How your next period is predicted
Your menstrual cycle is counted from the first day of one period to the first day of the next. Most adult cycles fall between 21 and 35 days, with 28 being a common (but by no means universal) average.
The calculator above adds your average cycle length to the first day of your last period to project the next one - and then keeps going to give you a 6-month forecast. The more accurate your average cycle length, the more accurate the predictions.
Why your period might come early or late
Cycle length naturally varies. Common reasons your period might shift by a few days:
- Stress - high cortisol delays ovulation, which delays your next period.
- Travel and time zones - circadian disruption affects hormone timing.
- Weight changes - significant gain or loss affects estrogen production.
- Intense exercise - high-volume training can suppress ovulation.
- Illness - even a bad cold can shift a cycle by a few days.
- Hormonal birth control - starting, stopping, or switching methods changes your cycle for several months.
- Perimenopause - cycles become less predictable as you approach menopause.
For a deeper dive, see our guide on why your period might be late.
Frequently asked questions
How does the period calculator work?
We take the first day of your last period and add your average cycle length to find the next period. Then we add the cycle length again to project the period after that, and so on for six cycles. The result is a rolling forecast of when you can expect your period each month.
How accurate is a period calculator?
Date-based predictions are most accurate when your cycles are regular (varying by less than 7 days). For irregular cycles or after big life changes - stress, weight changes, travel, illness, stopping birth control, postpartum, perimenopause - predictions can drift by several days. The Femora app re-learns your average each month, so its predictions sharpen over time.
Why is my period late?
Late periods can be caused by stress, intense exercise, weight changes, hormonal birth control changes, thyroid issues, PCOS, perimenopause, or pregnancy. If you're sexually active and your period is more than a week late, consider a pregnancy test. See our guide on causes of late periods for more.
What counts as a normal cycle length?
A normal adult cycle is anywhere between 21 and 35 days, measured from the first day of one period to the first day of the next. Teens often have longer, less predictable cycles for the first few years after their first period. Cycles shorter than 21 days, longer than 35 days, or that vary widely month to month are worth discussing with a doctor.
Can I predict my period without a calculator?
Yes - if you track the first day of each period for 3+ months, average the gaps between them, and add that average to your last period start date. The calculator just automates this. The more cycles you have logged, the better the prediction.
What's the difference between a period calculator and a fertility tracker?
A period calculator only predicts when your period will start. A fertility tracker (like Femora) also tells you when you're ovulating, identifies your fertile window, logs symptoms across cycles, and flags patterns over time. It's the difference between a single number and a full picture of your reproductive health.
These calculators give estimates based on cycle averages and standard formulas. They are for general information only and are not medical advice. For anything concerning your health or pregnancy, talk to a qualified healthcare provider.