How to Calculate Your Pregnancy Due Date
7 min read · Updated September 26, 2026 · by the SolveCalcPro editorial team
Pregnancy dating uses a few simple rules that you can do by hand: add 280 days to the last period, count back from the next period to find ovulation, and subtract 266 days from a due date to find conception. This guide shows each.
Due date from your last period (Naegele’s rule)
Add 280 days (40 weeks) to the first day of your last period. For 1 January 2026 that gives Thursday, October 8, 2026. If your cycle is longer or shorter than 28 days, adjust by the difference: a 32-day cycle gives Monday, October 12, 2026.
From conception or IVF
Add 266 days to the conception date, or 261 days to a day-5 embryo transfer date (263 days for day 3).
Ovulation and the fertile window
Ovulation happens about 14 days before the next period: first day of last period + (cycle length − 14). With a 28-day cycle starting 1 March 2026 that is Sunday, March 15, 2026, with a fertile window of Tuesday, March 10, 2026 – Monday, March 16, 2026.
Working back to conception
Conception ≈ due date − 266 days. For a due date of 8 October 2026: Thursday, January 15, 2026.
How reliable are the dates?
Only about 4–5% of babies are born on the due date; most arrive between weeks 37 and 42. An early ultrasound can adjust the date — follow your clinician. These tools are educational, not medical advice: due date, ovulation, conception.
Practice questions
Test yourself, then tap to check. Answers are calculated by the tools linked below.
- The first day of the last period was 14 April 2026 and the cycle is 30 days. What is the due date?
Show answer
Estimated due date: Thursday, January 21, 2027 · Gestational age on the “as of” date: 6 weeks 6 days
- The last period began 5 May 2026 and the cycle is 30 days. When is ovulation?
Show answer
Estimated ovulation day: Thursday, May 21, 2026 · Fertile window: Saturday, May 16, 2026 – Friday, May 22, 2026
- The due date is 20 December 2026. When was conception likely?
Show answer
Estimated conception date: Sunday, March 29, 2026 · Likely conception window: Thursday, March 26, 2026 – Monday, March 30, 2026
Frequently asked questions
How do I calculate my due date?
Add 280 days to the first day of your last period, or 266 days to the conception date.
When do I ovulate?
About 14 days before your next period, so day 14 of a 28-day cycle.
How long is the fertile window?
About six days: five days before ovulation, plus ovulation day and the day after.
Why is pregnancy counted from the last period?
It is the date most people know; conception is typically about two weeks later.
Try it yourself
Pregnancy Due Date CalculatorEstimate your due date from your last period, conception date or IVF transfer, with your week of pregnancy and key milestones.Ovulation CalculatorEstimate your ovulation day, fertile window and next period from your cycle, with the next three cycles mapped out.Conception Date CalculatorWork out when you probably conceived from your due date or last period, with the likely conception window.Keep learning
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