Women's Health
Ovulation Calculator
Find your fertile window — the handful of days each cycle when conception is actually possible.
Ovulation Calculator
These dates are estimates from an average cycle. Real cycles vary month to month, so treat every result as approximate and confirm anything you are relying on with a midwife or doctor.
About the Ovulation Calculator
Conception is only possible on a handful of days each cycle. This calculator estimates when those days fall — your fertile window — from the first day of your last period and your usual cycle length.
It works the way clinicians do, by counting backwards. The luteal phase, from ovulation to the next period, is far more consistent than the first half of the cycle, so ovulation is estimated as roughly 14 days before your next period is due rather than 14 days after the last one started. On a 28-day cycle those give the same answer; on any other length they do not.
Three cycles are projected so you can plan ahead. You can adjust the luteal phase if you know yours differs from the 14-day default.
How to use the Ovulation Calculator
- Enter the first day of your last period.
- Enter your average cycle length.
- Adjust the luteal phase if you know yours — otherwise leave it at 14 days.
- Select Find my fertile window.
- The most fertile days are the two before ovulation plus the day itself.
How the fertile window is worked out
Next period = last period + cycle length
Ovulation = next period − luteal phase (default 14 days)
Fertile window = ovulation − 5 days … ovulation + 1 day
Sperm can survive around five days in the reproductive tract; the egg stays viable for about 24 hours.
Worked example
Last period started 3 September, cycle length 32 days.
Next period: 3 September + 32 = 5 October.
Ovulation: 5 October − 14 = 21 September — that is cycle day 19, not day 14.
Fertile window 16–22 September, ovulation around 21 September
Counting forward instead would have put ovulation on 17 September, four days early — and possibly missed the window entirely.
Why the window is about six days long
The egg itself survives only around 12 to 24 hours after release. What stretches the window is sperm survival: in fertile cervical mucus, sperm can remain viable for up to about five days.
So intercourse several days before ovulation can still result in conception, while intercourse two days after generally cannot. Research on the timing of conception consistently finds the highest chance in the two to three days immediately before ovulation and on the day itself.
| Day relative to ovulation | Relative chance of conception |
|---|---|
| 5 days before | Low but real |
| 4 days before | Rising |
| 3 days before | High |
| 2 days before | Highest |
| 1 day before | Highest |
| Ovulation day | High |
| 1 day after | Very low |
| 2+ days after | Essentially none |
Because the peak sits before ovulation, waiting until you detect ovulation often means acting too late. Guidance for couples trying to conceive is generally to have regular intercourse every two to three days throughout the cycle rather than trying to time a single day precisely.
The luteal phase (luteal phase calculator)
The luteal phase runs from ovulation to the start of your next period. It is usually 12 to 14 days and varies little for an individual, which is why it makes a reliable anchor for the calculation.
A luteal phase consistently shorter than about 10 days is sometimes called a luteal phase defect and is worth raising with a doctor if you are trying to conceive. You can measure yours if you track ovulation directly — count from your positive ovulation test to the first day of your next period — then enter it above instead of the default.
Signs that support the estimate
A calendar prediction is a starting point. Basal body temperature, a home ovulation predictor kit and cervical mucus narrow it considerably:
- Ovulation predictor kits detect the luteinising hormone surge, which precedes ovulation by roughly 24 to 36 hours.
- Cervical mucus becomes clearer, stretchier and more slippery as ovulation approaches — often described as resembling raw egg white.
- Basal body temperature rises slightly after ovulation, so it confirms what happened rather than predicting it.
- Mittelschmerz, a one-sided twinge, is noticed by some people around ovulation.
Combining a calendar estimate with one or two of these is considerably more reliable than either alone.
The cycle length this calculator depends on comes from tracking your periods — the period calculator projects those forward and gives you the average to use here. If you are already pregnant, the due date calculator works forward from conception or your last period instead.
Can you ovulate more than once in a cycle?
More than one egg can be released, but only within a window of about 24 hours — this is how non-identical twins occur. What cannot happen is a second, separate ovulation later in the same cycle, because the hormonal shift after ovulation prevents it.
What can happen, and causes real confusion, is ovulating at an unexpected time. Stress, illness or travel can delay it, which shifts the fertile window later than any calendar predicts.
Do not use this as contraception. Calendar-based prediction is one of the least reliable methods of avoiding pregnancy — cycles shift with stress, illness, travel, weight change and age, and ovulation can happen earlier or later than any average predicts. If you are trying not to conceive, talk to a doctor or sexual health service about a method with a proper effectiveness rate.
