How cycle counting works
A menstrual cycle is counted from the first day of one period to the first day of the next. That first day is day 1, and it is the only date this calculator needs to anchor everything else: add one full cycle length and you reach the expected start of the next period. If your last period began on September 15 and your cycle runs 28 days, the next period is expected on October 13, whether those 28 days felt long or short. Your period length, the number of days of bleeding, is counted separately: a five-day period starting October 13 ends on October 17, its fifth day.
Most cycles are not exactly 28 days. Anything from about 21 to 35 days is common in adults, and the same person often varies by a few days from month to month. For the best estimate, enter the average of your last three to six cycles rather than your single most recent one, because one unusual month, a trip, an illness or a stressful week can stretch or shorten a cycle on its own. The table below the calculator projects the same average forward over your next three cycles, so you can plan around the likely dates while remembering they are projections, not promises. Everything on this page is an estimate only, not medical advice.
The formulas this calculator uses
Ovulation is worked out backwards from the next period, not forwards from the last one. The second half of the cycle, called the luteal phase, runs from ovulation to the next period and is fairly steady at about 14 days for many people, while the first half of the cycle is the part that usually stretches or shrinks. So the day your next period is due, minus 14 days, is the best calendar estimate of ovulation. All dates are calculated in UTC so time zones never shift a result by a day.
The fertile window, explained
The fertile window is the stretch of days when pregnancy is possible. Sperm can survive in the reproductive tract for up to about five days, while an egg can be fertilised for roughly 12 to 24 hours after ovulation. Adding those two facts together gives a window of about six days: the five days before ovulation, when sperm could already be waiting, plus ovulation day itself. This calculator displays the window through one day after ovulation as a cautious outer edge, because the exact hour of ovulation cannot be known from calendar dates alone. People trying to conceive often aim for the two or three days just before ovulation and ovulation day itself, when the chances in that cycle are highest.
Worked example
Last period starts September 15, 2026, with a 28-day cycle and a 5-day period length. Next period = Sep 15 + 28 days = October 13, 2026, and it ends on day 5, October 17. Ovulation = Oct 13 − 14 days = September 29, 2026. The fertile window runs from five days before ovulation to one day after: September 24 to September 30, 2026.
How to use this calculator
- Enter the first day of your last period.
- Enter your average cycle length in days, ideally averaged over your last few cycles.
- Enter how many days your period usually lasts.
- Read your next period start date in the headline panel.
- Check the breakdown for the period end date, ovulation day and fertile window, and the table below for your next three cycles.
Why real cycles refuse to follow the calendar
Even people who describe themselves as regular ovulate a few days earlier or later in some cycles. Stress, illness, travel across time zones, significant weight change, intense training, disrupted sleep and plain natural variation can all move a cycle, and an occasional cycle may have no ovulation at all. A calendar calculator cannot see any of that: it projects your average forward and assumes the coming cycle behaves like the average. Signs from your own body in the current cycle, such as changes in cervical mucus, basal body temperature patterns and ovulation test kits, respond to what is actually happening this month and are widely used alongside calendar estimates.
One warning deserves to be plain: do not use this calculator, or any calendar method, to try to prevent pregnancy. The gap between an estimated ovulation day and the real one, combined with how long sperm survive, makes calendar-only prediction far too unreliable for contraception. Talk to a qualified health professional if your cycles are frequently shorter than 21 or longer than 35 days, if periods stop, if bleeding is very heavy or painful, or if you have been trying to conceive for a year without success, or six months if you are over 35. Those situations deserve personal assessment, not a date estimate. Estimates only, not medical advice.
Important note
For information only — not medical advice. This calculator uses standard published formulas to estimate values for general education and planning. It does not diagnose, treat, or replace the advice of a doctor or other qualified health professional. If you have a health concern, are pregnant, or are making decisions about treatment, medication, or a major diet or exercise change, please talk to a healthcare professional first.