Ovulation & Fertile Window Calculator
Find your most fertile days and estimated ovulation date from your last period and cycle length.
How to use this tool
- Enter first day of last period and cycle length in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your estimated ovulation and the full breakdown beneath it.
Ovulation typically occurs about 14 days before the next period. The fertile window spans five days before ovulation through one day after.
⚠ This tool is for general information and education only and is not medical advice. Consult a qualified healthcare professional before making health decisions.
Formula
Ovulation day = LMP + (cycle length − 14) days
Fertile window start = ovulation − 5 days
Fertile window end = ovulation + 1 day
How it works
The calculator estimates ovulation by subtracting 14 days from the total cycle length and counting forward from the LMP, reflecting the convention that ovulation occurs approximately 14 days before the next expected period regardless of cycle length. The fertile window spans the five days before ovulation (when sperm can survive waiting) plus the day of ovulation and one day after. This method assumes a regular cycle and a consistent luteal phase length; irregular cycles reduce the reliability of the estimate.
Worked example
- LMP = 1 June 2026, cycle length = 28 days.
- Ovulation day offset = 28 − 14 = 14 days after LMP.
- Ovulation date = 1 June + 14 days = 15 June 2026.
- Fertile window start = 15 June − 5 days = 10 June 2026.
- Fertile window end = 15 June + 1 day = 16 June 2026.
Ovulation: 2026-06-15 | Fertile window: 2026-06-10 to 2026-06-16
Common mistakes to avoid
- Assuming ovulation always falls on day 14 of the cycle regardless of cycle length — the formula uses cycle length minus 14, so a 32-day cycle gives ovulation around day 18.
- Treating the fertile window as fixed at 5 days before ovulation only, forgetting the day after ovulation (ovulation day +1) is also included.
- Using average cycle length from memory rather than tracking actual cycles — even one extra or fewer day shifts the predicted ovulation date by the same amount.
Key terms
- Ovulation
- The release of a mature egg from the ovary, typically occurring once per menstrual cycle and marking the most fertile point.
- Luteal phase
- The phase of the menstrual cycle after ovulation, lasting approximately 14 days in most women, ending with menstruation.
- Fertile window
- The days in a cycle when conception is possible — up to five days before ovulation (sperm survival) through the day after ovulation (egg viability).
- Cycle length
- The number of days from the first day of one period to the first day of the next, commonly 21–35 days.
- Follicular phase
- The first part of the cycle from menstruation to ovulation, during which a follicle matures in the ovary; its length varies between women.
Frequently asked questions
- How accurate is this ovulation calculator?
- Cycle lengths vary, and ovulation can shift by several days due to stress, illness, or hormonal changes. Use this as a guide, not a certainty. Ovulation predictor kits give same-day confirmation.
- When is the best time to try to conceive?
- The 2–3 days just before ovulation are statistically the most fertile, as sperm can survive several days in the reproductive tract.