This is an estimate, not medical advice. It works from average cycle figures, and real bodies vary a great deal month to month. Nothing here replaces a consultation. For anything you are relying on — a pregnancy, fertility treatment, a health concern, or a decision about contraception — speak to a midwife, doctor or sexual health service who can assess your own situation.
Ovulation day by cycle length
Quick answers: with a 26 day cycle, ovulation day is around day 12. With a cycle of 30 days, you ovulate around day 16. With a 31 day cycle, around day 17. The second half of the cycle (the luteal phase) is fairly steady at about 14 days. So ovulation is roughly cycle length minus 14. Day 1 is the first day of your period.
| Cycle length | Likely ovulation day | Fertile window (most fertile days) |
|---|---|---|
| 21 days | Day 7 | Days 2–8 |
| 24 days | Day 10 | Days 5–11 |
| 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 |
| 34 days | Day 20 | Days 15–21 |
| 35 days | Day 21 | Days 16–22 |
| 38 days | Day 24 | Days 19–25 |
The two or three days just before ovulation, and ovulation day itself, carry the highest chance of pregnancy.
How to track ovulation with irregular periods
If your cycle length jumps around, a calendar estimate is weak. These steps help:
- Use your shortest and longest cycles. Take the last 6 cycles. Your fertile window could start as early as (shortest − 18) and end as late as (longest − 11).
- Add ovulation test strips (LH tests). They detect the hormone surge 24–36 hours before ovulation.
- Watch your body. Clear, stretchy cervical mucus and a small rise in morning temperature after ovulation both help confirm it.
Example: cycles from 27 to 35 days give a possible window from day 9 to day 24. Testing across that range is more reliable than any single calendar date. If your cycles are often shorter than 21 or longer than 35 days, talk to a doctor.
Ovulation after a miscarriage
Ovulation can return quickly — sometimes as soon as two weeks after an early pregnancy loss. Your first period may come 4 to 6 weeks later. Because the first cycle is hard to predict, ovulation tests are more useful than this calculator at first. Ask your doctor when it is safe to try again for you.
Related guides
Other ways to use this calculator
- Fertile window calculator and ovulation day predictor — the result shows both.
- Fertility calculator — the most fertile days are the two before ovulation and ovulation day.
- 35 day cycle ovulation day — around day 21.
- Ovulation after miscarriage — see the section above; cycles may take a few months to settle.
Frequently asked questions
The two to three days immediately before ovulation, plus the day of ovulation itself. On a 32-day cycle with a 14-day luteal phase, ovulation falls around cycle day 19, making days 16 to 20 the peak. Because the highest chance comes before ovulation, waiting to detect it often means acting too late — regular intercourse every two to three days is the usual advice.
About six days: the five days before ovulation plus the day itself. The egg survives only around 12 to 24 hours, so the length comes almost entirely from sperm, which can remain viable for up to five days in fertile cervical mucus. Intercourse a few days before ovulation can result in conception; two days afterwards, essentially cannot.
The luteal phase runs from ovulation to the start of your next period, usually 12 to 14 days. It matters because it stays fairly constant for an individual, while the first half of the cycle varies — which is why ovulation is estimated by counting back from the next period rather than forward from the last. A luteal phase consistently under 10 days is worth discussing with a doctor.
Not as a separate second event. More than one egg can be released within about 24 hours, which is how non-identical twins happen, but the hormonal changes after ovulation prevent another release later in the same cycle. What does happen is ovulating at an unexpected time — stress, illness or travel can delay it, shifting the fertile window away from any calendar prediction.
Usually around day 21, counting the first day of your period as day 1. Your most fertile days would be about days 16 to 22. Ovulation tests can confirm the exact day.
Use your shortest and longest recent cycles to set a wide possible window, then use LH ovulation test strips and watch for fertile cervical mucus. A calendar estimate alone is not reliable when cycle lengths vary.
Sources and further reading
- ACOG — Trying to get pregnant and fertility awareness — the fertile window and how it is identified
- NICHD — How to determine when ovulation occurs — ovulation timing, LH surge and cycle tracking
- NHS — Trying to get pregnant — guidance on timing intercourse when trying to conceive
- WHO — Family planning and contraception methods — effectiveness of fertility awareness methods
- ACOG — Early pregnancy loss — recovery and the return of ovulation