How to Calculate a Pregnancy Due Date
Short answer
Naegele’s rule: due date ≈ first day of last menstrual period + 280 days (40 weeks). Example: LMP 1 January → about 8 October the same year.
A due date is an estimate, not a deadline. Clinicians refine it with ultrasound. For a quick LMP-based date, use the due date calculator.
Naegele’s rule
Add 280 days to the first day of the last menstrual period (LMP), assuming a 28-day cycle. Irregular cycles need clinical dating.
Worked example
LMP = 12 March 2026. Adding 280 days lands near 17 December 2026. Confirm with your midwife or doctor.
Counting weeks
Pregnancy is usually counted in weeks from LMP. The date difference calculator can show days between two dates; the age calculator is for birthdays after delivery.
Important
This guide is educational only. Seek prenatal care for personal medical advice.
See also: how to calculate age from date of birth.
See also: how to count business days between dates.
See also: how to calculate sleep cycles.
More worked numbers
Change one input at a time so you build intuition. Write the formula, substitute values, then confirm with the free due date calculator. If the tool disagrees with your hand math, check units first (months vs years, percent vs decimal, inclusive ranges).
Common mistakes to avoid
- Mixing up percent and decimal forms (15% is 0.15, not 15).
- Using the wrong base when calculating a change or a reverse percentage.
- Ignoring fees, tax or daylight saving when the real-world total depends on them.
- Rounding too early in multi-step arithmetic.
How to practise
Invent three mini-problems from your own life — a bill, a grade, a trip, a loan quote — and solve them on paper before you open the calculator. Teaching the steps to someone else is one of the fastest ways to lock in the method.
Related tools on ViralScopeHub
After you finish this guide, explore the linked tools above and the related articles for neighbouring skills. Our calculators run privately in your browser with no sign-up, so you can check homework, budgets or work estimates without uploading data.
Quick reference card
Keep the short answer box at the top of this page as your cheat sheet. When a search result brought you here for a specific phrase, that box is designed to match the query with the formula first, then the examples below fill in the judgement calls.
Frequently asked questions
How is a due date calculated?
Often LMP + 280 days, then adjusted with ultrasound if needed. Double-check units and confirm with the linked tool on this page before you rely on the figure for money or school decisions.
Is the due date exact?
No. Only a minority of births occur on the estimated day. Double-check units and confirm with the linked tool on this page before you rely on the figure for money or school decisions.
What if I do not know my LMP?
An early dating scan is the usual alternative — ask your clinician. Double-check units and confirm with the linked tool on this page before you rely on the figure for money or school decisions.
Can I calculate it online?
Yes — our due date calculator uses the LMP method for estimates. Double-check units and confirm with the linked tool on this page before you rely on the figure for money or school decisions.