--- title: "Conception Date Calculator: When Did I Conceive?" description: "Estimate when you conceived, and your fertile window, from your due date or last period, with Hijri dates. Free, no sign-up, and private." url: https://periodcalc.app/conception-date-calculator/ lang: en --- # Conception date calculator Count back from your due date to the days you most likely conceived. - Gregorian + Hijri (Umm al-Qura) - Nothing leaves your browser - Free, no sign-up You can estimate your conception date by counting 280 days back from your due date to the first day of your last period ([ACOG](https://www.acog.org/womens-health/faqs/when-pregnancy-goes-past-your-due-date)). Then count forward to ovulation, which usually comes about 14 days before your next period ([NHS](https://www.nhs.uk/pregnancy/trying-for-a-baby/trying-to-get-pregnant/)). It is an estimate: fertilisation takes place in a fallopian tube, so the day cannot be seen ([ACOG](https://www.acog.org/womens-health/faqs/how-your-fetus-grows-during-pregnancy)). By [Hossam Hamouda](https://periodcalc.app/about/), developer of Period Calc · Last updated: 25 October 2026 ## How is a conception date worked out? [ACOG](https://www.acog.org/womens-health/faqs/when-pregnancy-goes-past-your-due-date) (the American College of Obstetricians and Gynecologists) gives the average length of a pregnancy as 280 days, or 40 weeks. It counts from the first day of the last period. So 280 days before your due date is the first day of your last period. If you enter your last period instead, the calculator starts there. Conception, or fertilisation, is the union of an egg and a sperm into a single cell ([ACOG](https://www.acog.org/womens-health/faqs/how-your-fetus-grows-during-pregnancy)). The NHS (the UK’s National Health Service) says you are more likely to get pregnant around the time you ovulate (release an egg). This is usually about 14 days before the start of your next period ([NHS](https://www.nhs.uk/pregnancy/trying-for-a-baby/trying-to-get-pregnant/)). For most women the gap is 10 to 16 days ([NHS](https://www.nhs.uk/conditions/periods/fertility-in-the-menstrual-cycle/)). ACOG says the days between ovulation and the next period are the most consistent part of a cycle ([ACOG](https://www.acog.org/womens-health/faqs/fertility-awareness-based-methods-of-family-planning)), so the calculator counts back from the next period. To reach ovulation it counts forward your cycle length minus 14 days from the first day of the last period. An egg survives about 24 hours after ovulation ([ACOG](https://www.acog.org/womens-health/faqs/fertility-awareness-based-methods-of-family-planning)), so the calculator names the ovulation day as the estimated conception day. ## The formula First day of the last period = due date − 280 days Ovulation ≈ first day of the last period + cycle length − 14 days Estimated conception ≈ the ovulation day Fertile window = ovulation − 5 days to ovulation + 1 day (starting no earlier than the day after a 5-day period) ## A worked example Your due date is **20 May 2027** and your cycle is **28 days** long. - **First day of the last period**: 13 August 2026 (20 May − 280 days) - **Ovulation**: around 27 August 2026 (13 August + 28 days − 14 days) - **Estimated conception**: around 27 August 2026 - **Fertile window**: 22–28 August 2026 - **In the Hijri calendar**: last period on 30 Safar 1448, ovulation around 14 Rabi I 1448, due date 14 Dhu al-Hijjah 1448 Change the cycle to 35 days and the same due date gives ovulation around 3 September 2026: seven days later, because the cycle is seven days longer. ## Why is conception a window, not one day? Several things keep the date uncertain. The NHS says it is difficult to pinpoint exactly when ovulation happens ([NHS](https://www.nhs.uk/conditions/periods/fertility-in-the-menstrual-cycle/)). Sperm can survive for several days: about 3 days, sometimes up to 5, according to ACOG ([ACOG](https://www.acog.org/womens-health/faqs/fertility-awareness-based-methods-of-family-planning)), and up to 7 days according to the NHS ([NHS](https://www.nhs.uk/conditions/periods/fertility-in-the-menstrual-cycle/)). So the day you had sex is not always the day you conceived, and conception can happen some days after sex. ACOG says you can become pregnant if you have sex anywhere from 5 days before ovulation until 1 day