Conception Calculator
Find out when you most likely conceived. Enter the first day of your last period and your average cycle length to pinpoint your conception date window and estimated due date.
Day 14
TYPICAL CONCEPTION DAY
+9 Days
TYPICAL IMPLANTATION
266 Days
TO ESTIMATED DUE DATE
Your Details
📌 Conception ≈ Last Period Start + (Cycle Length − 14) days. Result is a single best-estimate date plus a 6-day viable conception window.
Estimated Conception Date
Jun 15, 2026
Conception window: Jun 10 – Jun 16
Jun 24
Implantation
Mar 8
Due Date
Jun 15
Conception Est.
Jun 21–Jun 27
Implantation
Jun 27
Earliest Test
Day 14
Of Cycle
Estimated Due Date — Two Cross-Checks
Both methods should land within a day or two of each other; a larger gap usually means an irregular cycle length.
Mar 8, 2027
From Conception + 266 Days
Mar 8, 2027
Naegele's Rule (from LMP)
Conception Window Detail
★ marks the most likely conception day. Shaded cells show the 6-day viable window (5 days of possible sperm survival plus the ovulation day itself).
How This Calculator Works
Your conception date is the day sperm actually fertilized the egg — which happens within about 24 hours of ovulation, not necessarily the day of intercourse. Because ovulation typically falls 14 days before your next period regardless of overall cycle length, we can project it forward from the first day of your last period. Sperm can survive up to five days waiting for the egg, so this tool also reports a short window around the estimate rather than a single guaranteed day. This is different from working backward from an already-known due date — see our pregnancy conception calculator for that direction, or the ovulation calculator if you're trying to time intercourse around an upcoming fertile window instead.
The Formulas Used
Most Likely Conception Date = Last Period Start + (Cycle Length − 14) daysConception Window = Conception Date − 5 days to Conception Date + 1 dayEstimated Due Date = Conception Date + 266 daysDue Date Cross-Check (Naegele's Rule) = Last Period Start + 280 days + (Cycle Length − 28)A note on precision: this is a calendar-based estimate, not a diagnosis. Real ovulation timing varies cycle to cycle, and the further your cycle strays from regular, the wider the true uncertainty. An early-pregnancy ultrasound remains the most accurate way to confirm gestational age and conception timing.
📆
Cycle Regularity
The 14-day-before-next-period rule assumes a fairly consistent cycle. Irregular cycles shift ovulation unpredictably, widening the real-world margin of error around this estimate.
🧬
Sperm Lifespan
Sperm can survive in the reproductive tract for up to 5 days, which is why the conception window extends backward from ovulation rather than being a single point in time.
🥚
Egg Viability Window
Once released, an egg is typically viable for only 12-24 hours, which is why the window barely extends past the ovulation day itself.
🌱
Implantation Timing
After fertilization, the embryo travels to the uterus and implants roughly 6-12 days later — the point at which hCG production, and pregnancy symptoms, can begin.
— FAQ