Find out when you ovulate and which days are most fertile. Enter the first day of your last period and your cycle length to get an estimated ovulation date and fertile window, useful when you’re trying to conceive.
Enter the first day of your last period and your usual cycle length (the number of days from one period’s start to the next — typically 21–35). The fertile window is the days you’re most likely to conceive.
Estimates only, not medical advice or contraception. Cycles vary month to month; ovulation predictor kits and tracking with your doctor give more precise timing.
When Do I Ovulate on a 28 Day Cycle?
On a 28-day cycle, ovulation is estimated at about 14 days after your period starts, because it is assumed to happen 14 days before the next one. A Jan 1 start points to Jan 15, with a fertile window of Jan 10 to Jan 16.
| Cycle length | Ovulation (days after period starts) | Fertile window (days after period starts) |
|---|---|---|
| 24 days | 10 | 5 to 11 |
| 26 days | 12 | 7 to 13 |
| 28 days | 14 | 9 to 15 |
| 30 days | 16 | 11 to 17 |
| 32 days | 18 | 13 to 19 |
| 35 days | 21 | 16 to 22 |
How Many Days After Your Period Do You Ovulate?
Roughly your cycle length minus 14, counted from the first day of your period. The tool adds that many days to your start date, then marks the 5 days before and the day after as fertile.
- Take your average cycle length, say 30 days.
- Subtract 14: 30 − 14 = 16.
- Add 16 days to your period start: Feb 1 becomes Feb 17.
- Count 5 days back and 1 day forward for the fertile window: Feb 12 to Feb 18.
Formula: Ovulation day (counted from last period start) = cycle length − 14. Estimated ovulation date = last period start + (cycle length − 14) days. Fertile window = last period start + (cycle length − 14 − 5) days through last period start + (cycle length − 14 + 1) days. Estimated next period = last period start + cycle length. Estimated due date if conceived = ovulation date + 266 days.
- Last period start: the first day of your most recent period.
- Average cycle length: days from the start of one period to the next, a whole number from 20 to 45 (no default, so enter your own).
Example Scenarios
| Last period start | Cycle length | Estimated ovulation | Fertile window |
|---|---|---|---|
| Jan 1, 2026 | 28 days | Jan 15, 2026 | Jan 10 to Jan 16, 2026 |
| Jan 1, 2026 | 32 days | Jan 19, 2026 | Jan 14 to Jan 20, 2026 |
| Feb 1, 2026 | 30 days | Feb 17, 2026 | Feb 12 to Feb 18, 2026 |
| Mar 1, 2026 | 24 days | Mar 11, 2026 | Mar 6 to Mar 12, 2026 |
| Nov 1, 2026 | 26 days | Nov 13, 2026 | Nov 8 to Nov 14, 2026 |
The ovulation window on a 32 day cycle runs 13 to 19 days after your period starts.
Fertile Window vs Ovulation, and When the Estimate Slips
Ovulation is one day, while the fertile window covers about six, because sperm can survive several days. Real ovulation can also shift by several days from month to month.
- Use your average cycle length over several months.
- Ovulation test strips detect the LH surge, and fertile cervical mucus becomes clear and stretchy.
- Basal body temperature rises after ovulation, so it confirms rather than predicts it.
If your cycles are irregular, very long or absent, this ovulation calculator is much less reliable, so talk to a doctor. Seek advice too after a year of trying, or six months if you are 35 or older. It is not a form of contraception. This guide to the menstrual cycle explains the hormone changes behind the dates.
Common questions about ovulation timing
When do I ovulate on a 28 day cycle?
The estimate is about 14 days after your period starts, since ovulation is assumed to happen 14 days before the next period. With a period starting Jan 1, that points to Jan 15.What is the ovulation date on a 30 day cycle?
Subtract 14 from 30: ovulation falls about 16 days after your period starts, with a fertile window from 11 to 17 days after. A Feb 1 start gives an estimate of Feb 17.How many days after your period do you ovulate?
Counting from the first day of your period, it is roughly your cycle length minus 14. That is 10 days on a 24-day cycle and 21 days on a 35-day cycle.What is the fertile window vs ovulation?
Ovulation is the single day an egg is released, and the egg lasts about a day. The fertile window is wider, the 5 days before ovulation plus the day after, because sperm can survive several days waiting for an egg.Can I use ovulation timing like this to avoid pregnancy?
No. This tool is not designed or validated as contraception, and ovulation shifts from cycle to cycle. Speak with a healthcare professional about reliable contraception options.My cycles are irregular. How reliable is this estimate?
Less reliable, since the calculation depends entirely on your average cycle length. Ovulation test kits, basal body temperature or cervical mucus changes, or a doctor’s help, can give a clearer picture.Related Calculators
If your fertile window has passed and you’re now wondering about a possible pregnancy, the due date calculator for estimating from your last period or conception and the conception date calculator can help you work out the timeline. If you’re watching for very early signs, the early pregnancy symptoms guide covers what people commonly notice in the first weeks.