Ovulation calculator
Estimate ovulation, the fertile window and the next period from the first day of your last period and your cycle length, for the next 3 to 6 cycles.
Your estimated fertile days
Enter the first day of your last period and your average cycle length to estimate when you ovulate. Ovulation usually happens about 14 days before the next period starts, so the estimate is: ovulation = first day of last period + (cycle length − 14) days. For a period starting on 1 January 2026 and a 28-day cycle, estimated ovulation is 15 January, the next period is due on 29 January, and with a 32-day cycle ovulation moves to about 19 January.
The fertile window runs from 5 days before ovulation to 1 day after it (10 to 16 January in the example), because sperm can survive up to about five days and the egg for about a day after release. The two days before ovulation and ovulation day itself are the most fertile. The table repeats the estimate for the next 3 to 6 cycles, and the result also gives the earliest reliable day for a pregnancy test and the due date if you conceive this cycle.
This is a calendar estimate, not a measurement. Ovulation can move by several days from one cycle to the next, even when periods are regular, and stress, illness, travel or breastfeeding can shift it. Ovulation test strips (LH tests), basal body temperature and cervical-mucus changes show your own pattern better. This calculator is not a method of contraception. See a doctor if your cycles are shorter than 21 or longer than 35 days, vary a lot or stop, or if you have been trying to conceive for a year (six months if you are 35 or older). How we calculate this: see the formula table in the result. Last reviewed: 2026-09-21.
How to use it
- Enter the first day of your last period.
- Enter your average cycle length; if it varies, use the average of your last few cycles.
- Choose how many upcoming cycles to show (3 to 6).
- Read the estimated ovulation date, fertile window and next period, and the table of upcoming cycles; the “How we calculate this” table shows the formulas.
Frequently asked questions
How is ovulation calculated?
The phase after ovulation (the luteal phase) lasts about 14 days in most people, so ovulation is estimated at cycle length − 14 days after the first day of the period. For a 28-day cycle that is day 15 counting the first day of bleeding as day 1; for a 35-day cycle, day 22.
What is the fertile window?
The days when intercourse can lead to pregnancy: about five days before ovulation, ovulation day and the day after. The chance is highest in the two days before ovulation and on the day itself.
Can I use this calculator to avoid pregnancy?
No. Calendar estimates cannot predict ovulation reliably enough to be used as contraception, because ovulation can come earlier or later than expected. Talk to a doctor, pharmacist or family-planning service about effective methods.
My cycles are irregular. Will it work?
The estimate will be less reliable. Ovulation tests or tracking temperature and cervical mucus can help. Cycles shorter than 21 days, longer than 35 days, or that vary by more than a week or so are worth discussing with a doctor, as can be conditions such as polycystic ovary syndrome or thyroid problems.
When should I take a pregnancy test?
Most home tests are reliable from the day your next period is due, about 14 days after ovulation. Testing earlier can give a false negative; if the result is negative and your period does not come, test again a few days later.
When should we see a doctor about trying to conceive?
After 12 months of regular unprotected intercourse without pregnancy, or after 6 months if you are 35 or older. See a doctor sooner if periods are absent or very irregular, or if either partner has a known condition that affects fertility.
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