Ovulation Calculator - Fertile Window & Ovulation Date
Estimate your ovulation date and fertile window from your last period start date and average cycle length. Identify the days with highest conception likelihood.
Cycle Details
Ovulation ~14 days before next period.
Typical range: 21-35 days
Fertility Window
Estimated Ovulation
July 14, 2026
Most fertile day of your cycle
Methodology and limitations
Last reviewed:
Methodology
Estimates ovulation as cycle length minus 14 days from last period start, defines a fertile window from five days before ovulation through ovulation day, and projects the next period date.
Limitations
Calendar methods assume regular cycles and do not detect hormonal shifts from stress, illness, or PCOS. OPK tests, BBT tracking, or clinical evaluation improve accuracy.
Official sources
How Ovulation and Fertile Windows Are Estimated
Estimate your ovulation date and fertile window from your last period start date and average cycle length. Identify the days with highest conception likelihood.
Method used
The calculator estimates ovulation as cycle length minus 14 days from the last period start, defines a fertile window from five days before ovulation through ovulation day, and projects the next period date.
Ovulation day = LMP + (cycle length − 14); fertile window = ovulation − 5 days through ovulation
Practical example
With a period starting March 1 and a 28-day cycle, ovulation is estimated around March 15 with a fertile window of roughly March 10–15 and next period around March 29.
- Last period start date
- Average cycle length (21–45 days)
The result shows ovulation date, fertile window range, and next expected period date.
Assumptions
- Ovulation occurs about 14 days before the next period (luteal phase).
- Cycle length is relatively consistent month to month.
- Sperm viability supports conception in the five days before ovulation.
What this includes
- Estimated ovulation date, fertile window start and end, and next period projection.
What this excludes
- OPK test results, basal body temperature charts, cervical mucus tracking, PCOS irregular cycles, and fertility treatment protocols.
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Frequently Asked Questions
When does ovulation usually occur?
Ovulation typically happens about 14 days before the next period, regardless of cycle length. A 28-day cycle often ovulates around day 14; longer cycles ovulate later.
What is the fertile window?
The fertile window spans roughly five days before ovulation through ovulation day. Sperm can survive several days, so conception is possible before ovulation occurs.
How accurate is calendar-based ovulation tracking?
Calendar methods assume regular cycles. Stress, illness, and hormonal changes can shift ovulation. OPK tests, temperature tracking, or cervical mucus improve accuracy.
What cycle length should I enter?
Use your average over the last 3-6 months if cycles vary. Count from the first day of one period to the day before the next period starts.
Can I use this to avoid pregnancy?
Calendar methods alone are not reliable contraception. Many factors affect ovulation timing. Use approved birth control and consult a provider for family planning.