About the Due Date Calculator
Due Date Calculator helps estimate the key numbers involved in pregnancy decisions. It is designed for quick scenario comparison: enter a realistic set of values, calculate the result, then change one assumption at a time to see what has the greatest effect. Unlike a static table, the result responds to your inputs and keeps the calculation in your browser. Health and fitness results are population-based estimates. They can support a conversation or planning process, but they do not determine an individual diagnosis, treatment, or safe activity level.
How to use this calculator
Enter values that match your situation and select Calculate result. Review the main result and its supporting figures, then change one input at a time to compare scenarios. Results are rounded for readability while the calculation retains additional precision internally.
Information you will need
- Calculation Method
- Date
- Cycle Length
- Ultrasound Weeks
- Embryo Age
How the calculation works
Calculate due date and gestational age from 5 methods: Last Period, Due Date, Conception Date, Ultrasound Date, or IVF Transfer Date. Read the primary result together with the supporting values rather than focusing on one number alone. A useful estimate should make its assumptions visible. If the answer looks surprising, verify the unit, rate, time period, and whether the entered value is gross or net. Run a conservative and an optimistic scenario to understand the range of possible outcomes.
Formula or method
Due = LMP + 280 days (40 weeks). Gestational age from LMP. Cycle length adjusts for variations.
Worked example
Enter any date to calculate due date and current week.
The example is illustrative rather than a recommendation. Use your own verified values and keep all monetary or measurement units consistent. When comparing alternatives, save or note each result so the assumptions do not become mixed.
How to interpret the result
The primary output answers the main question posed by the due date calculator, while the additional cards provide context. A result with many decimal places is not necessarily more certain. The displayed precision makes comparison easier, but uncertainty in the inputs can be larger than the rounding difference.
Look for trends and ranges instead of treating a single estimate as a pass or fail result. If the output conflicts with symptoms, professional advice, or a direct measurement, rely on appropriate professional assessment.
Limitations and important notes
The due date calculator cannot account for every factor that affects an individual result. Measurement technique, hydration, body composition, medication, pregnancy, illness, training status, and normal biological variation may matter. Do not use an estimate to diagnose a condition, select medication dosage, decide whether to drive, or ignore symptoms. A qualified professional can interpret measurements in the context of history and appropriate testing.
Frequently asked questions
How is my due date calculated from my last period?
Using Naegele's rule, add 280 days (40 weeks) to the first day of your last menstrual period. This assumes a 28-day cycle with ovulation on day 14. The calculator adjusts for your actual cycle length: a 30-day cycle shifts the date about two days later, a 35-day cycle about seven days later. It is a solid starting point, but only about 4-5% of babies arrive exactly on their due date, so treat it as a target window rather than a deadline. Your provider can refine it.
Which is more accurate: ultrasound dating or a last-period calculation?
In most cases, a first-trimester ultrasound is more accurate. Measuring crown-rump length before 14 weeks dates a pregnancy within about 5-7 days, while LMP-based dating assumes a textbook 28-day cycle and can be off by more, especially with irregular periods. The American College of Obstetricians and Gynecologists recommends first-trimester ultrasound as the most accurate dating method and suggests updating the due date when it differs from the LMP calculation by more than 5-7 days. LMP remains a useful starting point before your first scan.
Why does my due date sometimes change after a scan?
Due dates can shift because scan measurements refine gestational age. If a first-trimester ultrasound differs from your LMP date by more than about 5-7 days, providers typically update the due date to match the scan, which is most accurate between about 8 and 13 weeks. Third-trimester scans are much less reliable for dating (uncertainty of roughly 2-4 weeks) and usually do not change an already confirmed due date. Small day-to-day differences between apps are also normal and not a cause for concern.
Can I calculate my due date from my conception or IVF transfer date?
Yes. If you conceived through IVF, add 266 days (38 weeks) to the transfer date, or use 263 days for a day-3 embryo and 261 days for a day-5 embryo. This is the most precise dating available. If you tracked ovulation and know your conception date, add 266 days from that date as well. Both methods remove the guesswork of cycle length and ovulation day, so they are more accurate than an LMP calculation for anyone who has those dates available.
How early can a dating ultrasound set my due date?
A dating ultrasound is typically performed between about 8 and 14 weeks, with many guidelines pointing to the 11-14 week window as most accurate. The embryo can often be measured as early as 6-7 weeks, but accuracy is best in the first trimester, when embryos grow at very similar rates. After about 20 weeks, natural variation in fetal size makes ultrasound dating progressively less reliable. Your provider schedules this early scan partly to confirm or set your estimated due date.
What counts as full term, and how late is 'late'?
Full term is defined as 37 to 42 weeks of pregnancy. Early term is 37-38 weeks, full term 39-40 weeks, late term 41 weeks, and post-term 42 weeks or beyond. Only about 4-5% of babies are born exactly on their due date; most healthy babies arrive within one to two weeks before or after it. So your due date is a planning anchor, and 'being late' is generally not a concern until you pass about 41 weeks. Discuss timing and induction options with your provider.
My cycle is longer than 28 days. How does that change my due date?
The standard 280-day (40-week) calculation assumes a 28-day cycle. If your cycle averages 32 days, ovulation happens about four days later than day 14, so your due date should move about four days later. You can adjust manually: cycle length minus 28 gives the number of extra (or fewer) days to add to the date. Entering your true average cycle length lets this calculator apply that adjustment automatically. For irregular cycles, ultrasound dating is a more reliable way to set the due date.