An ovulation calculator that instantly shows your fertile window, ovulation date, next period, and estimated due date for up to six future cycles — all from just two inputs. It is useful whether you are trying to conceive, avoiding pregnancy naturally, or simply want to understand your cycle better.
How it works
Every menstrual cycle has two phases separated by ovulation. The follicular phase runs from the first day of your period to ovulation; its length varies cycle to cycle. The luteal phase runs from ovulation to the day before your next period; it is remarkably stable, almost always 12–16 days.
The calculator uses this physiology directly:
Ovulation day = LMP + (cycle length − luteal phase)
With a 28-day cycle and the standard 14-day luteal phase, ovulation falls on cycle day 14. With a 35-day cycle it falls on day 21. With a 24-day cycle it falls on day 10.
The fertile window is the six-day span ending on ovulation day: five days before, plus ovulation itself. Those five preceding days matter because sperm survive in the fallopian tubes for 3–5 days, meaning intercourse several days before ovulation can still lead to fertilisation when the egg is released. The egg itself is viable for only about 12–24 hours post-ovulation, so the window closes quickly afterwards.
The estimated due date uses the standard gestational length of 266 days from conception, equivalent to Naegele’s rule (LMP + 280 days) adjusted for your cycle. It is written as:
Due date = ovulation date + 266 days
All computation runs locally in your browser using JavaScript date arithmetic — no data ever leaves your device.
Worked example
Suppose your last period started on 1 June 2026, your cycle is 32 days, and your luteal phase is 13 days:
- Ovulation day = 1 June + (32 − 13) = 1 June + 19 = 20 June 2026
- Fertile window = 15 June – 20 June (six days)
- Next period = 1 June + 32 = 3 July 2026
- Estimated due date (if conceived) = 20 June + 266 = 12 March 2027
Compare that to a standard 28-day cycle starting 1 June:
| Cycle length | Luteal phase | Ovulation | Fertile window |
|---|---|---|---|
| 28 days | 14 days | 15 June | 10–15 June |
| 32 days | 13 days | 20 June | 15–20 June |
| 24 days | 14 days | 10 June | 5–10 June |
The table shows why cycle length matters: a 32-day cycle delays ovulation by nearly a week compared with a 28-day cycle. Using 28 days when your average is 32 would cause you to miss your window entirely.
Improving accuracy
The calendar method works best when your cycles are regular. To sharpen the estimate, track two or three additional signals:
- Basal body temperature (BBT): your resting temperature rises 0.2–0.5 °C after ovulation and stays elevated through the luteal phase. Charting it confirms ovulation has occurred.
- Ovulation predictor kits (OPKs): detect the LH surge 24–36 hours before ovulation, giving a near-real-time alert.
- Cervical mucus: becomes clear and stretchy (“egg white”) in the days just before ovulation.
Using any of these alongside this calculator gives you a much more complete picture.
Important: This tool is for educational and informational purposes only. It is not medical advice and cannot replace clinical fertility assessment, ovulation monitoring, or guidance from a qualified healthcare professional.