, 8 on 16:8, 6 on 18:6, 4 on 20:4 and 1 on OMAD. The rest of the day is your fast.
2. Your maintenance calories. It estimates the calories you burn in a day from your weight, height, age and sex with the Mifflin-St Jeor equation, then multiplies by your activity level: 1.2 for sedentary, 1.375 for light, 1.55 for moderate and 1.725 for very active.
3. Your daily target. You choose the pace: keep your weight steady, or lose 0.5, 1 or 1.5 pounds a week. Each half pound a week takes 250 calories a day off maintenance (in metric, each 0.25 kg takes 275). The schedule does not change the target. A shorter window makes eating less easier, but it does not burn extra fat on its own. The target never drops below 1,200 calories a day for women or 1,500 for men.
4. Your meals. It splits the target across one, two or three meals inside your window, with the biggest share in the last meal, and gives each meal a clock time. Protein is set at 1.6 g per kg of a healthy weight for your height, shared evenly between the meals.
5. Your fast and your projection. It lists what happens during the fast at your own clock times. Then it projects your weight week by week and recalculates your calorie burn as you get lighter, so the line flattens the way real weight loss does. Add a goal weight to see how many weeks it takes and the date you would get there.

Example: a 35-year-old man who is 5 ft 8 in, weighs 180 pounds and exercises lightly burns about 2,373 calories a day. To lose about 1 pound a week on 16:8 with a first meal at noon, he would eat between 12 PM and 8 PM and aim for 1,873 calories a day: about 840 at noon and 1,033 at 7:15 PM, with 60 g of protein in each meal. At that pace he would reach 165 pounds in about 17 weeks.

Two things to keep in mind. Early weight loss on any new diet includes some water, so the scale often drops faster in the first week or two. And every projection is an estimate: weigh yourself once a week and adjust your calories if the trend runs faster or slower than the plan.

When the plan looks right, start it as a timer in FastingOS, our free fasting app, or add the eating window to your calendar from the results.

Want protein, carb and fat targets, or separate numbers for training and rest days? Use our IF macro calculator. For a full weight-loss timeline that accounts for missed days, use the fasting weight loss calculator.

What is Intermittent Fasting?

Intermittent fasting (IF) is an eating pattern that cycles between eating and fasting. It doesn’t tell you which foods to eat. It tells you when to eat. On 16:8, for example, you fast for 16 hours and eat within the other 8 hours of the day. Other approaches, such as the 5:2 diet or alternate-day fasting, restrict calories on certain days instead.

People have fasted for religious and cultural reasons for thousands of years, and for most of human history food wasn’t available around the clock.

Which Intermittent Fasting Schedule Should You Choose?

If you are new to fasting, start with 12 or 14 hours overnight and move to 16:8 once that feels easy. Your age matters too: see our intermittent fasting by age chart. To compare all nine schedules side by side, from 12:12 up to a 48-hour fast, see our fasting protocols.

Intermittent Fasting Benefits & Research

Intermittent fasting works well for weight loss, mainly because a shorter eating window helps many people eat less. In head-to-head trials, people on intermittent fasting tend to lose about the same weight as people on a regular calorie-controlled diet. Pick the approach you can stick to.

Benefits that studies have found or are still testing include:

• Weight and fat loss
• Better insulin sensitivity and lower blood sugar
• Lower blood pressure and blood fats in some people
• Less time spent planning and preparing meals

After about 12 hours without food, the body has used much of its stored glucose and starts burning more fat and making ketones. Researchers are studying whether this “metabolic switch” protects the brain and slows ageing, but most of that evidence still comes from animals. (1)

Growth hormone also rises during a fast, which may help the body spare muscle while it burns fat. (2, 3) That does not make fasting a shortcut for building muscle; total protein and training still matter most.

Intermittent Fasting Concerns

Intermittent fasting isn’t for everyone. Many people find it hard to keep up, and some overeat in their eating window.

“It’s human nature for people to want to reward themselves after doing very hard work, such as exercise or fasting for a long period of time, so there is a danger of indulging in unhealthy dietary habits on non-fasting days,”

says Dr. Frank Hu, chair of the department of nutrition at the Harvard T.H. Chan School of Public Health. (4)

Ease into it. Jumping straight to a long fast often brings headaches, hunger and low energy in the first week. Start with a shorter window, drink plenty of water, and eat mostly whole foods with enough protein in your eating window.

Who Should Not Do Intermittent Fasting?

Talk to your doctor before you start if you:

• Have diabetes, or take medication that lowers blood sugar or blood pressure
• Have heart or kidney disease or another serious health condition
• Are pregnant, breastfeeding or trying to conceive (5)
• Are under 18 or underweight
• Have, or have had, an eating disorder

What Can I Consume While Fasting?

• Drinks with no calories: water, black coffee, plain tea
• Supplements that have no calories and don’t need to be taken with food. Check what else breaks a fast.

Can I Exercise While Fasting?

Yes. Light cardio and weight training while fasted are fine for most healthy people. Fasted cardio burns more fat during the session, but over a whole day it doesn’t lead to more fat loss than training after a meal, so train when you perform best. If you lift hard, many people prefer to train late in their eating window so they can eat afterwards. Training in the morning before your first meal? Here is how to set up intermittent fasting with morning workouts.

Will I Lose Muscle With Fasting?

No more than on any other diet that puts you in a calorie deficit. Losing weight always costs some muscle, but eating enough protein in your eating window and lifting weights keep the loss small.

Intermittent Fasting Can Be A Viable Approach

Intermittent fasting works for many people, mostly because it makes eating less simpler. Use the calculator to see your eating window and calorie target, weigh yourself once a week, and adjust if the scale moves faster or slower than expected.

Frequently Asked Questions

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