Pregnancy Due Date Calculator
Enter the first day of your last menstrual period and average cycle length, or use a known conception date. The calculator returns an estimated due date, estimated conception date, current gestational age in weeks and days, and trimester boundaries. This is informational; use your clinician’s dating scan or assessment for medical decisions.
How the estimate is made
-
1
Choose the starting point
Use the first day of your last period or a known conception or ovulation date.
-
2
Apply standard pregnancy dating
The LMP method adds 280 days (40 weeks). The conception method adds 266 days.
-
3
Adjust for cycle length
In LMP mode, a cycle longer or shorter than 28 days shifts the estimate by the same number of days.
-
4
Show current gestation
Today's date minus the adjusted last-period date gives weeks plus days of gestation.
How the due date is estimated
The classic LMP calculation is usually called Naegele’s rule. It assumes a 28-day menstrual cycle, with ovulation around day 14.
Estimated due date = first day of last period + 280 days
= first day of last period + 9 months + 7 days
If you know the likely conception or ovulation date instead, the estimate is:
Estimated due date = conception date + 266 days
In LMP mode, the calculator also adjusts for cycle length. A 35-day cycle shifts the estimate 7 days later than a 28-day cycle; a 25-day cycle shifts it 3 days earlier.
Trimester milestones
| Gestational weeks | Trimester | Common checkpoints |
|---|---|---|
| 0-13 | First | Implantation, positive pregnancy test, early heartbeat, first-trimester dating scan when arranged |
| 14-27 | Second | Anatomy scan around 18-22 weeks, fetal movement often noticed between 16 and 25 weeks |
| 28-birth | Third | Growth checks, glucose screening where routine, GBS screening where recommended locally |
Why your clinician’s date may differ
- Early ultrasound dating. A first-trimester scan is often the most accurate way to establish gestational age, especially when period dates are uncertain.
- Irregular cycles. If your cycle is 35 days instead of 28, ovulation likely happened around day 21, not day 14. That shifts the due date about one week later.
- Assisted reproduction. With IVF or another documented fertility treatment, the transfer or procedure date may be the best clinical dating anchor.
What “due date” actually means
Only about 4-5% of babies arrive on the exact due date. Pregnancy is commonly described as lasting 37 to 42 weeks from the first day of the last period, and many babies arrive in the weeks around the estimate. Treat the date as a planning anchor, not a prediction.
Frequently Asked Questions
It is a useful average, not a guarantee. It works best for people with regular cycles who remember the first day of the last period. Early ultrasound dating can be more accurate for an individual pregnancy.
Enter your average cycle length. The calculator shifts the assumed ovulation day accordingly: a 35-day cycle means ovulation around day 21 instead of day 14.
Follow the due date documented by your clinician. This calculator only estimates from a last period or conception date, so it should not override a medical dating scan.
No. It is for planning and general education. Ultrasound findings, fertility-treatment dates, fundal height and clinical judgment should guide care.
No. Dates stay in your browser and are not transmitted.
Related Tools
BMI Calculator
Calculate body mass index from height and weight. Shows WHO category, healthy-weight range and limitations of BMI.
GPA Calculator
Calculate grade point average from letter grades and credit hours. Supports US 4.0 and 5.0 honors scales with the standard A/B/C/D letter table.
Inflation Calculator
Calculate the equivalent purchasing power of a dollar across any two years using historical CPI data from 1913 to today.
Scientific Calculator
Full scientific calculator with trig, log, exponents, factorial, memory, parentheses and DEG/RAD toggle. Runs in your browser.
Binomial Probability Calculator
Compute exact, cumulative and tail probabilities for a binomial distribution. Enter n trials, p success rate and k successes.
Calorie Calculator
Estimate daily calorie needs for your goal using Mifflin-St Jeor BMR and activity factors. Includes deficit and surplus targets.