Ovulation Calculator
Ovulation and fertile window estimates from your cycle, as a table and calendar
Runs in your browser · nothing is uploaded
Updated 2026-10-04The starting date (a period that began 10 days before today), the 28-day cycle and the 14-day luteal phase are only an example. Replace them with your own records.
The first day of your most recent period.
Days until the fertile window are counted from this date. It starts as today.
Days from the first day of one period to the first day of the next, on average. Enter 21 to 45.
Ovulation usually happens about 14 days before the next period (Korea Disease Control and Prevention Agency, understanding the normal menstrual cycle). You can change it from 10 to 16.
From 1 to 6 cycles in a row.
If your cycles vary a lot, fill in both. Leave them blank to use the average only.
Do not use this as birth control. It is an estimate from an average cycle, and your real ovulation day can differ.
Result
Enter the start of your last period and your cycle length to see ovulation and the fertile window.
This is a rough estimate for reference. It does not diagnose or treat anything, so talk to a doctor if you are worried. Do not use it as birth control. Stress, illness, sleep, childbirth and breastfeeding can all move ovulation, and with irregular cycles this prediction is not reliable. If you are planning a pregnancy or have concerns, ask a clinician.
Sources: KDCA National Health Information Portal, understanding the normal menstrual cycle, Wilcox AJ et al., N Engl J Med 1995;333:1517-1521. Ovulation about 14 days before the next period follows the KDCA guidance. Counting the fertile window from 5 days before ovulation through ovulation day follows Wilcox and colleagues. (Accessed 2026-10-04)
What it is
The ovulation calculator turns the start of your last period and your average cycle into a plan for the weeks ahead. For up to six cycles it lists the period start, the estimated ovulation day, the fertile window and the next period, and it draws a calendar for each month with those days marked. It also counts the days from the date you check, today by default, to the next fertile window. The page opens with an example, a period that began ten days ago, so you see a result at once. Change the numbers and everything recalculates. Nothing you enter leaves your browser. Do not use it as birth control.
How to use
- Enter the first day of your last period.
- Enter your average cycle length. It starts at 28 days, and you can use 21 to 45.
- Change the luteal phase if you need to. It starts at 14 days.
- Pick how many cycles to show. If your cycles vary, also enter the shortest and the longest recent cycle.
- Read the next fertile window, the cycle table and the calendar.
How it works
- Next period = period start + cycle length.
- Ovulation = next period - luteal phase. Ovulation about 14 days before the next period follows the Korea Disease Control and Prevention Agency guidance on the normal menstrual cycle.
- Fertile window = 5 days before ovulation through ovulation day, six days in all, after Wilcox AJ et al., N Engl J Med 1995;333:1517-1521.
- Irregular cycles: when the longest and shortest cycles differ by more than 7 days, a strong warning appears. The widest possible range runs from the fertile start worked out with the shortest cycle to the ovulation day worked out with the longest.
- Limits: cycle lengths 21 to 45 days, luteal phase 10 to 16 days, 1 to 6 cycles.
- Dates are plain calendar days, with no time zone or daylight saving effects.
Examples
A last period starting on 1 January 2026, a 28-day cycle and a 14-day luteal phase.
| Item | Math | Result |
|---|---|---|
| Next period | 1 Jan + 28 days | 2026-01-29 |
| Ovulation | 29 Jan - 14 days | 2026-01-15 |
| Fertile window | 5 days before to ovulation day | 2026-01-10 to 2026-01-15 |
With a 35-day cycle, ovulation falls on day 21 after the start, 2026-01-22. These dates are an example only.
Good to know
This is a rough estimate for reference. It does not diagnose or treat anything, and it must not be used as birth control. If you are worried about your health or planning a pregnancy, talk to a doctor.
FAQ
How is ovulation estimated?
The calculator takes your next expected period and subtracts the luteal phase, 14 days by default. A period starting 1 January with a 28-day cycle puts the next period on 29 January and ovulation on 15 January. You can change the luteal phase from 10 to 16 days.
How long is the fertile window?
Six days, counted from 5 days before ovulation through the day of ovulation. This follows the research of Wilcox and colleagues in the New England Journal of Medicine in 1995. The page shows no conception probabilities.
What if my cycles are irregular?
Enter your shortest and your longest recent cycle. If they differ by more than 7 days you get a strong warning that the prediction is unreliable, plus the widest possible window from those two cycle lengths. For a firmer answer, talk to a clinician.
Can I use this for birth control?
No. It is an estimate from an average cycle, real ovulation shifts with stress and health, and sperm can survive for days. If you need contraception, ask a doctor or pharmacist.
How accurate is the ovulation date?
Not very. It assumes an average cycle, and illness, sleep, childbirth and breastfeeding can all move it. Treat it as a rough guide and leave diagnosis and treatment to a doctor.
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