Ovulation Calculator — Find Your Fertile Window
Enter your last period date, cycle length, and luteal phase to find your ovulation day and fertile window instantly.
Enter the first day of your last period to see your fertile window.
These are estimates, not medical advice — consult your healthcare provider.
3-cycle projection
| Cycle | Ovulation | Fertile window | Period |
|---|
What is ovulation calculator?
The ovulation calculator estimates your fertile window using the luteal-phase-anchored model: ovulation is placed at your next expected period minus your luteal phase length. For a 28-day cycle with a 14-day luteal phase, that puts ovulation on day 14 — exactly 14 days before the next period. The 6-day fertile window then runs from 5 days before ovulation through ovulation day itself, reflecting the 5-day lifespan of sperm and the short viability window of a released egg.
The fertile window exists because sperm deposited before ovulation can remain viable and available when the egg is released. The egg itself is viable for only about 12–24 hours after ovulation, making the pre-ovulation days the highest-probability time for conception. The calculator’s projection table shows three consecutive cycles so you can plan ahead — each cycle simply starts on the previous cycle’s next-period date and applies the same cycle length and luteal phase.
For irregular cycles, these dates are estimates rather than certainties. Cycle length variation directly shifts the ovulation window, sometimes by several days in either direction. Ovulation predictor kits, basal body temperature charting, or cervical mucus monitoring can provide additional real-time confirmation of when ovulation actually occurs. All results from this calculator are for informational purposes only and are not a substitute for medical advice from a qualified healthcare provider.
When to use a ovulation calculator
- Find the fertile window for conception — The fertile window is the 6-day period ending on ovulation day — the 5 days before ovulation plus ovulation day itself. Entering your last period date and cycle length instantly shows the days when conception is most likely.
- Estimate your ovulation day — Ovulation is estimated by subtracting the luteal phase length from your next expected period. For a standard 28-day cycle with a 14-day luteal phase, ovulation falls on day 14 of the cycle. Adjust cycle length and luteal phase to match your own pattern.
- Predict your next period — The calculator shows the estimated start of your next period as LMP plus your cycle length, giving you a simple way to track when your next cycle begins.
- Plan ahead with a 3-cycle projection — The projection table shows the ovulation day, fertile window, and expected period date for the current cycle and the two following cycles — useful for planning around travel, appointments, or conception timing.
How to use the Ovulation Calculator — Find Your Fertile Window
- Enter the first day of your last period — Type or pick the date your most recent period began. This is your LMP (last menstrual period) and is the anchor for all calculations.
- Set your average cycle length (and luteal phase if known) — Cycle length defaults to 28 days (valid range 20–45). If you track your luteal phase, adjust the luteal phase field (default 14 days, range 10–16). The calculator clamps out-of-range values and notes any adjustments.
- Read your ovulation day, fertile window, and projections — Your estimated ovulation date (with day of week), fertile window, and next period appear instantly in the results panel. The 3-cycle projection table below shows the same data for the next two cycles. Use 'Copy summary' to save the result to your clipboard.
Worked examples
LMP January 1, 2026 — standard 28-day cycle
Input: LMP: 2026-01-01 / Cycle: 28 days / Luteal: 14 days
Output: Ovulation: January 15, 2026 (Thursday) / Fertile window: January 10–15, 2026 / Next period: January 29, 2026Ovulation = next period (Jan 29) − luteal phase (14 days) = Jan 15. Fertile window = 5 days before ovulation through ovulation day.
LMP January 1, 2026 — longer 32-day cycle
Input: LMP: 2026-01-01 / Cycle: 32 days / Luteal: 14 days
Output: Ovulation: January 19, 2026 / Fertile window: January 14–19, 2026 / Next period: February 2, 2026A longer cycle pushes ovulation later. Next period = Jan 1 + 32 = Feb 2; ovulation = Feb 2 − 14 = Jan 19.
LMP January 1, 2026 — short luteal phase (11 days)
Input: LMP: 2026-01-01 / Cycle: 28 days / Luteal: 11 days
Output: Ovulation: January 18, 2026 / Fertile window: January 13–18, 2026 / Next period: January 29, 2026A shorter luteal phase moves ovulation later in the cycle even when cycle length stays the same.