How to use this calculator
- Enter or choose first day of your last period.
- Enter or choose usual cycle length.
- Enter or choose luteal phase (if known).
- The estimated ovulation updates as you type. Use Copy result or Share to keep it.
Method
Ovulation ≈ LMP + (cycle length − luteal phase); fertile window = 5 days before ovulation to 1 day after; next period = LMP + cycle length
- LMP
- first day of the last menstrual period
- luteal phase
- days from ovulation to the next period (14 by default)
Worked example
- Next period: 5 Jan + 28 days = 2 Feb.
- Ovulation: 28 − 14 = 14 days after 5 Jan = 19 Jan.
- Fertile window: 5 days before (14 Jan) to 1 day after (20 Jan).
With a 28-day cycle starting 5 January 2026, ovulation is estimated around 19 January, with the fertile window from 14 to 20 January.
What the result means
Sex during the fertile window, especially the two or three days up to and including ovulation, gives the best chance of conception. Because ovulation varies from cycle to cycle, treat the dates as a guide rather than a certainty.
Assumptions and limits
- Your cycles are fairly regular and the length you enter is typical.
- The luteal phase is 14 days unless you enter your own figure.
- Sperm can survive up to about 5–7 days and the egg about a day, giving a fertile window of about six days.
- Not a method of contraception: calendar estimates miss many ovulations, especially in irregular cycles.
- Cycles after stopping hormonal contraception, after pregnancy, while breastfeeding, near menopause or with conditions such as PCOS can be unpredictable.
- Ovulation tests, basal body temperature or cervical mucus tracking show ovulation more directly.
Questions
My cycle varies. What length should I use?
Use your average over the last six cycles. If cycles vary by more than about a week, calendar estimates become unreliable and ovulation tests are more useful.
Can I get pregnant outside the fertile window?
It is less likely but possible, because ovulation can come earlier or later than predicted. Do not rely on these dates to avoid pregnancy.
Sources
Please note: This is a general screening estimate for adults, not medical advice or a diagnosis. Talk to a qualified health professional about your health.