Due Date Calculator
Estimate your pregnancy due date from your last menstrual period or conception date, adjusted for cycle length.
Formula last reviewed 4 August 2026 · How we verify our calculators
Estimated due date
2024-10-07
- Gestational age today (days)
- 946
Updates live as you type
Frequently asked questions
In LMP mode it uses Naegele’s Rule: 280 days (40 weeks) from the first day of your last menstrual period, adjusted for your cycle length. In conception mode it adds 266 days to the conception date.
Naegele’s Rule assumes a 28-day cycle with ovulation on day 14. This tool shifts the date by the difference, so a 35-day cycle pushes the due date about a week later.
It is an estimate — only about 1 in 20 babies arrives exactly on the due date. Most births occur within two weeks either side.
Early ultrasound dating is often more accurate than LMP, especially with irregular cycles. If they differ, your doctor will usually rely on the scan.
Only 1 in 20 babies actually arrives on this date
An LMP of 1 January 2024 with a standard 28-day cycle produces an estimated due date of 7 October 2024 — 280 days later. But treat that as a target, not a deadline: only around 5% of babies are born on their exact estimated day, with most arriving somewhere within two weeks either side of it. The number is a statistically grounded estimate, not a prediction of a specific day.
The 200-year-old rule behind that number
This calculator uses Naegele's Rule, an obstetric method dating to 1812 that's remained the standard starting point for due-date estimation ever since. In LMP mode, it adds 280 days (40 weeks) to the first day of your last period, then adjusts for your actual cycle length — because the rule's underlying assumption is a 28-day cycle with ovulation on day 14, and a longer or shorter cycle shifts real ovulation, and therefore conception, away from that assumption. In conception mode, when the actual conception date is known, it instead adds 266 days directly from that date, skipping the cycle-length adjustment entirely.
Why your actual cycle length changes the answer
Run a 35-day cycle — seven days longer than the standard — through the same LMP, and the calculator pushes the due date forward by that same seven days, because a longer cycle means ovulation likely happened later relative to the LMP than Naegele's Rule assumes by default. This is exactly why entering your real cycle length rather than accepting the 28-day default meaningfully changes the estimate for anyone whose cycle isn't exactly average. Early ultrasound dating, typically done in the first trimester, tends to be more precise than an LMP-based estimate — especially for irregular cycles, where the LMP is a weaker anchor to begin with — so if a scan date and this calculator disagree, the scan is generally the one to trust.
This is an estimate based on standard assumptions, not medical advice. Pregnancy dating and growth can vary — consult your doctor for personalised assessment, especially if cycles are irregular.
Sources
- Naegele's Rule (1812) — standard obstetric method for estimating gestational due date from LMP