Calculation details
Planning estimate only, not financial, tax, or legal advice. Verify assumptions and current rules before making decisions.
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Use this result
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What does this calculator estimate?
A due date calculator estimates when a baby is due from your last menstrual period or conception date. Enter the date to see the estimated due date and trimester.
- Due date = LMP + 280 days (40 weeks)
- Conception ≈ LMP + 14 days
- Only about 5% of babies arrive on the exact due date
How due dates are estimated
The standard method counts 280 days (40 weeks) from the first day of the last menstrual period, assuming ovulation around day 14. Estimating from a known conception date adds 266 days instead. Ultrasound dating in early pregnancy is the most accurate confirmation.
Limitations to watch for
The estimate assumes a regular 28-day cycle and day-14 ovulation. Irregular cycles shift the real due date. Fewer than 1 in 20 babies arrive on their due date — it's an estimate, not a deadline. Providers may adjust the date based on early ultrasound.
How to use it in practice
Use the estimate to plan maternity leave, prenatal appointments, and the third-trimester window. Expect labor anywhere from about 3 weeks before to 2 weeks after the date. Confirm the dating with your provider's ultrasound.
['Choose the input mode (LMP or conception date).', 'Enter the date.', 'The tool returns the estimated due date and conception date.']
How this calculator works
Formula
Due date = last menstrual period (LMP) + 280 days (40 weeks). Conception date ≈ due date − 266 days. From conception date mode: due date = conception + 266 days.
Worked example
With LMP of January 1: due date = January 1 + 280 days = October 8. Conception ≈ January 16.
Assumptions to verify
- A regular 28-day cycle with ovulation at day 14.
- The pregnancy is a single baby (multiples often deliver earlier).
- The entered LMP or conception date is accurate.
Frequently asked questions
How is my due date calculated?
From your last menstrual period: add 280 days (40 weeks). From conception: add 266 days. The tool supports both.
Why 280 days?
A typical pregnancy is about 40 weeks from the first day of the last period — 280 days. That assumes ovulation at day 14 of a 28-day cycle.
How accurate is the due date?
Only about 5% of babies arrive on the exact date. A due date is an estimate; full-term labor ranges from about 37 to 42 weeks.
What if my cycle is irregular?
The LMP method assumes a regular 28-day cycle. Your provider may adjust the date using an early ultrasound, which is more accurate.
What is the conception date?
The estimated day of fertilization — about 14 days after the LMP, or 266 days before the due date.
When does the third trimester start?
Around 28 weeks. The tool shows the estimated trimester at the end of the calculation.
Do twins have a different due date?
The estimated due date is the same, but multiples often deliver earlier — typically planned around 37–38 weeks.
Cite this tool
BoringToolsKit. “Due Date Calculator.” boringtoolskit.com/due-date-calculator/ (reviewed 2026-08-25). Free to reference in articles, syllabi, and answer posts with a link.
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