Ovulation Calculator
Predict ovulation and your fertile days across the next several cycles.
About Ovulation Calculator
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
Worked example
With the default values below, Ovulation Calculator returns Next ovulation of Jul 15, 2025. Change any input above and every figure here is recalculated in your browser.
Inputs used
- First day of last period
- 2025-07-01
- Average cycle length (days)accepts 21–45
- 28
- Number of cycles to projectaccepts 1–6
- 3
Results
- Next ovulation
- Jul 15, 2025
- Fertile window opens
- Jul 10, 2025
- Fertile window closes
- Jul 15, 2025
- Cycle length
- 28 days
Step by step
- 1LMP= Jul 1, 2025
- 2Ovulation day of cycle= Day 14
- 3Fertile window
Assumptions and limits
- The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax or fees or charges into account.
- Figures are rounded for display to whole numbers, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
- This tool provides estimates based on average cycle patterns and is not a reliable contraceptive method. Consult a healthcare provider for fertility guidance.
- This is an estimate for general information and not professional medical advice. See the full disclaimer for the limits that apply to this kind of calculator.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
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.