Intermittent Fasting Calculator for Women

The mechanism of fasting is the same for everyone — a shorter eating window helps you eat less, and that deficit drives the loss — but the arithmetic starts from a different place. The Mifflin-St Jeor equation this tool uses subtracts a fixed constant of 161 for women where it adds 5 for men, which gives a lower resting burn and therefore a lower maintenance figure for a woman of the same weight, height and age. For the worked body — a lightly active woman near 75 kg — maintenance lands around 1,990 calories, and a 16:8 window trims that to about 1,590 for a deficit near 400, projecting roughly 0.36 kg a week. Because the starting maintenance is lower, the same percentage cut produces a smaller absolute deficit than it would for a man, which is worth knowing when you compare schedules. Enter your own numbers and the calculator does the sums for you.

Because a woman's maintenance is lower to begin with, fasting windows reach the safe-intake floor sooner. This calculator sets that floor at about 1,200 calories a day — the NHS lower limit for unsupervised dieting — and flags any schedule that would push your estimated intake below it. A 16:8 window usually leaves you comfortably above the line, but the deeper windows trip it faster: 20:4 or OMAD can land a smaller woman's intake at or under 1,200. When the tool raises that flag, it isn't a verdict, just a signal to ease the window or seek guidance rather than press on. Confirm the maintenance the floor is measured against with the BMR Calculator.

Protein Still Matters

Whatever window you choose, protein does the heavy lifting of keeping the loss lean and hunger manageable. Around 1.6 grams per kilogram of bodyweight is a well-supported target for protecting muscle while dieting (Morton et al., 2018), and it matters more, not less, as the eating window narrows and there is less time to reach it. Split your daily calories into protein, carbohydrate and fat with the Macro Calculator.

With a lower maintenance and a nearer floor, the gentle end of the fasting range is often the smarter starting point for women. A 14:10 or 16:8 window creates a real, sustainable deficit while leaving plenty of room to eat well and stay clear of the minimum-intake line. There is no medal for the longest fast, and a schedule you can hold for months will always out-lose one you quit in weeks. Use the dropdown in the calculator above to compare the projected weekly loss from each window for your body, then choose the fastest one that still feels easy to live with.