Due Date Calculator
Estimate your due date from the first day of your last period, and see how many weeks along you are.
How it works
A compact workflow from input to download.
Enter your figures
Fill in first day of last menstrual period and average cycle length (days). Every value stays in your browser — nothing is sent to a server.
Read the result
The result updates as soon as your inputs are valid, with the headline figure highlighted and the supporting numbers broken out beneath it.
Change the inputs and compare
Adjust any value to see immediately how it moves the result — the quickest way to understand which input the outcome is actually most sensitive to.
Frequently asked questions
How is the due date calculated?
Why is it counted from my last period rather than conception?
How likely am I to give birth on the due date?
Are my numbers sent anywhere?
A date that is really a range
The estimated due date is one of the most misunderstood numbers in medicine, largely because it is presented as a single day. In reality only around one baby in twenty-five arrives on it, and a full-term birth is defined as anywhere from 37 to 42 weeks — a five-week window. The date is a statistical midpoint, useful for scheduling scans, tracking growth against expected milestones and deciding when intervention might be warranted. It is not a prediction of when labour will start. Understanding this in advance spares a great deal of anxiety in the final weeks, when a baby who has not arrived 'on time' is very often entirely normal and simply somewhere else in the distribution.
Why dating by ultrasound is more reliable
Naegele's rule assumes a textbook 28-day cycle with ovulation on day 14, and a great many people do not have one. If you ovulate on day 20, your dates are pushed by nearly a week and every subsequent milestone shifts with them. This is why clinicians increasingly rely on an early ultrasound for dating: in the first trimester, embryos grow at a remarkably consistent rate, so measuring the crown-rump length estimates gestational age within a few days regardless of cycle length. If your ultrasound dating differs materially from your last-period dating, the ultrasound is generally the one your care team will trust, and it is worth knowing that in advance rather than being surprised by it.
This is an estimate, not medical advice
A calculator works from a formula and a handful of numbers, and a formula cannot examine you. The results here are population-level estimates derived from studies of large groups, and any individual can sit far from the average for entirely healthy reasons. They are a useful starting point for a conversation and a reasonable way to track a trend over time, but they are not a diagnosis and they cannot account for your medical history, medication, body composition or anything else a clinician would consider. If a result concerns you, or if you are making a decision about your health, speak to a doctor rather than to a web page.
Related tools
Continue the workflow with adjacent tools.