Fasting for women

How to use the women’s fasting planner

The planner builds an intermittent fasting schedule for women around the cycle phase or life stage they are in. Start by picking your situation from the five buttons, which run from a regular cycle to trying to conceive. With a cycle, you also enter two numbers. One is your usual cycle length, from 21 to 40 days. The other is today’s cycle day, counting the day your period started as day 1. Your result has the likely phase, a fasting window, three tips and the signs that mean back off.

The result changes as soon as you change the day. Check it again each week, or whenever your period starts.

How the planner places you in your cycle

The planner estimates your ovulation day as your cycle length minus 14. It counts the day before and the day after as part of ovulation. Days 1 to 5 are your period, and the final five days of the cycle are late luteal. Everything between falls into the follicular or luteal phase.

Phase	24-day cycle	28-day cycle	32-day cycle	Suggested window
Period (menstrual)	days 1 to 5	days 1 to 5	days 1 to 5	12:12 to 14:10
Follicular	days 6 to 8	days 6 to 12	days 6 to 16	16:8
Ovulation	days 9 to 11	days 13 to 15	days 17 to 19	14:10 to 16:8
Luteal	days 12 to 19	days 16 to 23	days 20 to 27	14:10
Late luteal (premenstrual)	days 20 to 24	days 24 to 28	days 28 to 32	12:12 to 13:11

Worked example: your cycles last 32 days and today is day 20. The planner puts ovulation at day 18 (32 minus 14), so days 17 to 19 count as ovulation. Day 20 is the first luteal day, and the suggested window is 14:10. If you fast from 8 pm, a 14:10 window means eating from 10 am to 8 pm.

If your period is late and the day number runs past your usual length, the planner keeps showing the gentlest window.

What the suggested windows are based on

We wrote the phase windows ourselves, as our own framework. Fasting by cycle phase has almost no research behind it, and no health body has issued advice on it. The 14-day second half is a textbook figure. Real cycles vary a lot, and most of that variation is in the first half, before ovulation. Treat the phase dates as estimates and let how you feel overrule them.

The planner keeps the longest window, 16:8, for the follicular phase and eases off towards your period. Some women find they are hungrier in the days before a period, so the late luteal window is the shortest.

Other life stages in the planner

You chose	Suggested window
On hormonal contraception	14:10 to 16:8, kept steady
Perimenopausal	12:12 to 14:10
Postmenopausal	14:10 to 16:8
Trying to conceive	12:12 or no fasting

After menopause, falling oestrogen can lead to rapid bone loss. That is why the postmenopause tips put protein and strength training ahead of window length. For your protein range in grams, use the TDEE calculator, which bases it on your weight, age and goal.

When to ease off or stop

The planner lists signs to back off with each result. A period that becomes irregular or lighter, or stops, is the main one. Periods can stop after sudden weight loss or too much exercise, and three missed periods in a row is a reason to see a doctor.

Pregnancy, breastfeeding or an age under 18 means no fasting without advice from your doctor or midwife. With a past or current eating disorder, we advise against fasting by any plan, and a doctor should be your first stop.

Related guides and tools

Our women’s guide covers what trials in women found and how needs change after 40 and 50. To turn your window into clock times, use the fasting calculator. If you are new to fasting, start with 12 hours and build up from there.