Online Tool Store
Ovulation Calculator
Estimate ovulation day, fertile window, and next period from your last period date and average cycle length, privately in your browser.
🔒 This tool runs entirely in your browser. Your files are never uploaded to a server.
Example shown - replace it with your own dates. This is an estimate, not medical advice or contraception.
How it works
- Enter the first day of your most recent period.
- Enter your average cycle length, counted from one period's first day to the next.
- Review the estimated ovulation date, fertile window, and next period.
The estimate
The next period is estimated by adding the cycle length to the last period date. Ovulation is then estimated 14 days before that next period:
ovulation = last period + cycle length - 14 days
The fertile window shown starts five days before that estimate and ends one day after it. Biology varies, so treat every result as an educational estimate.
FAQ
How accurate is an ovulation calculator?
It is a calendar estimate based on an average cycle. Ovulation can vary from cycle to cycle, especially when cycles are irregular.
What is the fertile window?
The estimate covers the five days before ovulation, the estimated ovulation day, and the following day because sperm and egg survival create a multi-day window.
Can I use this to avoid pregnancy?
No. A calendar estimate is not reliable contraception. Use a medically accepted contraceptive method and consult a qualified clinician for personal guidance.
Is my cycle information saved?
No. The dates and cycle length are calculated only in this browser tab and are not stored or uploaded.
How we compare
| Feature | Online Tool Store | Period-tracking app | Paper calendar |
|---|---|---|---|
| No account or install | Yes | Usually needs an app | Yes |
| Instant fertile-window dates | Yes | Yes | Manual counting |
| Long-term symptom tracking | No | Usually included | Manual notes |
| Data stays in this tab | Yes | Depends on the app | Yes |
For a quick private calendar estimate without creating a health profile, Ovulation Calculator is the simplest option; use a clinical method when accuracy affects medical or contraceptive decisions.