Conception Calculator

Determine when conception most likely occurred by working backwards from your due date or forwards from intercourse/LMP.

Calculate Conception Window

Choose your calculation method below.

Calculated on your device. Nothing is saved.
Key Takeaway: Clinical pregnancy dating counts from your LMP (40 weeks total), while fetal biological age begins at conception (~38 weeks total).

Conception Timeline Formulas

Conception Date = Due Date − 266 days
Due Date = Conception Date + 266 days
Implantation = Conception Date + 8 to 10 days

When to See a Doctor

Online calculators provide general estimates based on biological averages. Consult a healthcare provider (such as an OB/GYN or general practitioner) if you notice:

  • Your periods suddenly become irregular after being consistent
  • Your cycle is consistently shorter than 21 days or longer than 35 days
  • Your period bleeding lasts longer than 7 days or is unusually heavy (soaking through a pad/tampon every hour)
  • You experience severe, debilitating pelvic pain or cramping that disrupts daily life
  • You experience bleeding between periods or after intercourse
  • You are under 35 and have been trying to conceive for 12 months (or 6 months if 35+)
  • Your period has stopped completely for 90 days or more (and you are not pregnant)
Next Logical Step

Explore Due Date & Milestones

Calculate prenatal ultrasound windows, trimester markers, and week-by-week progress.

Open Due Date Calculator

Frequently Asked Questions

Evidence-based answers to common questions about your cycle and fertility.

Does conception happen on the exact day of intercourse?

Not necessarily. Sperm can live in fertile cervical fluid for up to 5 days. If intercourse occurred on Monday and ovulation occurred on Thursday, actual conception (fertilization) takes place on Thursday when the egg is released.

How do you calculate conception date from a due date?

Human pregnancy lasts an average of 266 days (38 weeks) from conception. Subtracting 266 days from your clinical due date provides your estimated conception date.

Explore other free, private tools for tracking your menstrual cycle and fertility.

Pregnancy & Due Dates

Implantation Calculator

Calculate your estimated implantation window, earliest pregnancy test date, and clinical milestones based on ovulation or conception timing.

Calculate now
Pregnancy & Due Dates

Due Date Calculator

Calculate your estimated baby due date, current gestational age, trimester, and weekly milestones from your LMP, conception, or ultrasound date.

Calculate now
Ovulation & Fertility

Ovulation Calculator

Calculate your estimated ovulation date, 6-day fertile window, and peak conception days. 100% private, client-side calculator with zero tracking.

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Pregnancy & Due Dates

Pregnancy Test Date Calculator

Find the earliest possible and most reliable dates to take a home pregnancy test based on ovulation timing or your missed period date.

Calculate now

Medical & Scientific Sources

  1. American College of Obstetricians and Gynecologists (ACOG)(2017). "Methods for Estimating the Due Date"
  2. Wilcox AJ, et al. (NEJM)(1995). "Timing of Sexual Intercourse in Relation to Ovulation"
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