Ovulation Calculator
Estimate your ovulation date, fertile window and next period from your last period and average cycle length.
The three dates and how each is worked out
You give the first day of your last period and your average cycle length (28 days by default). The tool then counts forward on the calendar:
- Estimated ovulation: first day of last period + (cycle length − 14) days.
- Fertile window: the five days before that estimated ovulation day, plus the day itself.
- Next period: first day of last period + cycle length.
For a period that started on 1 October 2026 with a 28-day cycle, ovulation is estimated for 15 October, the fertile window runs 10 to 15 October, and the next period is due on 29 October. Every date here is an estimate from averages, not a diagnosis or a test result.
How your cycle length moves every date
The tool always places ovulation 14 days before the next expected period, so a longer cycle pushes everything later. All of these start from a period on 1 October 2026:
| Cycle | Fertile window | Ovulation | Next period |
|---|---|---|---|
| 21 days | 3–8 Oct | 8 Oct | 22 Oct |
| 24 days | 6–11 Oct | 11 Oct | 25 Oct |
| 28 days | 10–15 Oct | 15 Oct | 29 Oct |
| 32 days | 14–19 Oct | 19 Oct | 2 Nov |
| 35 days | 17–22 Oct | 22 Oct | 5 Nov |
With a short cycle the window can open during the days of your period: on a 21-day cycle it starts on day 3.
To get your average, count from the first day of one period to the day before the next period starts, for several cycles, and average them. Cycles of 29, 31 and 30 days average 30.
Why the window is six days, and how to widen it
The six-day window ending on ovulation day comes from the Wilcox studies of the timing of intercourse and conception (NEJM 1995, BMJ 2000). The weak point is the ovulation date itself. The NHS says ovulation usually happens around 10 to 16 days before the next period, and 14 is just the middle of that range.
If you want a margin, apply that range yourself. With a period on 1 October and a 28-day cycle, the next period is due 29 October, so ovulation could fall anywhere from 13 to 19 October. Counting five days back from the earliest date, the possible fertile days stretch from 8 to 19 October, twice as long as the tool's six-day window.
Do not use this as contraception
A calendar estimate must not be used to avoid pregnancy. The tool assumes every cycle is your average length and that ovulation falls exactly 14 days before your next period. A single cycle that runs early or late moves the real fertile days outside the dates shown.
If you want to prevent pregnancy, talk to a doctor, nurse or family-planning service about a reliable method of contraception.
Frequently asked questions
Can I use it if my cycles are irregular?+
It will still give dates, but they rest on one average length. One workaround is to run it twice, once with your shortest recent cycle and once with your longest, and treat everything between the two results as possible. With a period on 1 October, cycles of 26 and 32 days put ovulation on 13 and 19 October.
Does it predict several months ahead?+
No. It works out one cycle from the period date you enter. When your next period starts, enter the new date to get the following cycle's estimates.
Which cycle day is ovulation on?+
Counting the first day of bleeding as day 1, the tool puts ovulation on day (cycle length − 13). On a 28-day cycle that is day 15, and on a 32-day cycle it is day 19.
What does the next period date assume?+
It simply adds your average cycle length to the date you entered. If your period comes a few days either side of it, that is the normal variation the average hides.
Embed this tool on your website
Free to use on blogs, school sites and intranets. Paste this code into your page's HTML; please keep the credit line.
<iframe src="https://utilityapp.lat/embed/health/ovulation-calculator" title="Ovulation Calculator" width="100%" height="660" style="border:0;max-width:760px" loading="lazy"></iframe> <p style="font-size:13px"><a href="https://utilityapp.lat/health/ovulation-calculator">Ovulation Calculator</a> by UtilityApp</p>