after ([ACOG](https://www.acog.org/womens-health/faqs/fertility-awareness-based-methods-of-family-planning)). That is why the calculator shows a window around its estimated day. The calculator cannot know your own ovulation day. It uses the cycle length you type, and it works best when your cycles are regular. If they vary, take the date as a rough guide only. ## Is a scan more accurate than counting back? A date worked out from the calendar is only as good as the dates it starts from. The NHS says its 12-week scan can estimate more accurately how many weeks pregnant you are ([NHS](https://www.nhs.uk/pregnancy/finding-out/due-date-calculator/)). In England the dating scan is offered at around 10 to 14 weeks ([NHS](https://www.nhs.uk/pregnancy/your-pregnancy-care/12-week-scan/)). When the date of the last period is uncertain, ACOG says a first-trimester scan measures the embryo or fetus. A formula then estimates the gestational age from that measurement ([ACOG](https://www.acog.org/womens-health/faqs/how-your-fetus-grows-during-pregnancy)). With a due date from a scan, the cycle length you type still moves the estimate, even though a scan does not use it. After IVF (in vitro fertilisation), ACOG says the due date is set by the age of the embryo and the date it was transferred to the uterus ([ACOG](https://www.acog.org/womens-health/faqs/how-your-fetus-grows-during-pregnancy)). So use your clinic’s dates, not this calculator’s. ## When to see a doctor If you think you might be pregnant, take a test ([NHS](https://www.nhs.uk/pregnancy/trying-for-a-baby/doing-a-pregnancy-test/)); if it is positive, tell a doctor or midwife. A pregnancy test looks for the hormone hCG, which starts to be produced around 6 days after fertilisation ([NHS](https://www.nhs.uk/pregnancy/trying-for-a-baby/doing-a-pregnancy-test/)). The NHS says tests are most reliable from the first day of a missed period ([NHS](https://www.nhs.uk/pregnancy/trying-for-a-baby/doing-a-pregnancy-test/)). The [pregnancy test date calculator](https://periodcalc.app/pregnancy-test-date/) gives the day your next period is due. If you are not sure of your dates, speak to a midwife or GP (family doctor) ([NHS](https://www.nhs.uk/pregnancy/finding-out/due-date-calculator/)). Trying to get pregnant? The [ovulation calculator](https://periodcalc.app/ovulation-calculator/) shows your fertile days for the next cycles. Read about the [signs of ovulation](https://periodcalc.app/signs-of-ovulation/) too. Already expecting? The [pregnancy week calculator](https://periodcalc.app/pregnancy-week-calculator/) counts your weeks and days, and the [due date calculator](https://periodcalc.app/due-date-calculator/) turns a last period into a due date. Estimates only, not medical advice: no calculator can tell you the real day you conceived. ## Questions about your conception date ### Can I find the exact day I conceived? No. The NHS says it is difficult to pinpoint exactly when ovulation happens ([NHS](https://www.nhs.uk/conditions/periods/fertility-in-the-menstrual-cycle/)), and sperm can survive for days, so even a good estimate covers several days. A scan can estimate how many weeks pregnant you are more accurately than counting from your last period ([NHS](https://www.nhs.uk/pregnancy/finding-out/due-date-calculator/)). ### Is the conception date the day I had sex? Not always. Sperm can survive inside the body for about 3 days, sometimes up to 5, according to ACOG ([ACOG](https://www.acog.org/womens-health/faqs/fertility-awareness-based-methods-of-family-planning)), and up to 7 days according to the NHS ([NHS](https://www.nhs.uk/conditions/periods/fertility-in-the-menstrual-cycle/)). So conception can happen some days after sex. ### Why does my cycle length change the date? Ovulation happens about 14 days before the next period ([NHS](https://www.nhs.uk/pregnancy/trying-for-a-baby/trying-to-get-pregnant/)), so a longer cycle puts ovulation later. The NHS notes that being fertile around day 14 does not apply to women whose cycles are shorter or longer ([NHS](https://www.nhs.uk/conditions/periods/fertility-in-the-menstrual-cycle/)). ### My scan gave me a new due date. What happens to the estimate? The dating scan works out how many weeks pregnant you are and your due date ([NHS](https://www.nhs.uk/pregnancy/your-pregnancy-care/12-week-scan/)). Enter the new due date