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Ovulation Calculator
An ovulation calculator predicts your fertile days from your cycle. Ovulation usually happens about 14 days before your next period, and you can get pregnant from the 5 days before ovulation up to the day itself. With a 28-day cycle that started on 1 March 2026, your fertile window would be 10–15 March.
How to use the fertile window calculator
- Enter your last period. Pick the first day of full bleeding from your most recent period.
- Add your cycle length. Use your average over the last 3–6 cycles if you track them. If you don’t know, 28 days is a reasonable starting point.
- Choose how far ahead. Show the next 3 or 6 cycles to plan ahead.
- Confirm with your body's signs. Use ovulation tests, cervical mucus or temperature tracking to confirm the dates the calendar predicts.
When do I ovulate?
The second half of the cycle, from ovulation to your next period (the luteal phase), is fairly constant at about 12–16 days for most people. The first half varies much more. That is why ovulation is predicted by counting back about 14 days from your next expected period, rather than counting forward from the last one.
| Cycle length | Likely ovulation day | Fertile window (cycle days) |
|---|---|---|
| 25 days | Day 12 | Days 7–12 |
| 28 days | Day 15 | Days 10–15 |
| 32 days | Day 19 | Days 14–19 |
| 35 days | Day 22 | Days 17–22 |
Day 1 is the first day of your period. With a 28-day cycle that starts on 1 March, ovulation is likely around 15 March and the next period around 29 March.
Why the fertile window is six days
Sperm can survive in the reproductive tract for up to five days, while an egg survives for only 12–24 hours after ovulation. So sex in the five days before ovulation, and on the day itself, can lead to pregnancy. The chances are highest in the three days up to and including ovulation, which the calculator marks as your most fertile days.
The NHS advises that having sex every 2 to 3 days throughout the cycle gives the best chance of conceiving, so you do not need to time it exactly.
How to read your ovulation calendar
For each cycle the calendar shows your expected period, the fertile window, the most fertile days and the estimated ovulation date. It starts from your current cycle, so you can enter a period from a couple of months ago and it will roll forward. If your next period arrives early or late, enter the new date to recalculate.
Signs of ovulation you can track
- Ovulation predictor kits detect the surge in luteinising hormone (LH) in urine 24–36 hours before ovulation.
- Cervical mucus becomes clear, slippery and stretchy, like raw egg white, in the days before ovulation.
- Basal body temperature rises by about 0.2–0.5°C after ovulation. It confirms that ovulation happened rather than predicting it.
- Some people notice mild one-sided pelvic pain (mittelschmerz) or breast tenderness.
Combining a calendar estimate with one of these signs gives a much clearer picture than either alone.
Irregular cycles and when to get advice
Calendar predictions are least reliable when cycles vary by more than a week from month to month. Common causes include polycystic ovary syndrome (PCOS), thyroid problems, stress, big changes in weight and coming off hormonal contraception. If your cycles are irregular, or you have been trying for a year without success (six months if you are 35 or older), speak to your GP or doctor.
Ovulation calculator FAQs
How accurate is an ovulation calculator?
It is a reasonable estimate for regular cycles, but the actual day of ovulation can shift by several days from cycle to cycle. Ovulation tests or cervical mucus tracking make the timing more precise.
Can I get pregnant outside my fertile window?
It is unlikely but possible, especially if your cycle is irregular or ovulation comes earlier or later than predicted.
When should I take a pregnancy test?
Most tests are reliable from the first day of a missed period. Testing earlier increases the chance of a false negative.
Can I use this to avoid pregnancy?
No. Calendar predictions are not reliable enough for contraception. Speak to a doctor, nurse or sexual health clinic about contraception options.
Do I ovulate on day 14?
Only if you have a 28-day cycle. With a 32-day cycle ovulation is more likely around day 19, and with a 25-day cycle around day 12.
Sources and methodology
Next period = last period + cycle length. Ovulation is estimated as 14 days before the next period. The fertile window runs from 5 days before ovulation to the day of ovulation; the most fertile days are the 3 days ending on ovulation. Older start dates are rolled forward by whole cycles so the calendar begins with the current cycle. All example dates were produced by this calculator.
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