Ovulation Calculator

Worked example

A period starting 1 August on a 33-day cycle puts ovulation on 19 August — cycle day 19, not day 14. The fertile window runs 14 to 19 August. A calculator that assumes day 14 would have pointed at 14 August, the day the window opens rather than the day conception is most likely.

How to work it out yourself

1. 1.Enter the first day of your last period, not the day it ended.
2. 2.Set your usual cycle length. If it varies, use your shortest recent cycle: it opens the window earlier and misses less.
3. 3.Leave the luteal phase at 14 unless you have tracked it. It is the second half of the cycle, and it stays close to constant while the first half is what stretches or shortens. Ovulation lands on cycle day 14 of a 28-day cycle for exactly this reason.
4. 4.The window is the five days before ovulation plus the day itself. Sperm survive that long in fertile cervical mucus; the egg is viable for under a day.

The formula

1. Next period expected2026-10-02 + 33
2. Estimated ovulation2026-10-02 + 33 - 14 - 1
3. Cycle day this falls on2026-10-20 - 2026-10-02 + 1
4. Fertile window opens2026-10-20 - 5
5. Fertile window closes2026-10-20
6. Highest-probability days start2026-10-20 - 2
7. Days until ovulation2026-10-20 - 2026-10-02
8. Cycle day today2026-10-02 - 2026-10-02 + 1
9. Ovulation after that2026-10-20 + 33

Source: Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med 1995;333:1517-21, American College of Obstetricians and Gynecologists — trying to get pregnant, US Office on Women's Health — your menstrual cycle

Questions people actually ask

Why not just day 14?

Because day 14 is only correct on a 28-day cycle. Ovulation sits a roughly fixed 14 days before the next period, so on a 33-day cycle it falls near day 19 and on a 24-day cycle near day 10. Counting forward from the last period spreads the error across every cycle that is not 28 days; counting back from the next one does not.

How wide is the fertile window really?

Six days: the five days before ovulation and the day of ovulation itself. Wilcox, Weinberg and Baird tracked 221 women with daily hormone measurements and found no conceptions outside it. The probability of conception per act of intercourse rose from 0.10 five days before ovulation to 0.33 on the day itself.

Which days are best?

The two days before ovulation and the day of ovulation, where the Wilcox data puts the probability highest. Intercourse after ovulation contributed no pregnancies in that study, because the egg is viable for well under a day while sperm last up to five.

How accurate is this if my cycles are irregular?

Less accurate the more they vary, and that is a limitation of the arithmetic, not of the inputs. A cycle that swings between 26 and 34 days puts ovulation anywhere across an eight-day span. Ovulation predictor kits detecting the LH surge, or basal body temperature, measure the event rather than predicting it, and are the right tool when cycles are irregular.

Can this be used to avoid pregnancy?

Not as shown here. Calendar-based methods have a typical-use failure rate well above hormonal or barrier methods, because the window is predicted from an average rather than observed. Fertility awareness methods that work rely on daily observation of cervical mucus or temperature and on training, not on a date calculation.

Related

• Due Date CalculatorEstimated due date from a last menstrual period or a known conception date, with the current gestational age.
• BMI CalculatorBody mass index from height and weight in either metric or US units, with the CDC category cut-offs.
• Period CalculatorYour next six periods from the last one, with the prediction window widening the further out it goes — because cycle length varies month to month.
• Pregnancy Weight Gain CalculatorWhere your weight should be at this point in pregnancy, from the 2009 Institute of Medicine ranges for your pre-pregnancy BMI.
• Conception Date CalculatorWhen conception most likely happened, worked back from a due date or a last period, with the window either side that the single date hides.
• Dog Pregnancy CalculatorWhelping date from the mating date, on the 63-day canine gestation, with the dates each check and each stage of the pregnancy falls due.

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