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The estimated due date is traditionally calculated using the Naegele rule: 280 days (40 weeks) after the first day of the last period. If the conception date is known instead, this calculator counts 266 days (38 weeks) from that date - both methods land on the same assumed pregnancy length, just with a different starting point.
The calculation runs entirely locally in your browser, in pure JavaScript - nothing is uploaded and nothing is stored. Alongside the due date, the calculator uses today's reference date to show the current week of pregnancy (decimal, e.g. 12.4) and the days remaining until the due date. Change an input and everything updates instantly.
An honest note: this is a model-based estimate using a fixed formula, not medical advice. Only about 5 percent of births land exactly on the calculated date; irregular cycles, an uncertain conception date or a later ultrasound finding can change the actual estimate. For a reliable assessment, talk to a doctor or midwife.
Specifications
Specifications
Input formats
Form inputs (no file)
Processing
Locally in your browser (JavaScript)
File upload
None
In 3 steps
Choose the calculation method (last period or conception date).
Enter the corresponding date and today's reference date.
Read off the due date, week of pregnancy and days remaining.
Limitations: A model-based estimate using a fixed 280-/266-day formula, not medical advice. Only about 5 percent of births hit the calculated date exactly; cycle length, an uncertain conception date and individual factors are not accounted for. Talk to a doctor or midwife.
FAQ
Are my inputs uploaded?
No. The calculation runs entirely locally in the browser (pure JavaScript); nothing is sent or stored.
Is this medical advice?
No. This calculator only gives a rough, formula-based orientation and does not replace medical advice or examination.
Which method is more accurate?
Neither formula is inherently "more accurate" - both assume a fixed pregnancy length. In practice, a first-trimester ultrasound is usually considered a more reliable estimate.
Why does the actual birth often differ from the due date?
Because the formula assumes an average cycle length and pregnancy duration - individual variation is normal, and only about 5 percent of births land exactly on the calculated day.