Period calculator

Find out when your next period is due, plus your fertile window and ovulation, for the next three cycles. Free, private and no sign-up.

Your next period, worked out

Add the first day of your last period and you'll see your next three cycles: period dates, fertile window and estimated ovulation.

Estimates only, based on the calendar method. Not a method of contraception and not medical advice. Also try the ovulation calculator.

How this period calculator works

It counts forward from the first day of your last period by your cycle length to estimate when each next period starts. Day one of a cycle is the first day of bleeding.

Ovulation is estimated 14 days before each predicted period, because the stretch after ovulation (the luteal phase) is fairly steady, usually 12 to 14 days (Cleveland Clinic). The fertile window runs from five days before ovulation to one day after (Cleveland Clinic).

Example: if your last period started on 1 October and your cycle is 30 days, your next period is due around 31 October, with ovulation estimated around 16 October.

How accurate is it?

It's an estimate, and it works best when your cycles are fairly regular. Real cycles vary: in a study of more than 600,000 cycles, the average length was 29.3 days, not the textbook 28 (Bull et al., 2019).

Most of that variation comes from the first half of the cycle. In the same study the follicular phase (period to ovulation) ranged from about 10 to 30 days, while the luteal phase stayed closer to 7 to 17. So your next period is easier to predict than the exact day you ovulate.

For the best estimate, use your own average: add up the length of your last three to six cycles and divide by the number of cycles.

What counts as a normal cycle?

For adults, cycles of about 21 to 35 days are considered normal, and a period usually lasts 2 to 7 days, often around 5 (NHS, Cleveland Clinic). In the first few years after periods start, cycles can be longer or less regular, and it can take up to three years to settle.

A small change from month to month is normal. Your body isn't a calendar, and that's fine.

When to talk to a doctor

Your periods often come more than a week early or late, or your cycles regularly fall outside 21 to 35 days.
You change a pad or tampon every hour or two, pass clots the size of a coin or bigger, or bleed for more than a week (NHS: heavy periods).
You bleed between periods or after sex.
Period pain is severe and painkillers don't help (NHS: period pain).
You've missed three periods in a row (NHS: missed or late periods).

Questions people ask

How do I calculate my next period date?

Add your average cycle length to the first day of your last period. If your last period started on 1 October and your cycle is 28 days, your next period is due around 29 October. The calculator above does this for the next three cycles.

Can I use this if my periods are irregular?

Yes, but treat the dates as a rough guide. Use the average of your last few cycles. If your cycle length often changes a lot from month to month, or falls outside 21 to 35 days, it's worth asking a doctor why.

Why is my period late?

Common reasons include pregnancy, stress, sudden weight loss, being overweight, a lot of exercise, some hormonal contraception, breastfeeding, PCOS and perimenopause. If you could be pregnant, take a test. See a doctor if you miss three periods in a row.

Is this a form of birth control?

No. Calendar estimates can't tell you exactly when you ovulate, so they aren't a reliable way to prevent pregnancy. Talk to a doctor or nurse about contraception that suits you.

Do you save the dates I enter?

No. The calculator runs entirely in your browser. Nothing you enter is stored, added to the page address or sent to us, and it's gone when you close the page.

Sources

Menstrual Cycle (Normal Menstruation): Overview & Phases, Cleveland Clinic
Ovulation: Calculating, Process, Pain & Other Symptoms, Cleveland Clinic
Luteal Phase of the Menstrual Cycle: Symptoms & Length, Cleveland Clinic
Periods, NHS
Heavy periods, NHS
Period pain, NHS
Missed or late periods, NHS
Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles (Bull et al., 2019), npj Digital Medicine

Written by the Periodiva team for general information, not medical advice. If something about your cycle worries you, such as very heavy bleeding, severe pain or a missed period, talk to a doctor.