You can only conceive on about six days of each cycle: the five days before ovulation and the day itself. This calculator estimates those days from your last period and cycle length, ranks them by the chance of conception, and tells you when a pregnancy test will be reliable.
How it is calculated
- Next period = first day of last period + cycle length.
- Ovulation = next period − 14 days. The luteal phase (ovulation to period) is close to 14 days for most people, while the first half of the cycle varies. So for a 28-day cycle ovulation is around day 14; for a 35-day cycle around day 21, not day 17.
- Fertile window = the 5 days before ovulation plus ovulation day. Sperm survive up to 5 days in fertile cervical mucus; the egg lives 12–24 hours.
- Due date if you conceive = ovulation + 266 days (38 weeks), the same as the standard 40 weeks from the last period for a 28-day cycle.
Example
Last period started June 1, cycle 30 days. Next period: July 1. Ovulation: about June 17. Fertile window: June 12–17, with the best chances on June 15–17. A reliable pregnancy test from July 1.
Which days matter most
The landmark study by Wilcox and colleagues followed 221 women trying to conceive and found the probability of conception peaks at about 30% with intercourse one to two days before ovulation, is still high on ovulation day, and drops to near zero the day after. Intercourse every day or every other day across the window gives the best odds; timing a single attempt to the exact day is not necessary.
Getting a more exact date
Calendar estimates are within a day or two for regular cycles but can be off by several days if your cycle varies. Ovulation predictor kits detect the LH surge 24–36 hours before ovulation; a rise in basal body temperature confirms it afterwards. If your cycles are irregular, the calendar gives the range and the kits give the day.
Frequently asked questions
When am I most fertile?
The two days before ovulation and ovulation day itself. For a 28-day cycle that is roughly days 12–14; for longer cycles, count back 14–16 days from your next expected period.
How many days after my period do I ovulate?
It depends on cycle length, not on the period. Ovulation happens about 14 days before the next period. With a 28-day cycle and a 5-day period, that is about 9 days after bleeding stops.
Can I ovulate right after my period?
With a short cycle (21–24 days), yes: ovulation can be as early as day 7–10, which may overlap the end of a long period. This is why "safe days" based only on the calendar are unreliable for contraception.
When should I take a pregnancy test?
From the day your period is due, about 14 days after ovulation, most tests are reliable. Testing earlier can give a faint or false negative because hCG is still rising.
How accurate is an ovulation calculator?
For regular cycles, usually within 1–2 days. For irregular cycles it can be off by a week. Ovulation test strips and basal temperature tracking are more precise if timing matters.
Sources
- Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. NEJM, 1995
- ACOG: Evaluating Infertility (patient FAQ)
- Bull JR et al. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. NPJ Digital Medicine, 2019
By Calcelate Team. Formula from the sources above.
- 2026-09-14 · Formula and texts checked