How to use the due date calculator
- First day of last period (LMP) — pick the date your most recent menstrual period started, not when it ended.
- Average cycle length — leave it at 28 days, or enter your own typical length (the number of days from the start of one period to the start of the next).
- Read off your estimated due date, your gestational age today, and which trimester you are in — every figure updates the moment you change a value.
How the due date is worked out
The calculator uses Naegele's rule, the standard method used in clinics for over two hundred years: your due date is the first day of your last menstrual period plus 280 days, which is 40 weeks. Because that count starts at your last period — about two weeks before you actually conceive — a full-term pregnancy is described as 40 weeks even though the baby develops for roughly 38 weeks. If your cycle is longer or shorter than the textbook 28 days, you ovulate earlier or later, so the tool shifts the due date by the difference to keep the estimate realistic.
Gestational age and trimesters
Gestational age is how doctors and midwives describe how far along you are, counted in completed weeks and days from your last period. Pregnancy is then split into three trimesters: the first runs from week 0 to the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 until birth. Knowing your week helps you follow week-by-week guides, time your antenatal appointments and scans, and understand which developments to expect next.
How accurate is it?
Treat the result as a well-informed estimate rather than a fixed appointment. Only about 1 in 20 babies arrive on their exact due date, and most healthy births happen any time from 37 to 42 weeks. The estimate is most reliable when you know the date of your last period with confidence and your cycles are regular. If your periods are irregular, an early ultrasound dating scan will usually give a more precise date, and your healthcare provider may update the due date based on it.
Is it private?
Completely. The dates and cycle length you enter never leave your device — the whole calculation runs in JavaScript right inside this page, with no server call, no account and no tracking of what you type. Close the tab and nothing is saved.
Important: this tool is for general information only and is not medical advice, diagnosis or treatment. Always rely on the guidance of your doctor or midwife, whose dating scan takes priority over any estimate from a last-period date.
Frequently asked questions
Which date do I enter — the start or end of my period?
Enter the first day your last period started. The whole calculation, including gestational age and the due date, is measured from that day.
Why does changing my cycle length move the due date?
A longer cycle means you ovulate later, so conception and birth shift later too. The tool adds the gap between your cycle and the standard 28 days to the estimate, and a shorter cycle moves it earlier.
Can I use it if I do not know my exact last period date?
You can enter your best estimate to get a rough idea, but the result will be less reliable. In that case an early ultrasound dating scan is the most accurate way to set a due date.