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Pregnancy Due Date Calculator

A free online pregnancy due date calculator that estimates your due date from your last menstrual period (with cycle-length adjustment) or your conception date. Shows your current gestational age, trimester, estimated conception date, and full-term range.

How the due date is estimated

The most common method, called Naegele's rule, adds 280 days (40 weeks) to the first day of your last menstrual period. If you know your conception date instead, the calculator adds 266 days (38 weeks), which accounts for the roughly two weeks between the start of your last period and ovulation.

Adjusting for cycle length

Naegele's rule assumes a standard 28-day menstrual cycle with ovulation around day 14. If your average cycle is longer or shorter, the calculator shifts the due date and estimated conception date by the difference, since ovulation — and therefore conception — happens later in a longer cycle and earlier in a shorter one.

Gestational age, trimesters, and full-term range

Gestational age is measured from the first day of your last period, not from conception, which is why it's typically about two weeks ahead of your baby's actual age. The first trimester runs through week 12, the second through week 27, and the third from week 28 onward. A pregnancy is considered full-term between 37 and 42 weeks — the due date itself is just the midpoint estimate, not a fixed delivery date.

Common Mistakes to Avoid

Treating the due date as a guaranteed delivery date

Solution:

Only a small percentage of babies are born on their exact estimated due date — the full-term range (37–42 weeks) is a far more realistic window.

Forgetting that gestational age counts from the last period, not conception

Solution:

This is why a pregnancy is described as '40 weeks' even though conception happens roughly two weeks after the last period started — gestational age and fetal age aren't the same clock.

Ignoring cycle length when it is significantly different from 28 days

Solution:

A consistently longer or shorter cycle shifts the actual ovulation date, and the calculator's cycle-length adjustment accounts for this — leaving it at the default 28 when your cycle is very different will skew the estimate.

Assuming the conception-date method is more accurate than the last-period method

Solution:

Both methods are equally standard; the conception-date method is only more precise if the conception date is actually known, rather than estimated.

Using this calculator as a substitute for medical confirmation

Solution:

An ultrasound dating scan, typically done in the first trimester, is more accurate than a calendar-based estimate and is what healthcare providers use to confirm or adjust a due date.

Not accounting for irregular cycles

Solution:

This calculator assumes a regular, predictable cycle length — if your cycles vary significantly month to month, the estimate is less reliable and should be confirmed with a healthcare provider.

Frequently Asked Questions

By default, it's 280 days (40 weeks) from the first day of your last menstrual period, adjusted for your average cycle length if it differs from 28 days. If you use the conception-date method instead, it's 266 days (38 weeks) from conception.
It's an estimate, not a guarantee — only a small percentage of babies are born on their exact due date. Most arrive within the full-term range of 37 to 42 weeks.
Gestational age is counted from the first day of your last menstrual period. Fetal age (conceptional age) is counted from conception, which is roughly two weeks later — so gestational age is always about two weeks ahead of fetal age.
A longer cycle means ovulation — and conception — happens later than day 14, so the due date shifts later too. A shorter cycle shifts it earlier. Entering your actual average cycle length improves the estimate.
The first trimester runs from week 1 through week 12, the second from week 13 through week 27, and the third from week 28 through delivery.
A pregnancy is considered full-term between 37 and 42 weeks of gestation — the due date itself is the midpoint estimate of that range, not a fixed target.
No — every calculation runs locally in your browser. Nothing you enter is uploaded, logged, or stored on any server.

Free Online Pregnancy Due Date Calculator

Estimate your due date using either your last menstrual period or a known conception date. See your current week of pregnancy, trimester, estimated conception date, and the full-term range your baby is likely to arrive within.

What You Get

  • An estimated due date based on Naegele's rule (280 days from your last period) or 266 days from conception
  • An adjustment for average cycle length if it's longer or shorter than 28 days
  • Your current gestational age in weeks and days, and which trimester you're in
  • An estimated conception date
  • A full-term range (37–42 weeks) showing when delivery is medically considered full-term

Who Uses This Calculator

Anyone who is pregnant or expecting and wants a quick estimate of their due date and current stage of pregnancy, to discuss and confirm with their healthcare provider.