Calculating your estimated due date is straightforward once you know the Pregnancy Due Date formula and what each input means. This guide explains the method in plain language, walks through a manual calculation, and gives worked examples you can follow — then you can do it instantly with the Pregnancy Due Date Calculator.
What is Pregnancy Due Date?
The Pregnancy Due Date calculation tells you your estimated due date from a few simple inputs. The figure you are solving for here is the estimated due date.
The Pregnancy Due Date formula
The core formula is:
Estimated due date = First day of last menstrual period + 280Here is what each input means:
- First day of last menstrual period — a calendar date. Example: 2025-01-01.
How to calculate it step by step
- Note the first day of last menstrual period (for example, 2025-01-01).
- Apply the formula above to get your estimated due date.
- Double-check the result with the Pregnancy Due Date Calculator.
Worked examples
Example 1
| Input / Output | Value |
|---|---|
| First day of last menstrual period | 2025-01-01 |
| Estimated due date | 08 Oct 2025 |
| Estimated conception date | 15 Jan 2025 |
| Second trimester begins | 09 Apr 2025 |
| Third trimester begins | 16 Jul 2025 |
With first day of last menstrual period of 2025-01-01, the estimated due date works out to 08 Oct 2025.
Tips for an accurate result
- Keep your units consistent — mixing, say, months with years or grams with kilograms is the most common source of error.
- Round only at the very end. Rounding inputs early can shift the final answer noticeably.
- Re-run the numbers whenever an input changes, rather than estimating from an old result.
- These figures are general estimates, not medical advice — check with a qualified professional before acting on them.
Prefer not to do the maths by hand? — the Pregnancy Due Date Calculator does it instantly, for free, with the formula and a worked example built in.
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