Due Date & Pregnancy Week Calculator
Enter the first day of your last period, your ovulation date, or a known due date to get the estimated due date (Naegele's rule, 280 days), your current gestational age in weeks and days, the pregnancy month, and the dates of key milestones such as the start of full term. If your cycle is not 28 days long, enter its length to adjust the result. Everything runs in your browser and nothing you type is sent to a server. To count days between any two dates, try the Date Calculator, or check another health metric with the BMI Calculator.
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These results are estimates from standard formulas and are not a substitute for medical advice or diagnosis. Your actual due date is set by your healthcare provider, usually from an ultrasound. Please consult your doctor or midwife about your health and the course of your pregnancy.
The standard is 28 days. Enter a value from 21 to 45 and the due date shifts by the difference from 28.
How to use
- Under "Calculate from", choose which date you know: last period, ovulation, or due date.
- Enter the date. When calculating from your last period, also enter your cycle length if it is not 28 days.
- Change the reference date (today by default) to see the gestational age on any day.
- The due date, gestational age, and milestone dates appear. Treat them as estimates only.
Notes
- These results are estimates only and do not replace medical advice or diagnosis. Your provider sets the actual due date and gestational age, usually using ultrasound.
- When calculating from your last period, a cycle length other than 28 days shifts the due date by the difference. Irregular cycles make the estimate less reliable.
- When calculating from ovulation, the due date is ovulation + 266 days (week 0 day 0 is 14 days before ovulation). For IVF or other assisted conception, dating works differently, so follow your clinic's guidance.
- The pregnancy month counts 4 weeks per month and tops out at month 10 (weeks 36-39). Terms like the "stable period" vary from person to person.
- Your input is processed only in your browser and is never saved or sent anywhere.
Glossary
Naegele's rule
The classic way to estimate a due date from the first day of the last menstrual period: subtract 3 months and add 7 days, assuming a 28-day cycle. It is equivalent to adding 280 days (40 weeks). This tool adds days directly, so it can differ by a day in some years.
Gestational age
Pregnancy is dated from the first day of the last period (adjusted for cycle length), which is week 0 day 0. Conception happens about two weeks later, so a newly conceived pregnancy is already at about 2 weeks 0 days.
Preterm, term, post-term
Classification by timing of birth: term is 37w0d to 41w6d, preterm is 22w0d to 36w6d (before 37 weeks), and post-term is 42w0d or later.
Stable period
An informal Japanese term for roughly week 16 onward, when many people feel the early-pregnancy risks have eased. It is not a precise medical definition and varies by person.
FAQ
How is the due date calculated?
It adds 280 days (40 weeks) to the first day of your last period, which is equivalent to Naegele's rule. If your cycle is not 28 days, the date shifts by the difference. From an ovulation date, it adds 266 days.
What if my cycles are irregular?
With irregular cycles, a last-period estimate can be off by a lot. If you know your ovulation date, use that instead, or enter the due date your clinic gave you under "Known due date" to see the current week.
How are pregnancy weeks and months counted?
Weeks are counted from the first day of the last period (adjusted for cycle length) as week 0 day 0. Months are counted as 4 weeks each, so weeks 0-3 are month 1 and weeks 36-39 are month 10.
Why does my doctor give a different due date?
Providers often set the due date from ultrasound measurements, which can differ from the formula by several days or more than a week. This tool gives estimates and is not medical advice, so follow your provider.
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