Estimated due date calculator
Estimated birth date adjusted to your actual cycle length.
Calculator inputs
Results
Enter your values and press “Calculate” to see the result.
In short
- What it calculates
- Estimated birth date adjusted to your actual cycle length.
- Formula used
EDD = last period + 280 days + (cycle − 28)- Example
- Last period on 2026-02-01 with 30-day cycles:
Due date calculator
Naegele's rule places birth 40 weeks after the last period, but it assumes 28-day cycles. If yours is longer or shorter, the date shifts.
Here you can enter your real cycle length to refine the estimate.
How it works
280 days are added to the last period date and corrected by the difference between your cycle and the standard 28 days. Conception is estimated at ovulation, about 14 days into the cycle.
Formula
EDD = last period + 280 days + (cycle − 28)
Conception ≈ last period + 14 days + (cycle − 28)
Worked example
Last period on 2026-02-01 with 30-day cycles:
Adjustment: +2 days
EDD: 2026-11-10
Estimated conception: 2026-02-17
Explanation
Naegele's rule is two hundred years old
The standard calculation adds 280 days to the first day of the last menstrual period, a rule formulated by the German obstetrician Franz Naegele in the early nineteenth century. It assumes twenty-eight day cycles with ovulation on day fourteen, which does not hold for many women.
Almost nobody gives birth on the due date
Only around four or five per cent of births happen exactly on the estimated date. Most are spread across the two weeks before and the week after. That is why professionals increasingly speak of a 'birth window' rather than a specific date, and some countries have begun communicating a range.
If your cycles are not twenty-eight days
Naegele's rule shifts the date if your cycle is longer or shorter. With thirty-five day cycles, ovulation comes a week later and the estimated date should move back accordingly. The first-trimester scan corrects this automatically.
What the date is for
More than predicting the day, it serves to schedule check-ups, tests and clinical decisions: when to do each scan, when a birth counts as premature and when to consider induction.
Frequently asked questions
How many babies arrive on the due date?
Fewer than 5%. Most arrive within the two weeks either side of it.
Which date counts, the scan or this one?
The first trimester scan, which is the clinical reference. This calculator is a useful prior estimate.
What percentage of babies arrive on the due date?
Around four or five per cent. Most arrive in the window from two weeks before to one week after.
What if my cycles are longer than 28 days?
The calculated date falls earlier than the real one. The first-trimester scan is the reliable reference in that case.
Need to calculate something else?
These tools are often used alongside this calculator.
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