and the conception estimate moves with it. ### Can I use this after IVF (in vitro fertilisation)? No. After IVF, ACOG says the due date is set by the age of the embryo and the date it was transferred to the uterus ([ACOG](https://www.acog.org/womens-health/faqs/how-your-fetus-grows-during-pregnancy)). Your clinic knows these dates; this calculator counts from a last period. ## How this page was made Hossam Hamouda, the developer of Period Calc, wrote this page from the guidance of the health organisations listed under [Sources](https://periodcalc.app/conception-date-calculator/#sources). They have not reviewed it, and neither has a doctor. The calculator runs the [formula above](https://periodcalc.app/conception-date-calculator/#formula) in your browser. Its ovulation and fertile-window rules are the ones the Period Calc app uses; counting back from a due date is this page’s own. The calculator works from averages, so it cannot know how your own body differs from them, and it cannot diagnose anything: if something worries you, see [when to see a doctor](https://periodcalc.app/conception-date-calculator/#see-a-doctor). ## Sources 1. [Periods and fertility in the menstrual cycle](https://www.nhs.uk/conditions/periods/fertility-in-the-menstrual-cycle/) (NHS) 2. [Trying to get pregnant](https://www.nhs.uk/pregnancy/trying-for-a-baby/trying-to-get-pregnant/) (NHS) 3. [Fertility Awareness-Based Methods of Family Planning](https://www.acog.org/womens-health/faqs/fertility-awareness-based-methods-of-family-planning) (ACOG) 4. [How Your Fetus Grows During Pregnancy](https://www.acog.org/womens-health/faqs/how-your-fetus-grows-during-pregnancy) (ACOG) 5. [When Pregnancy Goes Past Your Due Date](https://www.acog.org/womens-health/faqs/when-pregnancy-goes-past-your-due-date) (ACOG) 6. [Pregnancy due date calculator](https://www.nhs.uk/pregnancy/finding-out/due-date-calculator/) (NHS) 7. [12-week scan](https://www.nhs.uk/pregnancy/your-pregnancy-care/12-week-scan/) (NHS) 8. [Doing a pregnancy test](https://www.nhs.uk/pregnancy/trying-for-a-baby/doing-a-pregnancy-test/) (NHS) ## Get reminders before your next period **Period Calc** for iPhone learns from your own cycles and reminds you two days before your period — discreetly, with no details unless you turn them on. - The Hijri date on every calendar day - Your fasting days, every month - A PIN and Face ID lock, and no account [Download on the App Store](https://apps.apple.com/app/id6814298894) ## Free calculators - [Period calculator](https://periodcalc.app/period-calculator/): Your next six periods, fertile days and ovulation, in Gregorian and Hijri dates. - [Ovulation calculator](https://periodcalc.app/ovulation-calculator/): Your fertile window and ovulation day for the next three cycles. - [Due date calculator](https://periodcalc.app/due-date-calculator/): Your due date, how many weeks pregnant you are and your trimester. - [Pregnancy week calculator](https://periodcalc.app/pregnancy-week-calculator/): How many weeks and days pregnant you are, your trimester and due date, from your last period or your due date. - [Pregnancy test date calculator](https://periodcalc.app/pregnancy-test-date/): Shows the day your next period is due. The NHS says most tests can be done from the first day of a missed period. ## Related guides - [Late period](https://periodcalc.app/late-period/): Pregnancy and stress are common reasons for a late period, and most home tests work from the first day you miss it. - [How long a period lasts](https://periodcalc.app/how-long-does-a-period-last/): A period usually lasts 2 to 7 days and a cycle 21 to 35 days. - [Signs of ovulation](https://periodcalc.app/signs-of-ovulation/): Mucus, temperature, tests and pain can show ovulation, but a temperature rise only confirms it afterwards. - [Irregular periods](https://periodcalc.app/irregular-periods/): The NHS calls periods irregular when the gap is under 21 or over 35 days, and says they are not usually a sign of a problem. Predictions are estimates, not medical advice or a diagnosis, and not a method of contraception. The prayer & purity guide gives the general rules of the four schools. It is not a fatwa — for your own case, ask someone whose knowledge you trust. This website uses no cookies and no analytics. The calculators run in your browser, and nothing you type is sent anywhere.