Ovulation Calculator
Last updated: 2026-06-25
Ovulation is estimated as last period start + (average cycle − 14), and the fertile window is about 6–7 days, from 5 days before ovulation through the day after.
This estimate assumes a regular cycle and is for reference only — it should not be relied on for birth control.
Estimate ovulation from your period and cycle
Estimated ovulation day
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- Fertile window
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- Next period date
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- Luteal phase (after ovulation to period)
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Please enter a valid last period date and cycle length.
This result is a reference estimate assuming a regular cycle and cannot be used as birth control. For accurate ovulation or conception planning, consult a healthcare provider.
How to use it
- Enter last period date — select the date your most recent period started.
- Enter cycle length — enter your average cycle in days. 28 is typical; 21–35 is normal.
- View result — press Calculate to see your ovulation day, fertile window and next period as dates.
How ovulation and the fertile window are calculated
A menstrual cycle runs from the first day of one period to the day before the next. Because ovulation happens about 14 days before the next period (the luteal phase), adding (cycle − 14) days to your last period start estimates your ovulation day.
Ovulation = last period start + (cycle − 14) days
Fertile window = ovulation −5 days to ovulation +1 day
Next period = last period start + cycle length
Ovulation & fertile window by cycle (day number from period start)
| Cycle | Ovulation day | Fertile window |
|---|---|---|
| 26 days | day 12 | days 7–13 |
| 28 days | day 14 | days 9–15 |
| 30 days | day 16 | days 11–17 |
| 32 days | day 18 | days 13–19 |
Good to know
- Sperm survive about 5 days and the egg about 24 hours after ovulation, so the fertile window is roughly 6–7 days.
- Stress, sleep and weight changes can shift your actual ovulation day each month.
- For more accurate confirmation, use ovulation predictor kits (LH), basal body temperature, or ultrasound.
- This calculator adds days based on UTC midnight, so results are identical regardless of time zone.
If you've confirmed a pregnancy, use the Due Date Calculator for your due date and gestational age, or the Date Calculator for day counts.
Frequently asked questions (FAQ)
How is the ovulation day calculated?
Ovulation is estimated about 14 days before your next period — that is, last period start + (cycle length − 14). For a 28-day cycle, day 14 from the start of your last period is the estimated ovulation day.
How long is the fertile window?
Sperm can survive up to about 5 days and the egg about 24 hours after ovulation, so the fertile window is roughly 6–7 days, from 5 days before ovulation through the day after. Pregnancy is most likely during this time.
Is this accurate? Can I use it for birth control?
No. This estimate assumes a regular cycle. Real ovulation can shift with stress, health and hormones, so it should not be relied on for contraception. For accurate contraception or conception planning, consult a healthcare provider.
What if my cycle is irregular?
If your cycle varies a lot month to month, this estimate is less accurate. Averaging the last few months can help, and ovulation predictor kits (LH tests), basal body temperature tracking or ultrasound give more accurate results.
Does the time zone change the dates?
No. The calculator converts your date to UTC midnight before adding days, so it always shows the same dates regardless of your time zone or daylight saving time.
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Last updated: 2026-06-25