Ovulation calculator
Your fertile window, likely ovulation day and next three periods, from your last period and cycle length.
An ovulation calculator estimates your likely ovulation day as cycle length minus 14 days after your last period, with a 6-day fertile window ending that day. For a 28-day cycle starting 1 January 2026, ovulation is likely on 15 January, with fertile days from 10 to 15 January.
Your fertile window
13–18 Oct
Likely ovulation: Sunday, 18 Oct 2026
- Likely ovulation
- 18 Oct
- Next period
- 1 Nov 2026
| Period starts | Fertile window | Ovulation |
|---|---|---|
| 1 Nov 2026 | 10–15 Nov 2026 | 15 Nov 2026 |
| 29 Nov 2026 | 8–13 Dec 2026 | 13 Dec 2026 |
| 27 Dec 2026 | 5–10 Jan 2027 | 10 Jan 2027 |
The fertile window is the 5 days before ovulation and ovulation day itself.
- Pregnancy dating starts
- —
- Due date
- —
Conception date = due date − 266 days. An estimate: the real date can differ.
An estimate from dates. Not a contraceptive method. Not medical advice. Nothing you type is stored.
Already pregnant? Find your due date and pregnancy week with the due date calculator.
How to use this ovulation calculator
- Type the first day of your last period as day, month and year.
- Enter your usual cycle length: the days from the first day of one period to the first day of the next. Use 28 if you're not sure.
- Read your fertile window and likely ovulation day. The table shows your next three periods and their fertile windows.
- Know your due date instead? Choose "Conception date from due date" to work backwards.
How ovulation is estimated
Ovulation day = first day of last period + (cycle length − 14) days
- Cycle length
- Days from the start of one period to the start of the next.
- 14
- The usual estimate of days from ovulation to the next period (the luteal phase).
- Fertile window
- The 5 days before ovulation plus ovulation day: 6 days in all.
- Next periods
- Last period + 1, 2 and 3 times your cycle length.
The 6-day fertile window comes from a 1995 study (Wilcox and others, New England Journal of Medicine). A 2019 study (Bull and others) found that the luteal phase varies. So the 14-day rule gives an estimate, not a certain date.
Worked example
Your last period started on 1 January 2026, and your cycle is 28 days.
- Ovulation = 1 January + (28 − 14) days = 15 January 2026.
- Fertile window = the 5 days before, plus 15 January: 10 to 15 January.
- Next periods = 1 January + 28, 56 and 84 days: 29 January, 26 February and 26 March 2026.
- Fertile window
- 10–15 Jan 2026
- Likely ovulation
- 15 Jan 2026
- Next period
- 29 Jan 2026
With a 32-day cycle starting on 10 February 2028, ovulation is likely on 28 February. The fertile window is 23 to 28 February. The next periods are 13 March, 14 April and 16 May 2028. That year has a 29 February, and the calculator counts it.
Ovulation day by cycle length
For a period starting on day 1, this is the likely ovulation day and fertile window for common cycle lengths.
| Cycle length | Likely ovulation | Fertile window |
|---|---|---|
| 24 days | Day 11 | Days 6–11 |
| 26 days | Day 13 | Days 8–13 |
| 28 days | Day 15 | Days 10–15 |
| 30 days | Day 17 | Days 12–17 |
| 32 days | Day 19 | Days 14–19 |
| 35 days | Day 22 | Days 17–22 |
Day 1 is the first day of your period. If your cycle length changes from month to month, the estimate is less reliable. Talk to a doctor if your periods are irregular.
Conception date from a due date
If you know your due date, the likely conception date is 266 days (38 weeks) before it. For a due date of 6 December 2026, that is 15 March 2026. A due date is itself an estimate, so the conception date is too.
Ovulation calculator FAQs
When do I ovulate?
A common estimate is 14 days before your next period. With a 28-day cycle, that is day 15, counting the first day of your period as day 1. With a 32-day cycle, it is day 19.
How long is the fertile window?
About 6 days: the 5 days before ovulation and ovulation day itself. This is based on a 1995 study in the New England Journal of Medicine.
Does this work with irregular periods?
Less well. The calculator assumes each cycle is the same length. If yours change a lot, the dates may be off by several days. Talk to a doctor if your periods are irregular.
Are "safe days" reliable for avoiding pregnancy?
No. This calculator is not a contraceptive method. Ovulation timing varies, so no calendar estimate can promise a safe day.
How do I find my conception date from my due date?
Take 266 days off the due date. Choose "Conception date from due date" above and type your due date to get it.
Is my data saved?
No. Everything is calculated in your browser. Nothing you type is sent to us or stored. The only place your dates go is the link you choose to copy or share.
Estimates only. Not a contraceptive method. For general information only, not medical advice. Talk to a doctor about your health.