HEALTH · CONCEPTION
Conception Calculator
Estimate your likely conception date and fertile window from a due date or last menstrual period (LMP). Informational only — not for contraception.
Not for contraception or fertility treatment — consult a healthcare provider.
About This Calculator
The conception calculator estimates the likely conception date and fertile window using Naegele's Rule — the clinical standard since the 19th century. Enter your last menstrual period (LMP) to estimate when conception occurred, or enter a known due date to work backwards. Both results are estimates; individual cycle variation means actual timing can differ by several days.
How It Works
In LMP mode, conception is estimated as LMP + 14 days (mid-cycle ovulation assumption for a standard 28-day cycle). The expected due date is LMP + 280 days (Naegele's Rule). In due-date mode, the calculator reverses the formula — LMP = EDD − 280 days, then conception = LMP + 14 days. The fertile window spans 5 days before the estimated conception date to 1 day after (reflecting sperm viability of ~5 days and egg viability of ~24 hours post-ovulation).
The Formula
Conception ≈ LMP + 14 days EDD = LMP + 280 days
- LMP
- first day of last menstrual period
- EDD
- estimated due date (280 days after LMP)
Frequently Asked Questions
- How accurate is this conception estimate?
- The estimate assumes ovulation occurs exactly 14 days after LMP in a 28-day cycle. In practice, ovulation timing varies — cycles shorter or longer than 28 days, or irregular cycles, shift conception by the same amount. The fertile window helps account for some of that uncertainty, but ultrasound dating by a healthcare provider is significantly more accurate.
- Can I use this to determine a legal or medical conception date?
- No. This is an informational estimate based on population-average cycle assumptions. For legal or clinical matters (such as establishing gestational age), only a healthcare provider — using ultrasound and a full history — can provide a reliable date.
- Why is conception estimated as LMP + 14 days?
- Naegele's Rule assumes a 28-day cycle in which ovulation occurs on day 14. Because conception can only occur around ovulation (eggs are viable for ~24 hours), day 14 is the standard clinical mid-cycle reference point. If your cycle differs from 28 days, actual conception timing shifts accordingly.
- Can I use this to avoid pregnancy?
- No. Fertile-window estimation based on average cycle assumptions is not a reliable contraceptive method. Consult a healthcare provider for contraceptive guidance.