Intermittent Fasting Calculator & Live Fasting Timer

Both are daily time-restricted eating plans. On 16:8 you fast for 16 hours and eat within an 8-hour window, for example noon to 8 PM. On 18:6 you fast for 18 hours and eat within 6 hours, for example 2 PM to 8 PM. 16:8 is the most popular starting point because it mostly uses the hours you are asleep. 18:6 adds two more hours in the fat-burning stage, but it is harder to keep up, so most people build up to it gradually from 14:10 or 16:8.

In the first 0–4 hours your last meal is digested and insulin brings blood sugar back to a steady level. From about 4–12 hours insulin falls and your body uses the sugar stored in your liver (glycogen) while digestion rests. Around 12–16 hours glycogen runs low and your body switches more to burning stored fat. After about 16 hours ketone levels rise (mild ketosis) and research suggests autophagy, the body's recycling of worn-out cell parts, increases. These timings are typical averages: your meals, activity and metabolism shift them.

Pick the time your last meal usually ends, for example 8 PM. Your fast then runs until 12 noon the next day, and you eat between noon and 8 PM. Sleep covers about half the fast. Water, black coffee and plain tea are fine while fasting. If 16 hours feels hard, start with 14:10 for a week or two and move the first meal later by 30 minutes at a time.

Intermittent fasting isn't suitable for children and teenagers, during pregnancy or breastfeeding, for people with a history of eating disorders, or for anyone underweight. If you have diabetes, take medicines that lower blood sugar or blood pressure, or have another medical condition, talk to your doctor before you start. Stop and eat if you feel dizzy, faint or unwell.

First your resting energy use (BMR) is calculated with the Mifflin-St Jeor equation: 10 × weight in kg + 6.25 × height in cm − 5 × age, plus 5 for men or minus 161 for women. That is multiplied by your activity level (1.2 for sedentary up to 1.9 for extra active) to give your TDEE, the calories you use in a day. For weight loss the calculator subtracts 250, 500 or 750 kcal depending on the pace you choose, but never suggests less than 1,200 kcal a day for women or 1,500 kcal for men.

Yes. Every calculation runs in your own browser. Your age, weight, goals, fasting timer and Habit Garden are saved only in your browser's local storage on this device, so they're there when you come back. Nothing is uploaded, there are no accounts, no analytics cookies, no tracking scripts and no ads. Clearing your browser data removes everything.