Ovulation Calculator
Predict ovulation and your fertile days across the next several cycles.
Inputs
Next ovulation
Jul 15, 2025
Fertile window opens
Jul 10, 2025
Fertile window closes
Jul 15, 2025
Cycle length
28days
Step by step
LMP
= Jul 1, 2025
Ovulation day of cycle
= Day 14
Fertile window
Day 9 – Day 14
Projected ovulation by cycle
| Cycle | Period starts | Ovulation | Fertile from | Fertile to |
|---|---|---|---|---|
| 1.00 | Jul 1, 2025 | Jul 15, 2025 | Jul 10, 2025 | Jul 15, 2025 |
| 2.00 | Jul 29, 2025 | Aug 12, 2025 | Aug 7, 2025 | Aug 12, 2025 |
| 3.00 | Aug 26, 2025 | Sep 9, 2025 | Sep 4, 2025 | Sep 9, 2025 |
How it works
Ovulation typically occurs 14 days before the next expected period (the luteal phase is roughly constant at 14 days). For a 28-day cycle that is day 14; for a 35-day cycle it is day 21. The fertile window runs from 5 days before ovulation through ovulation day, because sperm can survive in the reproductive tract for up to 5 days.
Formula
Ovulation day
Ovulation day = cycle length − 14
This tool provides estimates based on average cycle patterns and is not a reliable contraceptive method. Consult a healthcare provider for fertility guidance.
Frequently Asked Questions
Does ovulation always happen at the same time each cycle?
No. While the luteal phase is fairly stable, the follicular phase (and therefore ovulation timing) can vary with stress, illness, travel, and other factors. Calendar methods give a statistical estimate, not a guaranteed date.
What are the most fertile days?
The day before ovulation and ovulation day itself are the most fertile. The window extends back about 5 days because sperm can survive that long. The egg is viable for only 12–24 hours after ovulation.
How do I confirm ovulation?
Ovulation predictor kits (OPKs) detect the LH surge 12–36 hours before ovulation. Basal body temperature rises by 0.2–0.5 °C after ovulation. Transvaginal ultrasound can directly observe follicle development.