Conception Date Calculator
Estimate the day conception likely happened from your last period and your usual cycle length. Conception lines up with ovulation, which is timed by the luteal phase — the roughly two-week stretch after ovulation that stays fairly fixed even when cycles vary in length. This calculator dates conception forward from your last period using that convention. It is an estimate: the exact day depends on when ovulation actually occurred.
Calculate
Default result: Jan 15, 2026
Conception Date Calculator · Result
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Estimated conception date
2026-01-01 × 28
This calculator provides an estimate for general information only and is not medical advice. Due dates, gestational age, and fertility windows are estimates — babies rarely arrive exactly on the estimated due date, and individual cycles and pregnancies vary. Always confirm dates and any health decisions with your healthcare provider or OB-GYN.
Reviewed by the calculators.dev team · Last updated 2026-06-24
Formula reviewed against Luteal phase and ovulation timing; estimated date of conception (Wikipedia EDD; reproductive physiology references)
How to calculate
Enter the first day of your last menstrual period and your average cycle length. The calculator subtracts the fixed 14-day luteal phase from your cycle length to find ovulation, then dates conception to that day. A longer cycle pushes conception later; a shorter cycle pulls it earlier.
conception date = first day of last period + (cycle length − 14). The 14 is the luteal phase, the stretch from ovulation to the next period, which holds fairly steady across people. For a textbook 28-day cycle this is the familiar last period + 14 days.
Example calculation
From a last period on January 1, 2026 with a 28-day cycle, ovulation — and so conception — is estimated 14 days later, on January 15, 2026. Because the luteal phase is roughly fixed at 14 days, the offset is cycle length minus 14: for a 28-day cycle that is 28 − 14 = 14 days.
- conceptionDay
- January 15, 2026
Assumptions
- Ovulation, and therefore conception, is estimated as cycle length minus the 14-day luteal phase, counted from the first day of your last period.
- The luteal phase is treated as a fixed 14 days. In reality it varies by a day or two, so the conception date is an estimate, not an exact day.
- Cycle length must be between 20 and 45 days; values outside that range are reported as an error rather than a misleading date.
- Sperm can survive several days, so fertilization can occur on a day other than the exact ovulation day. This calculator reports the single most likely day.
- If your cycles are irregular, the estimate is less precise — an early ultrasound dates a pregnancy more reliably.
Common mistakes
- Assuming conception is always 14 days after the last period. That only holds for a 28-day cycle; the offset is your cycle length minus 14.
- Treating the estimated day as exact. Ovulation timing and sperm survival mean conception could be a day or two on either side.
- Using it as a paternity or legal date. It is a rough estimate and not suitable for those purposes.
Frequently asked questions
How is the conception date estimated?
It is dated to your estimated ovulation day: the first day of your last period plus your cycle length minus the fixed 14-day luteal phase. For a 28-day cycle that is 14 days after your last period.
Why subtract 14 days?
The luteal phase — the time from ovulation to the next period — stays close to 14 days even when total cycle length varies. So ovulation is best estimated by counting back 14 days from the next expected period, which is cycle length minus 14 from the last one.
How accurate is this?
It is an estimate. Real ovulation can shift by a day or two, and sperm can survive several days, so conception may occur slightly before or after the calculated day. Irregular cycles make it less precise.
Can I use this to find a due date?
Yes — the Due Date Calculator can take a conception date and add 266 days to estimate the due date. Your provider will confirm dating, often with an early ultrasound.