Estimate when you’re most likely to ovulate and your most fertile days for the next three cycles. Enter the first day of your last period and your usual cycle length.
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How it works
Ovulation usually happens about 14 days before your next period, whatever your cycle length. The calculator counts forward one cycle from your last period, then back 14 days.
The fertile window is the 5 days before ovulation, the day itself and the day after. Sperm can survive for up to 5 days, but an egg lives for only about 12 to 24 hours after it’s released, so the days just before ovulation are often the most fertile.
Worked examples
A 28-day cycle
If your period started on 1 November, your next period is due around 29 November and ovulation is likely around 15 November. Your fertile window is about 10 to 16 November.
A 35-day cycle
With the same start date, ovulation moves to around 22 November, a week later. That’s why using day 14 for everyone can be misleading.
Signs of ovulation
- Cervical mucus becomes clear, slippery and stretchy, like raw egg white.
- A small rise in body temperature after ovulation, if you track it daily.
- Ovulation predictor kits detect the hormone surge 24 to 36 hours before ovulation.
If you’ve been trying for a year without success (or 6 months if you’re 36 or older), see a GP.
Frequently asked questions
When am I most fertile?
In the 2 to 3 days before ovulation and on the day itself. With a 28-day cycle, that’s usually around days 12 to 14.
Can I use this as contraception?
No. Cycles vary and ovulation can shift, so calendar methods alone aren’t reliable contraception.
What if my periods are irregular?
Calendar predictions are less accurate. Ovulation tests or tracking signs such as cervical mucus can help, and a GP can investigate irregular cycles.
How long is the fertile window?
About 6 days: the 5 days before ovulation and the day of ovulation, with a little chance the day after.
Sources
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