Ovulation Calculator
Estimate your ovulation day, peak fertility days and full fertile window from your last period and your typical cycle length. Ovulation is counted back from your next expected period — the way fertility awareness practitioners do it — rather than assumed to fall on day 14.
Your cycle details
Your estimates
- Estimated ovulation
- Sat, Oct 10, 2026 · cycle day 15
- Fertile window (6 days)
- Mon, Oct 5, 2026 → Sun, Oct 11, 2026
- Peak fertility
- Thu, Oct 8, 2026 → Sat, Oct 10, 2026
- Best days to start LH testing
- Tue, Oct 6, 2026
- Next period expected
- Sat, Oct 24, 2026
- If conception occurs, due date
- Sat, Jul 3, 2027
Estimates only. Confirm ovulation with body signs — see below.
How the ovulation calculation works
The second half of the cycle — the luteal phase, from ovulation to the next period — is the most stable part of the cycle, typically 12–14 days. The first half varies far more. So the reliable way to estimate ovulation is to take your expected next period and count the luteal phase backwards. With a 28-day cycle and a 14-day luteal phase, ovulation lands on cycle day 15; with a 33-day cycle it lands on day 20, not day 14.
Your fertile window
Sperm can survive up to five days in fertile-quality cervical mucus, while the egg is viable for only 12–24 hours after release. That makes the fertile window roughly six days long: the five days before ovulation plus ovulation day. The two days before ovulation and ovulation day itself carry the highest chance of conception.
Confirming ovulation (fertility awareness method)
- Basal body temperature: a sustained rise of about 0.2 °C held for three consecutive days confirms ovulation has already happened.
- Cervical mucus: clear, stretchy, egg-white mucus marks the approach of ovulation; the last day of it is the peak day.
- LH tests: a positive result usually precedes ovulation by 24–36 hours.
Frequently asked questions
Can I get pregnant outside my fertile window?
It is far less likely, but irregular cycles can shift ovulation, so calendar estimates alone are not contraception.
What if my cycles are irregular?
Use your shortest and longest recent cycles to get a range, and rely on daily body signs rather than the calendar. Tracking several cycles gives a much better estimate.
What is a short luteal phase?
Repeatedly under 10 days may make implantation harder and is worth discussing with a clinician. Tracking basal temperature is how you measure it.
See your own cycle data
Harmonie learns from your own logged cycles and morning basal temperatures to confirm ovulation instead of estimating it, and tracks your luteal phase length over time.
Start tracking freeHarmonie is an educational tracking tool. It is not a medical device, it does not diagnose any condition, and it must not be used as a method of contraception or to avoid pregnancy. It is not a substitute for medical, contraceptive, or fertility advice — always speak to a qualified healthcare professional.