Ovulation calculator
Estimate when ovulation is likely and which days make up the fertile window, from the first day of your last period and your usual cycle length.
Calendar estimate only; not a contraceptive method. Consult a clinician for medical advice.
How this calculator works
The ovulation calculator estimates the day an egg is released using the luteal phase: ovulation usually happens about 14 days before the next period, not 14 days after the last one. So for a 28-day cycle, ovulation is around day 14; for a 30-day cycle, around day 16. Sperm can survive up to about five days in the reproductive tract, and the egg is viable for roughly a day, which is why the fertile window is usually given as the five days before ovulation plus the day after.
This is a calendar estimate, not a measurement. Cycle length varies, stress and illness can delay ovulation, and the luteal phase is not always exactly 14 days. If you are trying to conceive, adding ovulation predictor kits, basal body temperature tracking or cervical mucus observations will give you much better timing. If you are trying to avoid pregnancy, do not use calendar methods alone — they have a high typical-use failure rate.
The formula
With cycle length L days:
ovulation = LMP + (L − 14) days
fertile window = ovulation − 5 days … ovulation + 1 day
next period = LMP + L daysThe fertile window reflects sperm survival (about five days) and egg viability (about 24 hours). The calculation assumes a regular cycle and a 14-day luteal phase.
Worked example
For a last period starting Oct 12, 2026 and a 28-day cycle, ovulation is estimated around Oct 26, 2026, with the fertile window running from Oct 21, 2026 to Oct 27, 2026. The next period would be expected around Nov 9, 2026.
Common mistakes
- Assuming ovulation is always day 14. That is only true for a 28-day cycle; the timing moves with cycle length.
- Using it as contraception. Calendar estimates fail often because ovulation shifts between cycles. Use a method designed for contraception.
- Ignoring cycle variability. If your cycles vary by more than a few days, the fertile window estimate is too narrow to rely on.
Frequently asked questions
How accurate is an ovulation calculator?
It is a reasonable estimate for people with regular cycles, but it cannot confirm ovulation. Ovulation predictor kits detect the hormonal surge directly and are more accurate for timing.
When is the fertile window?
Roughly the five days before ovulation plus the day after, because sperm survive several days while the egg lives for about a day. The most fertile days are the two before ovulation.
Can I get pregnant right after my period?
It is possible, especially with short cycles and long periods, because sperm can survive several days. The calendar estimate here shows when the fertile window is likely for your cycle length.
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