Ovulation Calculator — predict your fertile window and peak ovulation day based on your last period and cycle length. Ovulation occurs approximately 14 days before your next period. The fertile window is the 5 days before ovulation plus ovulation day itself — the 6 highest-probability days for conception.
Ovulation Calculator
Find your fertile window and peak fertility days. Forecast up to 6 cycles ahead for family planning.
How Ovulation Is Predicted
Unlike the follicular phase (which varies with cycle length), the luteal phase — the time between ovulation and the next period — is consistently about 14 days for most women. This means ovulation always occurs approximately 14 days before your next period, regardless of total cycle length.
| Cycle Length | Ovulation Day | Fertile Window (cycle days) |
|---|---|---|
| 21 days (short) | Day 7 | Days 2–8 |
| 28 days (average) | Day 14 | Days 9–15 |
| 32 days | Day 18 | Days 13–19 |
| 35 days (long) | Day 21 | Days 16–22 |
Worked example: Last period started June 1, 2026. Cycle length: 28 days. Ovulation = June 1 + 14 = June 15. Fertile window: June 10–16. Peak fertile days: June 13–15. Next period expected: June 29.
🩺 Fertility Expert Insight
Calendar Prediction Is an Estimate — Here's How to Confirm: This calculator predicts your most likely ovulation window based on cycle length, but it cannot account for cycle variability. For the most reliable results, combine calendar tracking with LH ovulation predictor kits (OPKs) — these detect the LH hormone surge that triggers ovulation 24–36 hours in advance. The best approach is to start OPK testing a few days before your predicted fertile window, have intercourse every 1–2 days during the window, and consider BBT charting to confirm ovulation occurred. Women with irregular cycles (varying more than 7 days) benefit most from OPK testing rather than calendar prediction alone.
Ovulation & Fertile Window Calculator
21–45 days; avg 28
Typically 3–7 days
Ovulation Calculator FAQ
What is the fertile window?
The fertile window is the 6 days with the highest chance of conception: 5 days before ovulation plus ovulation day itself. Sperm survive 3–5 days in the reproductive tract, while the egg survives only 12–24 hours. Intercourse 1–2 days before ovulation typically gives the best odds.
How can I confirm ovulation?
Ovulation predictor kits (OPKs) detect the LH surge 24–36 hours before ovulation — the most practical at-home method. Basal body temperature (BBT) charting shows a 0.2–0.5°F rise after ovulation. Clear, stretchy cervical mucus ("egg white") is a physical sign near ovulation. Ultrasound monitoring by a doctor is definitive.
Does this work for irregular cycles?
Calendar prediction is less reliable for cycles that vary by more than 7 days month-to-month. Women with PCOS, thyroid issues, or perimenopause often have irregular cycles where OPK testing or BBT charting is more accurate than calendar-based prediction.
What is the luteal phase?
The luteal phase is the period between ovulation and the next menstrual period — typically 12–16 days (usually 14). Unlike the follicular phase, it's consistent across women with different cycle lengths. A luteal phase shorter than 10 days may indicate a luteal phase defect that can interfere with implantation.
How long does the egg survive after ovulation?
An egg survives only 12–24 hours after release. This is why the fertile window extends 5 days before ovulation — sperm can be waiting. Intercourse 1–2 days before the predicted ovulation date typically offers the highest conception probability.