Ovulation Calculator — Your Fertile Window and Next Period
Enter the first day of your last period and your cycle length to see when you are most likely to ovulate, the six fertile days, and when to test.
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If you conceive this cycle, the estimated due date is —.
| Cycle | Period starts | Fertile window | Ovulation | Next period |
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An estimate from the calendar method, assuming regular cycles. Ovulation can shift with stress, illness, travel and after stopping contraception; ovulation tests and temperature tracking confirm the actual day. This is not medical advice.
Estimate your ovulation date and fertile window from the first day of your last period and your usual cycle length. The calculator shows the six days on which conception is possible, the two most fertile days, the expected date of your next period, the earliest day a home pregnancy test is reliable, and — if this cycle works — the estimated due date. A table lists the next six cycles.
It uses the luteal-phase method: ovulation is counted back from the next period, which is why a 35-day cycle ovulates around day 21, not day 14.
How to use Ovulation Calculator
- Enter the first day of your last period — the first day of full bleeding, not spotting.
- Set your cycle length: the number of days from the first day of one period to the first day of the next. Count it over a few months if you can; 28 is the average, not the rule.
- Leave the luteal phase at 14 unless you have measured yours with temperature tracking.
- Read the fertile window and plan from there. The table shows upcoming cycles.
How ovulation is estimated
The second half of the cycle — from ovulation to the next period, the luteal phase — is remarkably constant at about 14 days. The first half varies: it is what makes cycles long or short. So the reliable way to predict ovulation is to count back 14 days from when the next period is due, not forward 14 days from the last one. On a 28-day cycle those agree (day 14); on a 24-day cycle ovulation is around day 10 and on a 35-day cycle around day 21. Calculators that always say "day 14" are wrong for everyone whose cycle is not 28 days.
The fertile window
An egg survives about 24 hours after ovulation; sperm can survive up to five days in fertile cervical mucus. Conception is therefore possible from five days before ovulation to the day of ovulation itself — six days — and most likely in the two days before it. Timing intercourse every day or two across the window covers the uncertainty in the estimate.
How accurate is this?
For regular cycles, the estimate is usually within a day or two. Cycles are not clockwork, though: stress, illness, travel, weight change, breastfeeding and coming off hormonal contraception all shift ovulation, sometimes by a week. If your cycles vary by more than a few days, treat the window as a starting point and confirm ovulation directly — urine LH tests turn positive about a day before ovulation, and basal body temperature rises just after it. Cervical mucus becoming clear and stretchy is the free signal.
Testing and due dates
A home pregnancy test is reliable from the day the period is due; earlier tests can miss a pregnancy that is real. The due date shown is 40 weeks from the last period (Naegele's rule), the same estimate a clinic will start from before an ultrasound refines it.
This calculator is for planning and general information, not medical advice. For the due date once pregnant, see the due date calculator.
Frequently asked questions
How do I know my cycle length?
Count from the first day of one period to the day before the next one starts. Do it for three or four cycles and use the average; if they vary by more than a few days, the estimate here is rougher and ovulation tests are worth using.
Why does a long cycle ovulate late?
Because the phase after ovulation is fixed at about 14 days, the extra days of a long cycle all come before it. A 35-day cycle ovulates around day 21; the "day 14" rule only holds for 28-day cycles.
Can I get pregnant outside the fertile window?
Unlikely but not impossible — the window is an estimate, and ovulation can shift within a cycle. For contraception, calendar methods alone are not reliable; for conception, aim at the window but do not rely on the calendar to rule days out.
When should I take a pregnancy test?
From the day your period is due, shown as "earliest reliable test". Testing earlier often gives a false negative because the hormone level has not risen yet. If the result is negative and your period still does not come, test again a few days later.
Is this medical advice?
No. It is a calendar estimate for planning. If you have irregular cycles, have been trying for a year (six months over 35), or have symptoms that concern you, see a doctor.