Cycle and fertility
Ovulation calculator
Enter the first day of your last period and your usual cycle length to see your estimated ovulation day, your fertile window and when your next period is due, with the same dates for the next two cycles. It is a calendar estimate built on averages, useful for planning and unreliable as birth control.
Enter your values above to see your result and what it means.
How the ovulation calculator works
The calculator adds your cycle length to the first day of your last period to estimate your next period, then counts back 14 days to estimate ovulation. Fourteen days is the usual assumption for the phase between ovulation and the next period, the same one behind due-date calculations. The fertile window is the 5 days before ovulation plus the day of ovulation: in the study by Wilcox, Weinberg and Baird, which measured hormones in daily urine samples to find the day of ovulation, every pregnancy came from intercourse in that six-day span. Counting the first day of your period as day 1, a 28-day cycle looks like the table below.
| Cycle day | Calendar estimate |
|---|---|
| Day 1 | Period starts |
| Days 2 to 9 | Before the fertile window |
| Days 10 to 14 | Fertile window (5 days before ovulation) |
| Day 15 | Estimated ovulation, last day of the fertile window |
| Days 16 to 28 | After ovulation |
| Day 29 | Next period expected (day 1 of the next cycle) |
When do you ovulate? Fertile days by cycle length
Because the estimate counts back from the next period, a longer cycle moves ovulation later and a shorter one moves it earlier, while the time after ovulation stays the same. Find your cycle length to see which cycle days the calculator marks as fertile.
| Cycle length | Fertile window (cycle days) | Estimated ovulation (cycle day) |
|---|---|---|
| 21 days | Days 3 to 8 | Day 8 |
| 24 days | Days 6 to 11 | Day 11 |
| 26 days | Days 8 to 13 | Day 13 |
| 28 days | Days 10 to 15 | Day 15 |
| 30 days | Days 12 to 17 | Day 17 |
| 32 days | Days 14 to 19 | Day 19 |
| 35 days | Days 17 to 22 | Day 22 |
Why a calendar can only estimate ovulation
Real cycles vary more than the arithmetic assumes. Across 612,613 cycles logged in a fertility app, the phase after ovulation averaged 12.4 days, and most lasted between 7 and 17 days, so ovulation often falls several days away from the 14-day mark. In a study of 221 women that tracked ovulation with hormone tests, only about 30% had their fertile window entirely within days 10 to 17, the range many guidelines quote, and on every day from day 6 to day 21 at least 10% of women were in their fertile window. The authors concluded that the timing of the fertile window can be highly unpredictable, even when cycles are usually regular.
An ovulation calculator is not birth control
Because ovulation can move by several days from one cycle to the next, a calendar estimate can mark days as safe when they are fertile. Use these dates for planning a pregnancy or understanding your cycle. If you want to avoid pregnancy, talk to a doctor, midwife or family planning clinic about contraception.
Frequently asked questions
How many days after my period do I ovulate?
Count from the first day of your period. In a 28-day cycle the calendar estimate is day 15, 14 days after your period started. In a 32-day cycle it is day 19, and in a 25-day cycle day 12. Real ovulation often lands a few days either side of the estimate.
How long does the fertile window last?
About six days: the 5 days before ovulation and the day of ovulation. In the Wilcox study, conception followed intercourse only within that span, with the highest chance, about 33%, on the day of ovulation.
Can I get pregnant outside my fertile window?
Pregnancy can only start in the days around ovulation, and the trouble is knowing when those days are. In the BMJ study of 696 cycles, women had at least a 10% chance of being in their fertile window on every day from day 6 to day 21, so days the calendar marks as outside the window can still be fertile.
Can I use an ovulation calculator to avoid pregnancy?
No. A calendar estimate cannot detect a cycle in which you ovulate early or late, and the fertile window is often unpredictable even in regular cycles. For contraception, speak with a doctor, midwife or family planning clinic.
What if my cycles are irregular?
Use the average of your last few cycles, and expect the estimate to be less reliable. If your cycle length often changes by more than a few days, a body sign such as basal body temperature gives a closer picture of your own timing. Very irregular or absent periods are worth discussing with a doctor.
Sources
- Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. Effects on the probability of conception, survival of the pregnancy, and sex of the baby. N Engl J Med. 1995;333(23):1517-1521.
- Wilcox AJ, Dunson D, Baird DD. The timing of the "fertile window" in the menstrual cycle: day specific estimates from a prospective study. BMJ. 2000;321(7271):1259-1262.
- Bull JR, Rowland SP, Scherwitzl EB, Scherwitzl R, Danielsson KG, Harper J. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. NPJ Digit Med. 2019;2:83.
- American College of Obstetricians and Gynecologists. Methods for Estimating the Due Date. Committee Opinion No. 700. Obstet Gynecol. 2017;129(5):e150-e154 (reaffirmed 2025).