How to use the Pregnancy Due Date Calculator
- Enter the first day of your last menstrual period.
- Enter your usual cycle length in days. Leave it at 28 if you are not sure or if your cycle is regular at about 28 days.
- The “check weeks as on” date starts as today. Change it to see the weeks for any day.
- Read the due date, the weeks of pregnancy, the trimester and the approximate conception date.
Formula used
Naegele’s rule: due date = first day of last period + 280 days (40 weeks), adjusted by (cycle length − 28) days for longer or shorter cycles.
Worked example
Here is the Pregnancy Due Date Calculator worked through with the example values shown above, so you can check each step by hand.
| Inputs | |
|---|---|
| First day of last period | 2026-08-01 |
| Usual cycle length | 28 days |
| Result | |
| Estimated due date | 8 May 2027 |
| Pregnancy on that date | 12 weeks, 6 days |
| Trimester | First trimester |
| Estimated conception around | 15 August 2026 |
Due date by cycle length
The same last period date with different cycle lengths. Longer cycles move the due date later.
| Usual cycle length | Estimated due date |
|---|---|
| 24 days | 4 May 2027 |
| 26 days | 6 May 2027 |
| 28 days | 8 May 2027 |
| 30 days | 10 May 2027 |
| 32 days | 12 May 2027 |
| 35 days | 15 May 2027 |
How the due date is estimated
This calculator uses Naegele’s rule: the due date is 280 days, which is 40 weeks, after the first day of the last period. For an LMP of 1 August 2026, that gives 8 May 2027. Pregnancy is counted from the LMP, so you are considered about two weeks pregnant at conception, which is why “40 weeks” covers roughly nine months of actual development.
If your cycle is longer or shorter than 28 days, ovulation happens later or earlier, so the due date shifts by the difference. With a 35-day cycle, the same LMP gives 15 May 2027, and with a 24-day cycle it gives 4 May 2027.
Weeks and trimesters
Pregnancy is divided into three trimesters. The first runs 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. On 30 October 2026, an LMP of 1 August means 12 weeks, 6 days, which is in the first trimester.
Doctors use weeks and days for scheduling scans and tests, for example the early scan, the detailed anomaly scan around the middle of the second trimester and the growth checks near the end.
How accurate is a due date?
Only a small minority of babies are born on the estimated date. Most births happen within a few weeks either side, and a full-term pregnancy is considered to run from 37 to 42 weeks. First pregnancies, previous deliveries, the baby’s sex and many other factors play a part.
If your periods are irregular, or you are not sure of the LMP, an ultrasound in the first trimester gives a more reliable estimate, and your doctor may adjust the date based on it.
What to do with the date
Use the estimate to plan appointments, leave from work and preparations, but treat it as a window rather than a deadline. Tell your doctor the LMP you used and the cycle length so they can compare it with scan findings.
This tool does not diagnose anything or replace medical advice. If you have pain, bleeding or any concern, contact a doctor straight away.
Frequently asked questions
How is the due date calculated from the last period?
By adding 280 days (40 weeks) to the first day of the last menstrual period, with an adjustment if your cycle is not 28 days.
Is the due date exact?
No. It is an estimate, and most babies are born within a few weeks of it.
What if my cycle is irregular?
The estimate may be off. An early ultrasound gives a better dating. Talk to your doctor.
How many weeks is a full-term pregnancy?
Usually 37 to 42 weeks, counted from the first day of the last period.
What does the conception date mean?
It is an approximate date of ovulation, about two weeks after the start of the last period for a 28-day cycle. It is not exact.
Can I use this to find out if I am pregnant?
No. It only estimates dates if you are pregnant. Use a pregnancy test and see a doctor.
This calculator gives estimates for information only. Actual figures from a bank, employer, tax authority or doctor may differ. It is not financial, tax, legal or medical advice.