Curious when conception likely happened? This calculator works backwards from your due date or last menstrual period to estimate the date of conception - useful for paternity, leave planning and clinical records.
Estimated conception
23 August 2026
Most likely date
LMP
9 August 2026
Last menstrual period
Due date
16 May 2027
Likely fertile window · 18 August 2026 – 24 August 2026
Conception happens during this 6-day window. Sperm can survive up to 5 days in the female reproductive tract, so intercourse earlier in the window can still result in fertilisation on the ovulation day.
Conception date is a statistical estimate (±5-7 days). For paternity or precise dating, an early ultrasound provides a more accurate fetal age. IVF dating uses the embryo transfer date and should not use this calculator.
कुछ सामग्री अब भी अंग्रेज़ी में दिखती है।
Knowing the approximate conception date is useful for several reasons: clinical records that ask 'when did conception occur', insurance/maternity benefits that may require date evidence, paternity questions, and the basic curiosity of understanding when a pregnancy began. This calculator works backward from either your due date (subtracting 266 days for average gestation from conception) or your last menstrual period (adding 14 days for typical mid-cycle ovulation). Both methods give a date with a ±5-7 day window of uncertainty.
Conception cannot be pinpointed to a single calendar day without ovulation tracking. The fertile window - when conception is biologically possible - spans 5 days before ovulation through 1 day after, because sperm can survive in the female reproductive tract for up to 5 days while the egg lives only 12-24 hours after release. So the 'conception date' produced by this calculator is the most-likely date within that window; the actual fertilisation could have occurred a few days either side.
Choose whether to calculate from due date or LMP.
Enter the date.
Read the estimated conception date and the typical fertile window around it.
Conception typically occurs around ovulation, ~14 days after the LMP for a 28-day cycle. From a known due date, conception was 266 days earlier (38 weeks of fetal age). The fertile window covers 5 days before ovulation and 1 day after - sperm can survive that long.
From due date: Conception = Due − 266 days From LMP: Conception = LMP + 14 days (for 28-day cycle)
Two calculation paths are supported. From due date: Conception ≈ Due Date − 266 days. The 266 days reflect average fetal gestation from conception (vs the 280-day Naegele's rule which counts from LMP). From LMP: Conception ≈ LMP + 14 days for a 28-day cycle, adjusted for cycle length variation. A 32-day cycle pushes likely conception to LMP + 18 days; a 26-day cycle to LMP + 12 days. The cycle adjustment matters for accurate estimation when cycles aren't exactly 28 days.
Conception is the moment of fertilisation - when a sperm cell penetrates the egg, typically within 12-24 hours of ovulation. This is distinct from implantation, which occurs 6-12 days later when the early embryo (blastocyst) embeds into the uterine wall and pregnancy hormones become detectable. Pregnancy tests measure hCG, which the placenta produces only after implantation - which is why home pregnancy tests can be negative for 7-10 days after conception even though pregnancy has technically begun.
For IVF and IUI pregnancies, conception dating is precise rather than estimated. IVF: fertilisation happens in the lab; the embryo transfer date is the start of the in-vivo phase. IUI: the date of insemination is the closest to conception, with fertilisation occurring within 24-48 hours. For these pregnancies, this calculator's estimates aren't applicable - use the clinical records directly.
Conception ≈ 8 October − 266 days = 15 January 2026. The fertile window: 10-16 January 2026. Conception likely happened around 14-15 January but could have been anywhere in the 5-day window before.
Likely ovulation/conception: 14 January 2026. Fertile window: 9-15 January. Anyone using this date for paternity or planning purposes should remember there's a ±5-7 day uncertainty band.
Within 5-7 days. Conception happens during a fertile window, not a single day. Without ovulation tracking, the precise date can't be determined.
Yes - sperm survive 3-5 days in the female reproductive tract. Intercourse on Day 10 can result in fertilisation on Day 14 if ovulation is delayed.
No. Fertilisation (conception) happens within hours of ovulation. Implantation occurs 6-12 days later when the embryo embeds in the uterine wall.
It's a statistical estimate - within a 5-7 day window. If certainty matters (paternity, etc.), an early ultrasound provides better dating.
No - IVF conception happens in the lab, with a known transfer date. The 'conception date' for IVF is the date of fertilisation in the embryologist's record.
Use ovulation prediction kits (OPKs), basal body temperature tracking and the Ovulation Calculator. The fertile window is 5 days before ovulation through 1 day after.
Body composition, daily energy needs and pregnancy timelines all rest on formulas validated across millions of people, but no single number tells the whole story. The values from this calculator are point estimates with a known margin of error - body fat from skinfold or US Navy method has roughly a ±3% standard error compared to a DEXA scan, and BMR can vary ±10% person to person at the same height and weight. Use these numbers to track trends week over week, not to chase decimal precision day to day.
Pair this tool with the BMR, TDEE, calorie and macro calculators to turn a single measurement into an actionable plan. None of these tools are a substitute for a qualified doctor, registered dietitian or certified strength coach - but they remove the busywork so you spend more time on training and recovery, less on math.
The Conception Calculator is part of the Health & Fitness suite on Online Calculator Tools, alongside 6 related calculators. All tools share the same UI patterns, the same privacy-first rendering (nothing is sent to a server), and the same commitment to showing the formula and assumptions behind every result.
