Free tool
Ovulation Calculator
Estimate your fertile window and likely ovulation day from your cycle dates — and understand what the estimate can and can’t tell you.
By Clever Mum Editorial · Editorial standards
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Calendar estimates assume ovulation ~14 days before your next period, which varies between people and cycles. For accuracy, combine with ovulation tests or cervical-mucus tracking. If cycles are irregular or you've been trying for over a year (or 6 months over 35), talk to your GP. General information, not medical advice.
When do you ovulate?
The NHS says you are more likely to get pregnant around the time you ovulate, which is usually about 14 days before your next period starts. In a 28-day cycle that is around day 14; in a 32-day cycle it is closer to day 18. The calculator counts back 14 days from your expected next period.
What is the fertile window?
It is the few days when sex is most likely to lead to pregnancy: the five days before ovulation and the day of ovulation itself. Sperm can survive for several days, while an egg lasts for about a day after it is released, so the days before ovulation matter most.
| Cycle length | Likely ovulation | Most fertile days |
|---|---|---|
| 26 days | Around day 12 | Days 7 to 12 |
| 28 days | Around day 14 | Days 9 to 14 |
| 30 days | Around day 16 | Days 11 to 16 |
| 32 days | Around day 18 | Days 13 to 18 |
Day 1 is the first day of your period. Cycles vary from month to month, so treat these as estimates.
Should we only have sex on fertile days?
You do not need to. The NHS says that if you are aged 39 or under and have sex without contraception every 2 to 3 days, it is likely you will get pregnant within a year. Regular sex through the month avoids the pressure of timing and covers the days a calendar estimate might miss, especially if your cycles are irregular.
How can I tell when I am ovulating?
A calendar estimate is a starting point. Ovulation predictor kits detect the hormone surge that comes shortly before ovulation. Some people also notice clearer, slippery cervical mucus around the fertile days. If your cycles vary a lot, predictor kits are more useful than the calendar.
When should we see a GP?
Speak to a GP if you have been trying for a year or more, or sooner if you are 36 or over or already know of something that may affect fertility. Before trying, the NHS advises taking a daily folic acid supplement. Our trying to conceive guide covers preparation, and once you have a positive test the due date calculator takes over.
Ovulation Calculator: common questions
How many days after my period do I ovulate?
It depends on your cycle length, not your period length. Ovulation usually happens about 14 days before your next period, so around day 14 of a 28-day cycle and around day 18 of a 32-day cycle, counting the first day of your period as day 1.
What are my most fertile days?
The five days before ovulation and the day of ovulation. The two or three days just before ovulation are usually the most fertile, because sperm can survive for several days while an egg lasts about a day.
Is an ovulation calculator accurate?
It gives an estimate based on an average pattern. Ovulation can move from cycle to cycle, particularly with irregular periods, stress or illness. Ovulation predictor kits give a more direct signal, and having sex every 2 to 3 days avoids relying on one date.
Can I get pregnant straight after my period?
It is possible, particularly with a short cycle, because ovulation may come early and sperm can survive for several days. If you are not trying to get pregnant, use contraception throughout your cycle.
Sources & further reading
- Trying to get pregnant — NHS
- Infertility — NHS
Trying to conceive?
Our trying to conceive guide covers timing, preparation and when to see a GP, and our how long does it take article has the realistic statistics if you're feeling anxious about the wait.