Ovulation Calculator
Estimate your ovulation day and most fertile window from your last period and cycle length, plus when your next period is due.
Enter the first day of your last period to estimate your fertile window.
How to use this calculator
Enter the first day of your last menstrual period (LMP) and your average cycle length in days. The default is 28 days, but you can adjust this to match your pattern. The calculator then displays your estimated ovulation date, your fertile window, and the expected start date of your next period. If you are tracking multiple months, update the LMP each cycle for a fresh estimate.
How ovulation timing works
The menstrual cycle has two main phases separated by ovulation. The follicular phase runs from the first day of your period until ovulation and can vary considerably in length — from about 10 days to 21 days or more. The luteal phase runs from ovulation until your next period and is notably more consistent, typically lasting 12–16 days with an average close to 14 days.
Because the luteal phase is relatively stable, the most reliable way to estimate ovulation without physical testing is to count 14 days back from the expected start of the next period. That's what this calculator does. For a standard 28-day cycle, the next period is expected 28 days after the LMP, so ovulation is estimated around day 14. For a 35-day cycle, the next period is expected on day 35, placing ovulation around day 21.
Understanding the fertile window
An egg is only viable for fertilisation for about 12–24 hours after ovulation. However, sperm deposited in the reproductive tract can remain viable for up to five days under favourable conditions. This means that intercourse before ovulation can lead to conception if sperm are still present when the egg is released.
The combined fertile window is therefore approximately six days: the five days leading up to ovulation and the day of ovulation itself. Conception rates are highest when intercourse occurs in the two to three days immediately before ovulation.
Worked example
Suppose your last period started on May 1 and your average cycle is 30 days.
- Next expected period: May 1 + 30 days = May 31
- Estimated ovulation: May 31 − 14 days = May 17
- Fertile window: May 12 – May 17 (five days before ovulation through ovulation day)
- Peak fertile days: approximately May 15–17
How to improve accuracy
Calendar estimates like this one are a useful starting point, but they cannot account for cycle-to-cycle variation. To get a clearer picture, consider combining this estimate with one or more of the following:
- Ovulation predictor kits (OPKs): Detect the LH surge 24–36 hours before ovulation. Start testing a few days before your estimated ovulation date.
- Basal body temperature (BBT): Your resting temperature rises by about 0.2–0.5 °C (0.4–1.0 °F) after ovulation and stays elevated. Charting BBT daily can confirm that ovulation occurred, though it is a retrospective signal.
- Cervical mucus monitoring: Around ovulation, cervical mucus typically becomes clear, slippery, and stretchy — often described as resembling raw egg white. This is a real-time fertility sign.
- Cycle tracking over multiple months: Using your average cycle length from three or more recent cycles gives a more reliable estimate than a single cycle.
Common mistakes to avoid
- Assuming ovulation always occurs on day 14. Day 14 applies only to an exact 28-day cycle. If your cycle is longer or shorter, ovulation shifts accordingly.
- Relying on this calculator as contraception. Calendar methods cannot reliably prevent pregnancy. Ovulation can occur earlier or later than predicted, especially after illness, stress, or travel.
- Ignoring irregular cycles. Cycles that vary by more than a week from month to month make calendar prediction unreliable. Consult a healthcare provider if your cycles are consistently irregular.
- Testing too late with OPKs. The LH surge is brief. If you start testing too late in your cycle, you may miss it. Begin testing a few days before your estimated ovulation date.
All results are estimates for informational purposes only and are not medical advice or a reliable method of contraception. Consult a qualified healthcare provider for personalised guidance on conception or family planning.
How we calculate this
Ovulation is estimated by subtracting 14 days from the expected start of the next period (LMP + cycle length). The fertile window covers the five days before estimated ovulation through the day of ovulation. Next period date is LMP + cycle length. All results assume a consistent luteal phase of approximately 14 days and are estimates only — actual ovulation timing varies and these results are not medical advice or a reliable method of contraception.
Sources
Frequently asked questions
When do I ovulate?
Ovulation typically occurs about 14 days before your next period starts — not necessarily 14 days after your last period began. For a 28-day cycle that places ovulation around day 14, but for a 35-day cycle it falls around day 21. This calculator estimates your ovulation date by subtracting 14 days from your next expected period.
What is the fertile window?
The fertile window spans the five days before ovulation plus the day of ovulation itself — roughly six days in total. Sperm can survive in the reproductive tract for up to five days, so intercourse in the days leading up to ovulation can still result in fertilisation. The most fertile days are typically the two or three days just before ovulation.
How accurate is this calculator?
It is an estimate based on average cycle timing and the assumption that the luteal phase (after ovulation) is consistently about 14 days. Actual ovulation can vary by several days even in regular cycles. Ovulation predictor kits (OPKs), basal body temperature (BBT) charting, and cervical mucus monitoring give more individualised signals.
Can I use this calculator to avoid pregnancy?
No. Calendar-based estimates are not a reliable form of contraception. Ovulation timing varies from cycle to cycle, and no algorithm can predict it with the precision needed to prevent pregnancy. Speak with a healthcare provider about effective contraceptive options.
What if my cycles are irregular?
This calculator works best with fairly regular cycles. If your cycle length varies significantly from month to month, the estimate will be less accurate. Tracking your cycle over several months to find an average, or using physical signs like OPKs and BBT, will give a clearer picture. Irregular cycles can sometimes indicate an underlying condition worth discussing with a doctor.
How long does the luteal phase last?
The luteal phase — the time between ovulation and the start of your next period — typically lasts 12–16 days and averages about 14 days. Unlike the follicular phase (before ovulation), the luteal phase is relatively consistent for a given person, which is why subtracting 14 days from the expected next period is a reasonable ovulation estimate.
What is an ovulation predictor kit (OPK)?
An OPK detects the luteinising hormone (LH) surge that triggers ovulation roughly 24–36 hours before it occurs. A positive OPK result is one of the most reliable at-home signs of impending ovulation. Most brands recommend testing once or twice daily starting a few days before your estimated ovulation window.
Can stress or illness affect ovulation timing?
Yes. Physical illness, significant psychological stress, major weight changes, intense exercise, and certain medications can all delay or suppress ovulation in a given cycle. If ovulation is delayed, your period will be correspondingly late — the luteal phase length stays roughly constant, so any delay in ovulation pushes back the next period by the same amount.