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Ovulation Calculator
Find your most fertile days, estimated ovulation date and next period for the next six cycles, with a calendar of your current cycle.
If you are trying to get pregnant, timing matters: you can only conceive on a few days of each cycle. This ovulation calculator estimates the day you ovulate and your fertile window — the five days before ovulation plus ovulation day — from the first day of your last period and your average cycle length. It shows a colour-coded calendar of your current cycle and a table of the next six cycles with the fertile window, ovulation day and the date to expect your next period.
If you know the length of your luteal phase (the days between ovulation and your next period), you can enter it for a more personal estimate. Everything is calculated in your browser, and nothing is stored or sent anywhere.
How to use Ovulation Calculator
- Enter the first day of your last period.
- Enter your average cycle length (from the first day of one period to the first day of the next).
- Optionally enter your luteal phase length; leave 14 if you don't know it.
- See your fertile days, ovulation date and next period, and the next six cycles.
Features
- Estimated ovulation day and six-day fertile window.
- Calendar of the current cycle: period, fertile days and ovulation.
- Table of the next six cycles.
- Cycles from 21 to 45 days and a custom luteal phase.
- Next period date and the earliest day for a pregnancy test.
- Private: runs in your browser, nothing stored or sent.
How ovulation is estimated
After ovulation the luteal phase lasts fairly consistently, usually 12 to 14 days, while the phase before it varies from woman to woman. That is why ovulation is estimated by counting back from the next period rather than forward from the last one: ovulation day = first day of your period + cycle length − luteal phase. With a 28-day cycle and a 14-day luteal phase, that is day 15 of the cycle (counting the first day of the period as day 1).
| Cycle length | Estimated ovulation (cycle day) | Fertile window (cycle days) |
|---|---|---|
| 24 days | Day 11 | Days 6–11 |
| 28 days | Day 15 | Days 10–15 |
| 32 days | Day 19 | Days 14–19 |
| 35 days | Day 22 | Days 17–22 |
Why the fertile window is six days
Sperm can survive in the body for up to five days, while an egg lives for only 12 to 24 hours after ovulation. So the days on which sex can lead to pregnancy are the five days before ovulation and ovulation day itself. The chance is highest in the two or three days up to and including ovulation, which is why those days are often called the most fertile days.
Getting a more accurate picture
Calendar estimates assume your cycles are regular, but ovulation can shift from cycle to cycle with stress, illness, travel or breastfeeding. To pinpoint your own ovulation, combine the calendar with signs from your body: ovulation predictor kits detect the hormone surge a day or two before ovulation, basal body temperature rises slightly after it, and cervical mucus becomes clear and stretchy around the fertile days. Tracking for a few cycles shows your personal pattern, including your luteal phase length.
When to talk to a doctor
Most couples conceive within a year of trying. Doctors generally suggest getting checked after 12 months of trying, or after 6 months if you are 35 or older, or sooner if your cycles are very irregular, shorter than 21 days or longer than 35 days, or if periods stop. This calculator is a planning aid, not a medical test, and it must not be used as contraception.
Frequently asked questions
When do I ovulate?
Usually about 14 days before your next period. With a 28-day cycle that is around day 15; with longer cycles, later. Enter your cycle length to see your estimate.
What are my most fertile days?
The two to three days up to and including ovulation, within a six-day fertile window.
Can I use this as birth control?
No. Cycles vary and ovulation can come early or late, so calendar estimates are not reliable as contraception.
My cycles are irregular. Does it still work?
Less well. Use your average cycle length and combine the estimate with ovulation tests or temperature tracking.
What is the luteal phase?
The days from ovulation to your next period, usually 12 to 14. If you know yours from tracking, enter it for a more accurate date.
When can I take a pregnancy test?
From the day your next period is due. Some sensitive tests work a few days earlier, but results are more reliable after a missed period.
Is my information private?
Yes. Everything is calculated in your browser and nothing is stored on a server or sent anywhere.
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