Health calculator
Ovulation Calculator
Ovulation day, fertile window, and next period dates.
Quick answer
How do I calculate when I ovulate?
Ovulation usually happens about 14 days before your next period. With a 28-day cycle that started January 1, ovulation is about January 15 and your most fertile days are January 10–16. Your next period would be January 29.
Next period = last period + cycle length. Ovulation ≈ next period − 14 days. Fertile window = 5 days before ovulation through 1 day after.
⚡ Answer updates as you type · Enter = calculate · Esc = reset
- Next period = last period + 28 days = May 20, 2026.
- Ovulation ≈ 14 days before the next period = day 14 of your cycle = May 6, 2026.
- Fertile window = the 5 days before ovulation plus ovulation day and the day after (sperm can live up to 5 days).
- Upcoming periods: May 20, 2026 • June 17, 2026 • July 15, 2026.
- This is an estimate — cycles vary. It is not birth control. Talk to your doctor for medical advice.
Already pregnant? Try the Pregnancy Due Date Calculator.
Estimate your ovulation day, most fertile window, next period, and when to take a pregnancy test from your last period and cycle length.
About this Ovulation Calculator
The Ovulation & Period Calculator estimates your ovulation day, fertile window, next period, and the earliest day to take a pregnancy test from the first day of your last period and your average cycle length. This is an estimate, not birth control or medical advice.
What you'll need
- • First day of your last period
- • Average cycle length (21–45 days)
How it works
- 1. Pick the first day of your last period.
- 2. Enter your usual cycle length.
- 3. Read your ovulation day and fertile window.
- 4. See your next three period dates.
- 5. Know the earliest pregnancy test date.
Worked example
A 32-day cycle starting March 3: ovulation ≈ day 18 (March 21), fertile window March 16–22, next period April 4.
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Ovulation Calculator FAQ
What is the Ovulation Calculator?
The Ovulation Calculator is a free online tool that helps you ovulation day, fertile window, and next period dates.
How do I use the Ovulation Calculator?
Open the Ovulation Calculator, enter the requested values, and review the instant estimate on the page.
Is the Ovulation Calculator free?
Yes. The Ovulation Calculator is free to use on Fast Online Calculator and does not require an account.
How long is the fertile window?
About 6 days — the 5 days before ovulation and ovulation day — because sperm can live up to 5 days and an egg about 24 hours.
How accurate is an ovulation calculator?
It works best with regular cycles. Ovulation tests, basal temperature, and cervical mucus tracking give a more precise picture.
When should I take a pregnancy test?
On or after the day your period is due for the most accurate result.
Can I use this as birth control?
No. Calendar estimates are not reliable contraception. Talk to your doctor about options.
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