A 28-day cycle
Last period on 1 September 2026: next period 29 September, ovulation about 15 September, fertile window 10–16 September. The next ovulation is about 13 October.
Ovulation Calculator
Estimate your ovulation day, fertile window and next period from your cycle.
Result
Enter your values and select Calculate to see the result.
Runs in your browser — nothing you enter is sent to a server.
How the Ovulation Calculator works
Enter the first day of your last period and your usual cycle length. Ovulation usually happens about 14 days before the next period (the luteal phase), so the calculator counts forward to the next period and back 14 days. The fertile window runs from 5 days before ovulation to 1 day after, because sperm can live up to about 5 days and an egg about 1 day. The next three cycles are listed too.
If you know your luteal phase is shorter or longer (10 to 16 days), change it. Cycles vary, so treat the dates as a guide.
Formulas
Ovulation
ovulation = first day of last period + cycle length − luteal phase (14)
Fertile window
from ovulation − 5 days to ovulation + 1 day
Examples
A 35-day cycle
With the same start date, ovulation moves to about 22 September — longer cycles shift ovulation later, not the luteal phase.
Frequently asked questions
Can I use this to avoid pregnancy?
No. Calendar estimates can be days out, especially with irregular cycles. Talk to a doctor or pharmacist about contraception.
How can I know when I really ovulate?
Ovulation tests (LH strips), basal body temperature and cervical mucus changes give clearer signs than the calendar alone.
Is my information stored?
No. The dates are calculated in your browser and not saved.
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