# Ovulation and Fertile Window Calculator > Work out your likely day of ovulation and fertile window from your last period and cycle length. How the calculation works and why it is only an estimate. The tool below estimates your day of ovulation and your fertile window from the **first day of your last period** and your **cycle length**. It is an **estimate** and cannot be used as a method of contraception. **In short:** An ovulation calculation estimates the day of ovulation and the fertile window from the first day of the last period and the cycle length. Ovulation is placed about 14 days before the next period; the fertile window begins 5 days before ovulation and ends 1 day after it. The result is an estimate, is not reliable with irregular cycles and cannot be used as a method of contraception. ## Calculator **First day of your last menstrual period** Day select12345678910111213141516171819202122232425262728293031 Month selectJanuaryFebruaryMarchAprilMayJuneJulyAugustSeptemberOctoberNovemberDecember Year select20262025 Cycle length 21 days22 days23 days24 days25 days26 days27 days28 days29 days30 days31 days32 days33 days34 days35 days ## How is it calculated? The menstrual cycle has two phases. In the first, an egg matures, and **its length varies from woman to woman.** The second runs from ovulation to the next period and lasts **about 14 days**, however long the cycle is. The calculation therefore places ovulation **14 days before the next period**: - **Day of ovulation:** last period + cycle length − 14 days - **Fertile window:** begins **5 days before** ovulation and ends **1 day after** it The window is that wide because sperm can survive in the female reproductive tract for **up to 5 days**. The egg itself can be fertilised for about **24 hours** after it is released. **A common misconception:** that ovulation always happens on day 14 of the cycle. That holds only for a 28-day cycle. In a woman whose cycle is 32 days, ovulation happens not on day 14 but **around day 18**. Assuming a fixed day 14 therefore gives the wrong answer. ## Why is it only an estimate? - The day of ovulation **shifts from month to month**, even in the same woman. - Stress, illness, sleep patterns and weight changes all affect the timing. - In women with irregular cycles the calculation is **not reliable**. ## Physical signs of ovulation Alongside the calendar calculation, the body gives its own signs: - **A change in discharge:** as ovulation approaches, discharge becomes clear, slippery and the consistency of raw egg white. - **Basal body temperature:** rises slightly after ovulation. That rise shows ovulation **has happened**; it does not predict it in advance. - **One-sided pelvic pain:** some women feel a brief, mild pain at ovulation. **Not a method of contraception.** Working out the fertile window and avoiding intercourse on those days (the calendar method) is not a reliable way of preventing pregnancy; ovulation can happen earlier or later than expected. For the options, see [contraception: which methods are available](https://www.muratalpay.com.tr/en/blog/dogum-kontrol-yontemleri-nelerdir-gebelikten-korunma-yollari/). ## When should you see a doctor? - If your cycle is **shorter than 21 days or longer than 35** - If cycle length **varies markedly** from month to month - If pregnancy has not happened after **a year** of regular trying under 35, or **six months** at 35 and over ## Sources 1. Basic reproductive physiology: the luteal phase lasts about 14 days regardless of cycle length, so the day of ovulation is estimated by counting back from the next period. 2. Fertile-window studies: the window opens about 5 days before ovulation and closes 1 day after it, based on sperm survival in the female reproductive tract and the fertilisable lifespan of the egg. The information on this page is based on the national and international guidelines listed above. The page is reviewed whenever those guidelines are updated. ## Practice and Contact Details The information on this page is general. Please speak to your doctor about your own situation. - **Doctor**Op. Dr. Murat Alpay — Specialist in Obstetrics and Gynaecology - **Address**Op. Dr. Murat Alpay Practice, Tekelioğlu Cd. No:51 B Kat:2, Filo Apt., 07160 Muratpaşa / Antalya - **Telephone**0505 351 77 88 - **Hours**Monday – Saturday 09:00 – 18:00 · Closed on Sunday [← All treatments](https://www.muratalpay.com.tr/en/tedaviler/) --- Original page: https://www.muratalpay.com.tr/en/tedaviler/yumurtlama-hesaplama/ Last updated: 2026-09-29 Op. Dr. Murat Alpay — Specialist in Obstetrics and Gynaecology Op. Dr. Murat Alpay Practice, Tekelioğlu Cd. No:51 B Kat:2, Filo Apt., 07160 Muratpaşa / Antalya 0505 351 77 88 · Monday – Saturday 09:00 – 18:00 · Closed on Sunday *This information is for general guidance only and does not replace examination, diagnosis or treatment.*