Ovulation & Fertile Window Calculator
Find your most fertile days and next period dates.
Ovulation Calculator
Predict your ovulation date, six-day fertile window, and the next three cycles from your last period.
The first day of full bleeding counts as cycle day 1.
Count from the first day of one period to the day before the next.
Usually 12–16 days. Leave at 14 if you are unsure.
Fri, 30 October 2026
Predicted ovulation — cycle day 15
- Fertile window
- 2026-10-25 → 2026-10-31
- Peak fertility
- 2026-10-29 → 2026-10-30
- Next period due
- 2026-11-13
Sperm can survive up to five days, so the fertile window opens before ovulation. The egg is viable for roughly 12–24 hours after release.
Next three cycles
This is an estimate based on calendar dates, not a medical diagnosis or a dating scan. Only about 4% of babies arrive on their estimated due date. Speak to your midwife, GP, or obstetrician about dating and any symptoms that worry you.
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What This Calculator Does
The Ovulation Calculator predicts when you are most likely to ovulate and the six-day window in which conception is possible. It works from the first day of your last period, your average cycle length, and your luteal phase, and it projects the next three cycles.
How It Works
Ovulation is estimated by counting backwards from the next expected period: ovulation day equals cycle length minus luteal phase length. In a 28-day cycle with a 14-day luteal phase, that is day 15 by obstetric counting (14 days after the first day of bleeding). The fertile window opens five days before ovulation, because sperm can survive that long, and closes about a day after it, because the egg is viable for roughly 12–24 hours.
Example
With a last period starting on 1 May 2026, a 28-day cycle, and a 14-day luteal phase, ovulation is predicted for 15 May 2026. The fertile window runs 10–16 May, the next period is due 29 May, and the following cycle ovulates around 12 June.
Formula
Ovulation = LMP + (cycle length − luteal phase)The luteal phase is usually 12–16 days and is more stable than the follicular phase.
Window = ovulation − 5 days to ovulation + 1 dayNext period = LMP + cycle lengthReading the Results
Ovulation date
The single most likely day of ovulation. Real ovulation can move by a few days from cycle to cycle, even when your cycles are regular.
Fertile window
The days on which intercourse can lead to conception: five days before ovulation through about one day after.
Peak fertility
The two days with the highest probability — the day before ovulation and ovulation day itself.
Sources and References
ACOG — Fertility awareness and ovulation timing
Sperm survive up to 5 days; the egg is viable 12–24 hours
ACOG patient education describes the fertile window as the five days before ovulation plus ovulation day.
NICE NG201 — Antenatal care
Gestational age is counted from the LMP
The same LMP convention is used here so the cycle dates line up with antenatal dating.
References identify the method or data behind this calculator. Where a standard or tariff changes, the current edition applies, and the assumptions stated above take precedence over a remembered figure.
Frequently Asked Questions
How accurate is an ovulation calculator?
For regular cycles it is typically accurate to within a few days, which is enough to time the fertile window. Irregular cycles, PCOS, thyroid conditions, and recent hormonal contraception make calendar predictions unreliable — ovulation predictor kits, basal body temperature, or cervical mucus tracking are better then.
Does everyone ovulate on day 14?
No. Day 14 is the midpoint of an idealised 28-day cycle. Ovulation timing varies with cycle length and can shift between cycles, which is why the luteal phase is used to work backwards from the next period.
When is the best time to try to conceive?
The two days before ovulation and ovulation day carry the highest chance. Intercourse every one to two days across the fertile window covers the peak without relying on a single predicted date.
Can I use this calculator to avoid pregnancy?
No. Calendar methods are not reliable contraception, because ovulation can occur earlier or later than predicted and sperm survive for days. Use a method recommended by a clinician.