In a regular cycle, ovulation happens about 14 days before your next period, and the fertile window covers the five days before ovulation, the day itself and the day after. Enter the first day of your last period and your usual cycle length; the calculator shows your most likely ovulation day and fertile window for this cycle and the next three.
Ovulation day by cycle length
Days are counted from the first day of your period (day 1):
| Cycle length | Likely ovulation | Fertile days |
|---|---|---|
| 21 days | Day 7 | Days 2–8 |
| 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 |
The second half of the cycle, from ovulation to the next period, is fairly constant at about two weeks. It is the first half that stretches or shrinks, which is why a longer cycle moves ovulation later rather than the period earlier.
Why the window is about six days
A landmark study of couples trying to conceive (Wilcox and colleagues, 1995) found that pregnancies came from intercourse in the six days ending on the day of ovulation, with the best chances on the last two or three. Sperm can survive in the body for up to five days; the egg lives for about a day. The calculator adds the day after ovulation, because the exact day is never certain.
Planning ahead
Seeing four cycles at once helps with travel, work shifts and appointments. The later cycles assume your cycle stays the same length; if your next period arrives earlier or later, enter its first day for an updated forecast.
Signs and tests
- Ovulation test kits detect the rise in luteinizing hormone (LH) 24–36 hours before ovulation.
- Cervical mucus becomes clear and stretchy, like raw egg white, around the fertile days.
- Basal body temperature rises by about 0.2–0.5 °C (0.4–0.9 °F) after ovulation, confirming it has happened.
If you have been trying to conceive for a year (six months if you are over 35) without success, it is worth seeing a doctor. Your dates are calculated on your device and never sent anywhere, and once zet4u is saved there the calculator works offline.
Frequently asked questions
When do I ovulate?
In a regular cycle, about 14 days before your next period. In a 28-day cycle that is around day 14; in a 32-day cycle, around day 18.
How long is the fertile window?
About six days: the five days before ovulation and the day of ovulation itself, because sperm can live up to five days and the egg about one. The calculator includes the day after ovulation as a safety margin.
Can I use this to avoid pregnancy?
No. Cycles vary, and calendar estimates are not reliable contraception. Talk to a doctor about effective methods.
What if my cycles are irregular?
Calendar methods work poorly. Ovulation test kits, which detect the hormone surge before ovulation, or tracking basal body temperature give a much better picture.
Sources
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