Fertile Window Calculator: Your Most Fertile Days and Ovulation Date
Find your fertile window, most fertile days and estimated ovulation date from your last period, cycle length and luteal phase, with the day-by-day chance of conception (Wilcox study), an irregular-cycle range and your next six cycles.
How it works
Ovulation ≈ first day of last period + (cycle length − luteal phase) · Fertile window = the 5 days before ovulation + ovulation day (6 days) · Most fertile = the 2 days before ovulation + ovulation day · Irregular cycles: window = 5 days before the earliest ovulation (shortest cycle) to the latest ovulation (longest cycle) · Due date if you conceive = ovulation + 266 days
You can only get pregnant from sex in a short window each cycle. Sperm can survive in the fallopian tubes for up to 7 days, but the egg lives for only about a day after it is released. In the landmark study of couples trying to conceive (Wilcox and colleagues, 1995), every pregnancy came from sex during the six days ending on ovulation day. The chance of conception from sex on a single day rose from about 10% five days before ovulation to about 27%, 31% and 33% on the two days before ovulation and ovulation day itself, and no pregnancies followed sex after ovulation. Ovulation usually happens about 10 to 16 days before the next period. That second half of the cycle (the luteal phase) is fairly steady, so the calculator counts back from your expected next period by your luteal phase, 14 days unless you know yours. For irregular cycles it uses your shortest cycle for the earliest possible ovulation and your longest for the latest, which gives a wider window. Dates are estimates. Ovulation can shift by several days from one cycle to the next, even in women with regular cycles, so the window is a guide for timing sex when trying to conceive, not a way to avoid pregnancy. Ovulation tests, cervical mucus changes and basal body temperature charting show when you actually ovulate.
Worked example
With a last period on June 1, 2026, a 28-day cycle and a 14-day luteal phase, the next period is due June 29. Ovulation is 28 − 14 = 14 days after the period started, on Monday, June 15, 2026 (cycle day 15). The fertile window is the 5 days before plus ovulation day, June 10 – June 15, 2026 (cycle days 10–15), and the most fertile days are June 13 – June 15, when the chance of conception from sex on that day is about 27%, 31% and 33%. A pregnancy test works from June 29, and a pregnancy conceived this cycle would be due around March 8, 2027.
Sources: Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. NEJM 1995;333:1517-21 · Wilcox AJ, Dunson D, Baird DD. The timing of the “fertile window” in the menstrual cycle. BMJ 2000;321:1259 · NHS: Fertility in the menstrual cycle (ovulation 10–16 days before a period, sperm survive up to 7 days) · NHS: Natural family planning (fertility awareness) effectiveness · NHS: Infertility (when to see a GP) · NHS: Doing a pregnancy test (from the first day of a missed period)
Frequently asked questions
How many days is the fertile window?
About six days: the five days before ovulation and ovulation day itself. Sperm can live for several days in the reproductive tract, but the egg survives only about a day, so sex after ovulation rarely leads to pregnancy. The chance is highest in the two days before ovulation and on the day itself.
Which day am I most likely to get pregnant?
Sex in the two days before ovulation or on ovulation day gives the best chance, about 27% to 33% per day in the Wilcox study of healthy couples. Because ovulation is hard to pin down, sex every 2 to 3 days through the cycle covers the window without timing it.
Can I use my fertile window to avoid pregnancy?
Not reliably. Ovulation varies from cycle to cycle, and fertility awareness methods are only about 76% effective in typical use, so about 24 in 100 women get pregnant in a year. Talk to a doctor, nurse or sexual health clinic about more effective contraception.
How do I work out my fertile window with irregular periods?
Choose Irregular and enter your shortest and longest cycles from the last 6 to 12 months. The window then runs from 5 days before the earliest likely ovulation to the latest, so it can be long. Ovulation tests or temperature charting narrow it down.
What is the luteal phase and why does it matter?
It is the time from ovulation to the start of your next period, usually 10 to 16 days and fairly steady for each woman. Cycle length varies mainly before ovulation, so subtracting the luteal phase from the cycle length places ovulation better than assuming day 14.
When should we see a doctor about getting pregnant?
The NHS advises seeing a GP after a year of regular unprotected sex without a pregnancy, or sooner if the woman is 36 or over or either partner knows of a possible fertility problem. More than 8 in 10 couples where the woman is under 40 conceive within a year.
Related calculators
Estimates based on average cycles and published studies; every body is different. This is not medical advice and not a method of contraception. Your doctor, nurse or midwife can give guidance for your situation.