> ## Content Index > Fetch the complete content index at: https://verinourishwomen.com/llms.txt > Use this file to discover other available public pages before exploring further. # Ovulation Calculator: Fertile Window and Ovulation Date - URL: https://verinourishwomen.com/tools/ovulation-calculator/ - Published: 2026-07-08T09:55:12.000Z - Updated: 2026-09-06T16:31:16.000Z - Description: Free ovulation calculator: estimate your fertile window, ovulation date and next period from one date or your last 3 periods. - Author: VeriNourish Editorial Team - Tags: Tools Estimate your fertile window, likely ovulation date and next period from your cycle length or your last 3 period starts. An estimate for planning, not contraception. Nothing is stored. First day of your last period Average cycle length days Count from the first day of one period to the first day of the next. Previous period starts (optional) Previous Two periods ago Enter two earlier start dates to calculate your average from the last 3 periods. Your fertile window Pick your last period date Ovulation is estimated in the middle of that window. Your highest-probability days appear here. ◷ Your next period is estimated once you add a date. These are estimates based on an average cycle. Real ovulation shifts from month to month, so this is not a reliable way to avoid pregnancy. If your cycles are very irregular, see our guides to [PCOS](https://verinourishwomen.com/conditions/pcos/) and [progesterone](https://verinourishwomen.com/hormones/progesterone/). This tool is information, not medical advice. ## How it works Enter the first day of your last period and your usual cycle length. If you know the starts of your previous two periods, add them and the calculator will use the average of those two completed cycles. ## The formula and sources The calculator places ovulation about 14 days before your next expected period. It marks the fertile window as the five days before ovulation through ovulation day, and shows the final two days before ovulation through ovulation day as the highest-probability dates. ## Assumptions - Your cycles are fairly regular. - The luteal phase is close to the typical 14 days. ## Limitations This is an estimate for planning, not a contraception method. Ovulation shifts month to month, and stress, illness, and cycle changes move it. ## When to see a doctor If your cycles are very irregular, or you have been trying to conceive without success, speak to your doctor. Our guide to [perimenopause](https://verinourishwomen.com/menopause/perimenopause/) covers changing cycles later in life. ## Primary sources - [Frontiers in Medicine, 2018](https://pmc.ncbi.nlm.nih.gov/articles/PMC5767237/?ref=verinourishwomen.com). Sets the fertile window as the day of ovulation and the five days before it, based on sperm surviving up to five days in good cervical mucus and the egg surviving about 24 hours. The two to three days before ovulation are the most fertile. - [NHS, Periods and fertility in the menstrual cycle](https://www.nhs.uk/conditions/periods/fertility-in-the-menstrual-cycle/?ref=verinourishwomen.com). Ovulation often occurs 10 to 16 days before the next period; a 28-day cycle does not predict every woman’s timing. - [American College of Obstetricians and Gynecologists, When to have sex](https://www.acog.org/womens-health/experts-and-stories/the-latest/trying-to-get-pregnant-heres-when-to-have-sex?ref=verinourishwomen.com). Describes a six-day fertile window and advises sex every day or every other day when trying to conceive. Placing ovulation 14 days before your next period is an estimate that assumes a regular cycle. If your cycles vary, the predicted date moves with them, which is the main limitation of any calendar method. **Calculator version 1.1.** Figures and formula last checked 6 September 2026 against the sources above. This tool gives general information and does not replace advice from your own doctor. ## Frequently asked questions ### When am I most fertile? The fertile window is the five days before ovulation plus ovulation day. The two days before ovulation are the most likely for conception. ### How accurate is an ovulation calculator? It estimates from an average cycle. Real ovulation shifts with stress, illness and cycle changes, so treat it as a planning guide, not a precise date. ### How do I calculate ovulation from my last 3 periods? Enter the first day of your latest period and the two starts before it. The calculator works out the length of the two completed cycles, averages them, and uses that average for the next estimate. If those cycle lengths differ, the dates are less precise. ### When should we have sex to try to conceive? ACOG advises sex every day or every other day during the fertile window. A calendar estimate cannot confirm the exact day of ovulation, so do not wait for a single predicted date. ### Can I use it to avoid pregnancy? No. It is not a contraception method. Ovulation is too variable to rely on for preventing pregnancy. ### What if my cycles are irregular? The estimate is less reliable, and tracking signs like basal temperature or ovulation tests helps. Very irregular cycles are worth discussing with a doctor. See [perimenopause](https://verinourishwomen.com/menopause/perimenopause/).