Use the free Pregnancy Due Date Calculator below: your data never leaves your browser — all work happens on your device, and nothing is uploaded to any server. Results are educational estimates only, not medical advice.
What the pregnancy due date calculator does
If you are pregnant, or hoping to be, one of the first dates you want is your estimated due date. This free calculator gives you that date from the first day of your last menstrual period, using the standard 280-day rule that doctors have used for generations. It also shows an estimated conception date, the approximate ends of your first and second trimesters, how many weeks pregnant you are today, and how many days are left until the due date. The result is an educational estimate, not medical advice, and your own doctor or midwife is the right person for decisions about your care.
The maths behind it is simple. A typical pregnancy lasts about 40 weeks from the first day of the last period, which is 280 days. Because not every cycle is 28 days long, the tool adjusts the result when you give a different cycle length: a longer cycle pushes the due date later, and a shorter cycle pulls it earlier. That adjustment is approximate, but it is better than ignoring cycle length altogether.
How to use it
Enter the first day of your last menstrual period, using the calendar picker. If you know your usual cycle length and it is not 28 days, change that number too. Press Calculate. The big result is your estimated due date. Below it you will see the estimated conception date, which is roughly two weeks after the last period for a 28-day cycle, the trimester boundaries, and your current week and day count, worked out from today date.
If you are not sure of the first day of your last period, the result will be less reliable, and that is normal. Many people are unsure, especially if their periods are irregular. In that case treat the date as a rough guide and let an early ultrasound, if your care provider offers one, give a more precise estimate.
Why due dates move
A due date is an estimate, not an appointment. Only about one baby in twenty arrives exactly on the due date. Most arrive within the two weeks before or after it, and that range is completely normal. Doctors know this, which is why they talk about a due window rather than a single day, even though a single date is easier to plan around.
Several things can shift the estimate. Ovulation does not always happen on day fourteen, cycles vary, and the date of the last period can be misremembered by a few days. An early ultrasound measures the baby and can refine the date, which is why the date from a scan sometimes differs from the date calculated from the last period by a few days. Neither is wrong; they are two estimates of the same thing, and your care provider will tell you which one to use.
A worked example
Suppose the first day of your last period was 1 January 2026 and your cycle is 28 days. Add 280 days and the estimated due date is 8 October 2026. The estimated conception date is around 15 January, the first trimester ends around 2 April, and the second trimester ends around 9 July. If your cycle were 30 days instead, the due date would move two days later, to 10 October. Small changes in cycle length make small changes in the date, which is why the cycle box matters.
Common questions
Is 40 weeks the same as nine months? Roughly, but not exactly. Forty weeks is about nine calendar months plus a week, because calendar months are longer than four weeks. That is why counting in weeks is clearer than counting in months during pregnancy.
What if my periods are irregular? Then this estimate is rougher than usual, because the tool assumes a regular cycle. Talk to your doctor or midwife early, since they may use a scan to date the pregnancy instead.
Does the due date mean the baby must arrive by then? No. Many pregnancies go a little past the due date, and your care provider will monitor you and discuss options if that happens. Do not try to hurry things along on your own based on a calculator result.
Can I use this to plan work leave? You can use it as a starting point for planning, but confirm the dates with your care provider and your employer rules, because leave dates usually depend on the official expected date and on local law, not on a web estimate.
An important note
This tool gives educational estimates only. It is not medical advice, it cannot diagnose anything, and it cannot replace antenatal care. If you have pain, bleeding, worrying symptoms, or any concern about your pregnancy, contact your doctor, midwife or local emergency service promptly. Early care matters, and no calculator can substitute for a professional who knows your history.
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