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Period Calculator & Ovulation Dates

Enter the first day of your last period and your average cycle length into this period calculator to estimate your next three periods, likely ovulation dates and a six-day fertile window for each cycle.

Last period start

Average cycle days

Calendar estimates are uncertain, particularly with irregular cycles, and must not be used as contraception. Ovulation is modeled 14 days before the next period; the window is a rough six-day estimate. No dates or notes are saved.

Choose a date to see the estimate.

How the dates are estimated

Next periodlast period start + cycle length (× 1, 2, 3 for three cycles)
Possible ovulationnext period start − 14 days
Possible fertile windowovulation − 5 days → ovulation day (6 days)

This is the calendar method. It counts backward from the next period because the phase after ovulation tends to vary less than the phase before it. A long cycle usually means later ovulation, not a longer second half. Enter your average cycle length in whole days, from 15 to 60.

Worked example

Last period started Oct 1, average cycle 28 days:

Cycle	Period	Ovulation	Fertile window
1	Oct 29	Oct 15	Oct 10–Oct 15
2	Nov 26	Nov 12	Nov 7–Nov 12
3	Dec 24	Dec 10	Dec 5–Dec 10

Ovulation is placed on cycle day 15, and the fertile window covers days 10 to 15. The three cycles are simply 28, 56 and 84 days after the start date, so any error in the average repeats and grows.

How cycle length moves ovulation

Estimated ovulation by cycle length

Cycle length	Ovulation day	Fertile window
21 days	day 8	days 3–8
24 days	day 11	days 6–11
28 days	day 15	days 10–15
32 days	day 19	days 14–19
35 days	day 22	days 17–22

Day 1 is the first day of bleeding. Day 14 is only typical for a 28-day cycle. With a 35-day cycle, the model puts ovulation a week later.

Why the window is only a guide

The NHS says ovulation usually happens 10 to 16 days before the next period, not always 14. That alone can move the window by several days.
The six-day window comes from a 1995 study by Wilcox and colleagues. They found that conceptions came from sex in the five days before ovulation or on ovulation day itself.
A 2000 follow-up by the same group found that the timing of the window varied widely, even in women with regular cycles. Only about 30% had their window entirely within days 10 to 17.
Stress, illness, travel, breastfeeding, recent hormonal contraception and the years around puberty and menopause can all shift a cycle.

Getting a better average

The estimate is only as good as the cycle length you enter. Write down the first day of bleeding each month — spotting beforehand doesn't count as day 1. After a few cycles, subtract each start date from the next to get the lengths, then average them. If they range widely, say 26 to 34 days, try the shortest and longest lengths too. The real dates will probably fall between the two results. Then reset the start date each time a new period begins.

When to talk to a clinician

This is an estimate, not medical advice, and it can't tell you whether you are fertile. The NHS says most periods come every 21 to 35 days. See a GP, nurse or sexual health clinic if your periods change or stop, if you bleed between periods, after sex or after menopause, or if you've missed three periods in a row and a pregnancy test is negative. If you're trying to conceive, ovulation test kits or tracking basal body temperature give more information than a calendar.

Sources: NHS: Periods; NHS: Periods and fertility in the menstrual cycle; Wilcox AJ, Weinberg CR, Baird DD, N Engl J Med 1995;333:1517–21; Wilcox AJ, Dunson D, Baird DD, BMJ 2000;321:1259–62. Checked 15 October 2026.

Frequently asked questions

How do I work out when my next period is due?+

Add your average cycle length to the first day of your last period. With a period that started on October 1 and a 28-day cycle, the next is estimated for October 29, then November 26 and December 24. With a 32-day cycle, the next one moves to November 2.

How do I find my average cycle length?+

Count from the first day of one period (day 1) to the day before the next period starts. Do this for several cycles — six is a good number — and take the average. Use whole days. The calculator accepts 15 to 60 days.

When do I ovulate?+

The calculator assumes ovulation about 14 days before your next period. With a 28-day cycle starting October 1, that's October 15. The NHS says ovulation usually happens 10 to 16 days before the next period, so your real date can be a few days either side.

Do I ovulate on day 14?+

Only in a textbook 28-day cycle. The time from ovulation to the next period stays fairly constant, so cycle length mostly changes when ovulation happens. Starting October 1, the calculator puts ovulation on October 15 (day 15) for a 28-day cycle, October 19 (day 19) for a 32-day cycle and October 11 (day 11) for a 24-day cycle.

What counts as day 1 of my cycle?+

The first day of real menstrual bleeding, when you need a pad, tampon or cup. Light spotting before that doesn't count. Enter that day as the last period start.

How can I tell when I actually ovulate?+

A calendar can only estimate. Ovulation predictor kits detect the rise in luteinizing hormone (LH) in urine a day or so before ovulation; basal body temperature rises slightly after ovulation; and cervical mucus often becomes clear and stretchy around it. Tracking one or more of these over a few cycles gives a better picture than dates alone.

What is the fertile window?+

The days when sex can lead to pregnancy: roughly the five days before ovulation and ovulation day itself, because sperm can survive for several days and the egg for much less. The calculator shows this six-day window — October 10 to October 15 in the 28-day example. It is a rough estimate, not a guarantee.

Can I use this calculator as birth control?+

No. A calendar estimate can be several days off, even in regular cycles, and must not be used as contraception. If you want to avoid pregnancy, talk to a doctor, nurse or sexual health clinic about reliable methods.

Does it work with irregular periods?+

Only roughly. It uses one average cycle length, so if your cycles vary by a week or more, real dates can land well outside the estimates. The NHS describes cycles from about 21 to 35 days as common. See a GP or clinician if your periods change suddenly, stop, or you bleed between periods or after sex.

How far ahead does it predict?+

Three cycles. Errors add up with each cycle, so the third prediction is the least certain. Update the start date when your next period arrives for a fresh estimate.

Are my dates saved or shared?+

No. Nothing is uploaded, and no dates or notes are stored. Everything is calculated in your browser and cleared when you close or reload the page. Download summary saves a small text file to your own device.