Ovulation Calculator
Estimate your fertile window and ovulation day from your cycle.
Reviewed by the ToolNestr Editorial Team — July 2026
Dates are not stored — everything runs in your browser.
For general information only — not medical or contraceptive advice.
How ovulation is calculated
The calculation is based on the standard 14-day luteal phase that most women experience. The luteal phase (the time between ovulation and the next period) is relatively fixed at about 14 days, regardless of total cycle length. Using this known constant, we can work backwards and forwards:
Next period date
nextPeriod = lastPeriod + cycleLength
Simply add the number of days in your average cycle to the first day of your last period.
Ovulation date
ovulation = nextPeriod − 14
Subtract 14 days (the typical luteal phase length) from your predicted next period date.
Fertile window
fertile = ovulation − 5 to ovulation + 1
Sperm can survive up to 5 days in the reproductive tract, and the egg is viable for about 24 hours after ovulation. This gives you a 6-day fertile window.
Worked example
Last period started June 1, with a 28-day cycle:
When to use this tool
The Ovulation Calculator is a quick way to estimate your fertile window. Whether you are planning a pregnancy or simply want to understand your cycle better, knowing when ovulation is likely to occur helps you make informed decisions.
Planning conception
Identify your fertile window so you can time intercourse for the days when pregnancy is most likely. The fertile window is the six days ending on ovulation day.
Tracking cycle phases
Understand where you are in your cycle at any point. Knowing which phase you are in helps explain energy levels, mood changes, and physical symptoms.
Tips for accuracy
Track multiple cycles
Your cycle length may vary from month to month. Track at least three cycles to find your average before relying on the estimates. The more data you have, the more accurate the prediction becomes.
Use ovulation test kits
Ovulation predictor kits (OPKs) detect the surge in luteinising hormone (LH) that happens 24–36 hours before ovulation. Combine this tool with OPKs around your estimated fertile window for much greater accuracy.
Cycles vary — this is an estimate
The standard 14-day luteal phase is an average. Many women have luteal phases between 12 and 16 days. If you already know your luteal phase length, you can adjust the ovulation formula accordingly for a more personalised result.
Not reliable as birth control
This calculator is intended for conception planning and cycle education only. It should never be used as a method of contraception. Sperm can survive up to 5 days, so even intercourse outside the estimated fertile window can result in pregnancy.
Related tools
Frequently asked questions
When am I most fertile?
The few days before and on ovulation — roughly the 6-day fertile window this tool shows.
How accurate is this?
It is an estimate based on average timing; ovulation tests and tracking improve accuracy.
Can I use it as birth control?
No — it is not reliable for preventing pregnancy.
What if my cycle is irregular?
The estimate is less accurate for irregular cycles. Track multiple cycles for better predictions.
Sources & references
This tool uses standard formulas and reference values from:
- • NIH / National Institutes of Health and CDC reference values and formulas. nih.gov
- • Peer-reviewed equations (e.g. Mifflin-St Jeor, Harris-Benedict) as originally published.
- • ACOG for pregnancy, ovulation and due-date methodology. acog.org
Not medical advice. Estimates only — consult a qualified healthcare professional for any personal health decision.