What is an ovulation calculator?
An ovulation calculator estimates when your ovaries are likely to release an egg, and so which days you're most fertile. It's used by couples trying to conceive to time sex, and by anyone who wants to understand their cycle.
Give this calculator the first day of your last period and your usual cycle length. It estimates ovulation as 14 days before your next period, marks the fertile window from five days before ovulation to the day after, and repeats the calculation for your next three cycles, alongside your expected period dates.
How to use the Ovulation Calculator
1.Enter your last period
The first day of bleeding, not spotting.
2.Enter your cycle length
Days from the start of one period to the start of the next; 28 if you're not sure.
3.Read your fertile days
Three cycles of fertile windows, ovulation days and expected periods.
Example: period started 20 October 2026
A regular 28-day cycle:
- Next period
- 17 November 2026
- Likely ovulation
- 3 November (17 Nov − 14 days)
- Fertile window
- 29 October – 4 November
- Following ovulation
- 1 December 2026
Ovulation ≈ next period − 14 days
The best chances fall on 1–3 November; the window repeats every 28 days if the cycle stays regular.
The fertile window
An egg survives for about a day after ovulation, but sperm can live for up to five days, so the fertile window is the five days before ovulation plus the day itself. The chance of pregnancy is highest from sex in the two or three days before ovulation.
The calendar method assumes ovulation 14 days before your next period. Stress, illness, travel and conditions like PCOS can shift it, so this works best for regular cycles. Ovulation predictor kits detect the hormone surge a day or so before ovulation and are more precise.
Frequently asked questions
When do I ovulate in a 28-day cycle?
Around day 14, counting the first day of your period as day 1.
Can I use this to avoid pregnancy?
No. Calendar estimates aren't reliable enough for birth control; talk to a doctor about contraception.
My cycle isn't regular. Will this work?
Less well. Use your average cycle length, and consider ovulation tests; see a doctor if cycles vary by more than about 7–9 days.
What are signs of ovulation?
Clearer, stretchy cervical mucus, a small rise in resting temperature afterwards, and for some, mild one-sided pain.
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