How do I calculate my ovulation date?
- 1Enter the first day of your last period.
- 2Enter your average cycle length (28 days if you're not sure).
- 3Leave the luteal phase at 14 days unless you know your own.
- 4Read your ovulation day, fertile window, most fertile days and next period, plus the next 3 cycles.
How is the fertile window worked out?
- 01Next period = first day of last period + cycle length
- 02Ovulation = next period − luteal phase (usually 14 days)
- 03Fertile window = 5 days before ovulation to ovulation day (6 days)
- 04Most fertile = the 2 days before ovulation and ovulation day
After ovulation, the time to the next period (the luteal phase) is fairly steady — about 14 days for most people. Cycles differ mainly in the days before ovulation. So the calculator counts back from your next expected period: in a 28-day cycle ovulation is around day 14 to 15, in a 35-day cycle around day 21 to 22.
Sperm can survive in the body for up to about 5 days, and an egg lives for about a day after ovulation. A large study of couples trying to conceive (Wilcox and colleagues, New England Journal of Medicine, 1995) found pregnancies came only from sex in the 6 days ending on ovulation day, with the best chances in the last 3 days. That 6-day window is what this calculator shows.
Upcoming cycles repeat your average cycle length. Real cycles vary, so the further ahead a date is, the less certain it becomes.
Good to know
- Track a few cycles and use the average length — one unusual cycle can throw the dates off.
- Ovulation tests (LH strips) and cervical mucus changes can confirm when you actually ovulate.
- If you're trying to conceive, having sex every 2–3 days through the cycle covers the fertile window without timing.
- Cycles shorter than 21 days or longer than 35 days, or very irregular cycles, are worth discussing with a doctor.
Ovulation calculator — common questions
When do I ovulate?
Usually about 14 days before your next period. With a 28-day cycle that's around day 14; with a 32-day cycle around day 18. Enter your dates above to see your estimate.
What is the fertile window?
The days when sex can lead to pregnancy: the 5 days before ovulation and ovulation day itself, because sperm can live up to about 5 days and the egg about 1 day.
What are my most fertile days?
The 2 days before ovulation and the day of ovulation have the highest chance of pregnancy.
Can I use an ovulation calculator as contraception?
No. Ovulation can come earlier or later than expected, so a calendar estimate alone is not reliable birth control. Fertility awareness methods need training and daily tracking of body signs.
How accurate is an ovulation calculator with irregular periods?
Less accurate. If your cycle length changes by more than a few days from month to month, use ovulation tests or temperature tracking, or ask a doctor.
When can I take a pregnancy test?
Most home pregnancy tests work from the first day of your missed period — the "next period" date shown here.
Sources
- Wilcox AJ et al. Timing of sexual intercourse in relation to ovulation. N Engl J Med 1995
- NHS – Periods and fertility in the menstrual cycle
- ACOG – Fertility Awareness-Based Methods of Family Planning
- NHS – Doing a pregnancy test
Formulas last reviewed: September 30, 2026 ·How we check our formulas