How the Ovulation Calculator works
Ovulation is the release of an egg from the ovary. In most cycles it happens about 14 days before the next period starts — the second half of the cycle (the luteal phase) is fairly constant from cycle to cycle, while the first half varies more. The calculator therefore takes the first day of your last period, adds your average cycle length to find your next period, and counts back 14 days to estimate ovulation. If you know your luteal phase is longer or shorter, you can change it under “Luteal phase”.
From the ovulation date it works out your fertile window — the five days before ovulation plus ovulation day — and repeats the calculation for your next three cycles, along with the day to take a pregnancy test and the due date you would have if you conceived in that cycle.
Ovulation day by cycle length
| Cycle length | Estimated ovulation | Fertile window |
|---|---|---|
| 24 days | Day 10 | Days 5–11 |
| 26 days | Day 12 | Days 7–13 |
| 28 days | Day 14 | Days 9–15 |
| 30 days | Day 16 | Days 11–17 |
| 32 days | Day 18 | Days 13–19 |
| 35 days | Day 21 | Days 16–22 |
Day 1 is the first day of full bleeding. These are averages for a 14-day luteal phase.
Why the fertile window is six days
An egg can only be fertilised for roughly 12 to 24 hours after it is released, but sperm can survive in the reproductive tract for up to about five days. So intercourse in the five days before ovulation can still lead to pregnancy, and the chance is highest in the two to three days just before ovulation and on the day itself. Many people trying to conceive simply aim for sex every one to two days through the fertile window rather than trying to time a single day.
Signs that you are ovulating
- Cervical mucus becomes clearer, wetter and stretchy — often compared to raw egg white — in the days before ovulation.
- Ovulation predictor kits detect the surge of luteinising hormone (LH) that usually comes 24–36 hours before ovulation.
- Basal body temperature rises slightly after ovulation and stays higher until your period; charting it over a few months shows your own pattern.
- Some people notice one-sided twinges of pain (mittelschmerz), breast tenderness or a change in libido.
When cycles are irregular
If your cycle length varies by more than a few days, a calendar estimate is less reliable. Using your shortest and longest recent cycles gives a wider window, and ovulation tests become more useful. Very irregular or absent periods can have many causes, so it is worth talking to a doctor or nurse.
General guidance is to see a healthcare professional if you have been trying for a year without success (or six months if you are 35 or older), or sooner if you have irregular periods or a known health condition. Once you conceive, the Pregnancy Test Calculator shows when to test and the Pregnancy Calculator takes over from there.