Ovulation & Fertile Window Calculator
Your fertile window, ovulation day and test dates for the next six cycles.
Enter the first day of your last period.
Calendar
- Your last period
- Expected period
- Fertile window
- Estimated ovulation
- Today
Your next six cycles
| Cycle | Fertile window | Ovulation | Next period · test from | Start LH tests |
|---|
- These are estimates from averages, and ovulation often comes earlier or later than predicted. This calculator is not a method of contraception.
- If your cycles are often shorter than 24 or longer than 38 days, or vary a lot, a calendar estimate is unreliable — LH tests or a doctor can help.
For general information only, not medical advice. Consult a qualified health professional for advice about your situation.
About the Ovulation & Fertile Window Calculator
You can get pregnant only on a few days of each cycle: the fertile window is the six days ending on the day of ovulation. In a study of 221 women trying to conceive, every one of the 192 pregnancies came from intercourse in those six days (Wilcox, Weinberg & Baird, 1995). This calculator estimates ovulation as 14 days before your next period — or your own luteal-phase length — and shows the fertile window, the days to start ovulation (LH) tests and the first day a home pregnancy test is likely to work, for each of your next six cycles, on a calendar you can export.
The dates are estimates from averages. Ovulation moves from cycle to cycle, even in regular cycles, so this is not a method of contraception. Your dates stay in your browser.
How to use it
- Enter the first day of your last period — the first day of real bleeding, not spotting.
- Enter your usual cycle length: the days from the first day of one period to the first day of the next (21–45). The average of your last few cycles works best.
- Leave the luteal phase at 14 days unless you know yours (from LH tests or temperature charting), and set how long your period lasts so the calendar can show it.
- Read this cycle’s fertile window and the dates for LH and pregnancy tests, browse the next six cycles in the table and calendar, and download them as a calendar file (.ics) or CSV.
Examples
Fertile window 10–15 Oct · estimated ovulation 15 Oct · start LH tests 10–12 Oct · next period, and a pregnancy test if it does not come, 29 Oct
Ovulation about 20 days after the period starts (cycle day 21), so the fertile window is cycle days 16–21.
Ovulation about 21 days after the period starts (cycle day 22) — a week later than the “day 14” rule of thumb.
How the dates are worked out
- Next period = first day of the last period + cycle length.
- Ovulation = next period − luteal phase. The luteal phase (from ovulation to the next period) is the steadier half of the cycle: in 327 ovulatory cycles it averaged 14.13 days (standard deviation 1.41), and phases of 9 days or less were judged abnormal (Lenton et al.). For a 28-day cycle that puts ovulation 14 days after the period starts.
- Fertile window = the 5 days before ovulation and the day of ovulation.
- LH tests: an ovulation test detects the rise in luteinising hormone that triggers the release of an egg; MedlinePlus advises starting 3 to 5 days before the expected ovulation date, and a positive result means you should ovulate in the next 24–36 hours.
- Pregnancy test: the Office on Women’s Health suggests testing as soon as the first day of your missed period, with more accurate results after that day. In up to 10% of women implantation happens after the first day of a missed period, so a negative result should be repeated a few days later.
With an average cycle that is not a whole number (such as 28.5 days), each cycle’s start is rounded from your last period, so the dates do not drift.
How reliable is a calendar estimate?
Not very, for any single cycle. In the same group of women, the fertile window fell on a broad range of days: on every day from cycle day 6 to day 21 at least 10% of women were in their fertile window, and only about 30% had it entirely within days 10–17 (Wilcox, Dunson & Baird, 2000). The authors advise that the timing of the fertile window can be highly unpredictable, even when cycles are usually regular.
The chance of conceiving also rises through the window: it ranged from about 10% for intercourse five days before ovulation to about 33% on the day of ovulation (Wilcox et al.). If you are trying to conceive, LH tests or charting give a better fix on your own ovulation than averages can.
Irregular cycles
If your cycles are often shorter than 24 days or longer than 38 days, or vary by more than about 7–9 days from the shortest to the longest, they are outside the FIGO normal limits for adults, and a calendar estimate will be unreliable. MedlinePlus notes that LH tests may also miss the surge when cycles are irregular. These patterns are worth discussing with a doctor, especially if you are trying to conceive. The period tracker checks your logged cycles against these limits.
Sources
- Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med 333(23):1517-21
- Wilcox AJ, Dunson D, Baird DD. The timing of the “fertile window” in the menstrual cycle. BMJ 321(7271):1259-62
- Lenton EA, Landgren BM, Sexton L. Normal variation in the length of the luteal phase of the menstrual cycle. Br J Obstet Gynaecol 91(7):685-9
- MedlinePlus. Ovulation home test
- Office on Women’s Health. Pregnancy tests
- Munro MG et al. The two FIGO systems for normal and abnormal uterine bleeding symptoms. Int J Gynaecol Obstet 143(3):393-408
Limitations
- An estimate from averages: your ovulation day can differ by several days in any cycle.
- Not a method of contraception. It assumes natural cycles: if you take hormonal contraception, or have recently been pregnant or breastfeeding, ask your doctor instead.
- Cycle lengths of 21–45 days and luteal phases of 9–18 days are accepted. For irregular cycles use LH tests and ask a doctor.
Privacy
Everything happens in your browser. What you enter or open here is not uploaded or stored by MySmartCoPilot.
Frequently asked questions
When am I most fertile?
In the six days ending on the day of ovulation, and most of all in the last few: the chance of conception rose from about 10% for intercourse five days before ovulation to about 33% on ovulation day in the Wilcox study. For a 28-day cycle that is roughly days 10–15, counting the first day of your period as day 1.
How many days after my period do I ovulate?
It depends on your cycle length, because the time from ovulation to the next period is the more constant part — about 14 days. With 28-day cycles ovulation is about 14 days after your period starts; with 35-day cycles, about 21 days after.
Can I get pregnant outside the fertile window shown?
Yes, if you ovulate earlier or later than the estimate — which is common. The window shown is where ovulation is most likely on average, not a guarantee, so do not rely on it to avoid pregnancy.
When should I take a pregnancy test?
From the first day of your missed period — the “next period” date shown. Results are more accurate if you wait until after that day; if the test is negative and your period still has not come, test again a few days later.
What if my period is late?
If the next period should already have started, the calculator says how many days late it is instead of showing a future fertile window — later dates would assume a period that has not come. If it has started since, enter that day as the first day of your last period. If you could be pregnant, a home pregnancy test can be used from the first day of a missed period; if your period does not come, talk to a doctor.
Does it work with irregular periods?
Not well. With cycles that vary a lot, an average cycle length cannot predict ovulation. LH tests, charting your temperature, or advice from a doctor are better options.