Ovulation window estimate
Enter the first day of your last period and your usual cycle length. The result is an estimate.
General information, not medical advice. This result does not replace a doctor. If in doubt or if you have symptoms, see a health professional.
Result
Oct 4, 2026Estimated ovulation
- Ovulation likely between Oct 2, 2026 and Oct 6, 2026.
- Estimated fertile window: Sep 27, 2026 to Oct 7, 2026.
- Next period expected between Oct 16, 2026 and Oct 20, 2026.
- Estimate only. This calculation is not a method of contraception.
Inputs
- First day of last period
- 2026-09-20
- Cycle length
- 28
- Usual variation
- 2
How it is calculated
Ovulation happens about 14 days before the next period: period + (cycle − 14) days.
Result
Estimated ovulation
How it is calculated
- Ovulation happens about 14 days before the next period: period + (cycle − 14) days.
- Your usual variation widens the period: ± that many days.
- Fertile window: 5 days before ovulation (sperm survival) to 1 day after.
Example
Period on 20 September 2026, 28-day cycle ± 2: ovulation around 4 October, fertile window 27 September to 7 October.
Good to know
- The real ovulation day varies, even with regular cycles. It is never a reliable way to avoid pregnancy.
- Stress, illness, breastfeeding or some treatments change the cycle.
- For a pregnancy plan, irregular cycles or contraception, talk to a doctor or midwife.
Sources
- Wilcox A.J. et al. (1995), N Engl J Med
When to use it
- View an estimate of ovulation and the fertile window from cycle details.
- See how usual cycle length and variation widen the estimate.
Frequently asked questions
How is the ovulation date estimated?
The calculation places ovulation about 14 days before the next period. The actual date can vary, so this is only an estimate.
Can this fertile-window estimate be used as contraception?
No. A calendar calculation cannot reliably predict ovulation and is not a method of contraception.
What does the cycle-variation margin mean?
It widens the estimated period by the number of days entered, before and after the central dates.