How to use the due date calculator
- Choose whether you're calculating from your last period, a known conception date, or a due date you've already been given.
- Enter the date, and adjust your cycle length if it isn't the standard 28 days.
- See your estimated due date, current week and trimester, plus milestone dates.
How due dates are calculated
The standard method is Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. Pregnancy is counted from that day rather than conception, which is why you're considered "2 weeks pregnant" at conception. If your cycle is longer or shorter than 28 days, ovulation shifts accordingly, so this calculator adjusts for your cycle length. A dating ultrasound in the first trimester remains the most accurate method and may revise the estimate.
Benefits
- Estimates a due date from your last period, a conception date or a known due date.
- Shows how far along you are in weeks and days, plus your trimester.
- Lists milestone dates including the start of full term at 37 weeks.
- Adjusts for cycle lengths other than 28 days.
Limitations to know
- It is an estimate — only about 4% of babies arrive on their exact due date.
- An early dating ultrasound is more accurate than any date-based calculation.
- It cannot account for irregular cycles or individual medical factors.
Common mistakes to avoid
- Entering the end of your last period instead of the first day.
- Treating the due date as a deadline rather than the middle of a likely range.
- Using it in place of the dates your midwife or doctor gives you.
Alternatives
To count days to any date use Days Between Dates; to count down to the big day use the Countdown Timer.
Last updated: August 2026 · Reviewed by the AI Toolbox editorial team.
Frequently asked questions
How accurate is a due date?
Only about 4% of babies are born on their exact due date. Most arrive within two weeks either side — it's a best estimate, not a deadline.
Why count from my last period?
Because the LMP date is usually known, while the exact moment of conception rarely is. It's the standard clinical convention.
What if my cycle isn't 28 days?
Enter your actual cycle length — the calculator shifts the estimated ovulation date accordingly.
Is my data private?
Yes — everything is calculated in your browser and nothing is uploaded or stored.