How the ovulation calculator works
Your cycle has two halves. The first, from your period to ovulation, varies in length from person to person and month to month. The second, the luteal phase from ovulation to your next period, is steadier: about 14 days. So the calculator counts back 14 days from when your next period is due.
| Cycle length | Ovulation, days after your period starts | Fertile window, days after it starts |
|---|---|---|
| 24 days | 10 | 5 to 10 |
| 26 days | 12 | 7 to 12 |
| 28 days | 14 | 9 to 14 |
| 30 days | 16 | 11 to 16 |
| 32 days | 18 | 13 to 18 |
| 35 days | 21 | 16 to 21 |
The calculator also starts from the cycle you're in now if your last period was more than a cycle ago, and shows the due date you'd have if you conceived that cycle.
The fertile window, and the best days in it
Sperm can survive in the body for several days. In a study that tracked ovulation with daily hormone tests (Wilcox and colleagues, 1995), pregnancies only began from sex in the six days ending on the day of ovulation. The chance of conceiving ranged from about 10% for sex five days before ovulation to about 33% on the day itself. The Office on Women's Health puts the most fertile time at two to three days before ovulation through 12 to 24 hours after it.
Why it's only an estimate
Fourteen days is a convention. Cleveland Clinic describes a normal luteal phase as 10 to 17 days, and one analysis of more than 600,000 app-tracked cycles found an average of 12.4 days. The days before ovulation vary even more: the Office on Women's Health says anywhere from 13 to 20, and they can change from month to month in the same person. If your cycles vary by more than a few days, treat the dates as a rough guide.
Better signs of ovulation
- Ovulation predictor kits (LH tests) detect the surge in luteinizing hormone that triggers ovulation.
- Cervical mucus becomes clear, slippery and stretchy, like raw egg white, around your most fertile days.
- Basal body temperature, taken before you get up each morning, rises slightly (about 0.4 to 0.8°F) after ovulation. It confirms ovulation has happened rather than predicting it, but over a few months it shows your pattern.
If you've been trying for a year (or six months if you're 35 or older), or your periods are very irregular, talk to your doctor. Once you're pregnant, the due date calculator takes it from there.