Pregnancy Due Date Calculator
A pregnancy due date calculator estimates an expected delivery date from the first day of your last menstrual period, often shortened to LMP. This page uses the standard 280-day dating method, which is the same basic approach many due date wheels and pregnancy trackers start with. Enter one calendar date, and the tool returns an estimated due date plus an estimated conception date for planning reference. This estimate is most useful when you want a quick timeline for early appointments, maternity leave conversations, travel planning, or personal record keeping. It works best when you know your LMP with reasonable confidence and your cycle is close to the standard model used for pregnancy dating. If your cycle length varies a lot, if you are not sure which bleeding day counts as the true start of the last period, or if a clinician has already dated the pregnancy with ultrasound, use this result as a general reference instead of a final answer.
Quick answer
The calculator dates pregnancy from the first day of the last menstrual period, not from the day of conception.
What this tells you
- •The calculator dates pregnancy from the first day of the last menstrual period, not from the day of conception.
- •It adds 280 days to the LMP date to estimate the due date, which equals 40 weeks of gestation.
- •It also estimates conception as 14 days after LMP because the standard model assumes ovulation around day 14 of a 28-day cycle.
- •This is a planning estimate, so the exact delivery date can still fall earlier or later.
- •If a healthcare professional gives you a different gestational age based on ultrasound or treatment dates, that clinical dating usually carries more weight.
How to Use
- 1Enter the first day of your last menstrual period in the date field. Use the earliest day bleeding started, not the day the period ended.
- 2Check that the date is correct before you calculate. A one-day input mistake shifts the estimated conception date and the estimated due date by one day too.
- 3Review the estimated due date as the main result. This is the date the tool gets by counting 280 days forward from the LMP date.
- 4Review the estimated conception date as a secondary reference. The tool places conception 14 days after LMP because that is the assumption built into the formula.
- 5Use the result for rough planning only, then confirm pregnancy dating with a qualified healthcare professional, especially if your cycles are irregular, your dates are uncertain, or fertility treatment was involved.
How It Works
Formula
Due date = LMP + 280 days
Estimated conception date = LMP + 14 daysThis pregnancy due date calculator follows the exact logic in the linked formula file. First, it reads the LMP date as a calendar date in UTC so the day stays stable. Next, it adds 280 days, which equals 40 weeks, to produce the estimated due date. It also adds 14 days to estimate conception. That second step reflects the standard assumption that ovulation happens about two weeks after the start of the last menstrual period in a 28-day cycle. Plain-English version: count forward 14 days from the first day of the last period for an estimated conception date, then count forward 280 days for the estimated due date. Example: if LMP is 2026-01-01, adding 14 days gives 2026-01-15 and adding 280 days gives 2026-10-08. The method is simple and widely used for a first estimate, but it is still an estimate because real ovulation timing, implantation timing, and pregnancy dating can differ from the standard model.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
LMP on January 1, 2026
Start with the first day of the last menstrual period, which is 2026-01-01. Add 14 days to estimate conception and you get 2026-01-15. Add 280 days to estimate the due date and you get 2026-10-08. This is a clean example of the standard 40-week dating method with no month-end or leap-year complications.
LMP on March 15, 2026
Using 2026-03-15 as the LMP, the estimated conception date lands 14 days later on 2026-03-29. Counting forward 280 days gives 2026-12-20 as the estimated due date. This example shows how the tool handles a date that stays within the same calendar year while still crossing several months with different lengths.
LMP on July 4, 2026
For an LMP of 2026-07-04, add 14 days to get an estimated conception date of 2026-07-18. Then add 280 days to reach an estimated due date of 2027-04-10. This is a good reminder that pregnancies often span two calendar years, so a summer LMP can still lead to a spring due date the next year.
Leap-year LMP on February 29, 2024
A leap-day LMP is valid in 2024, so the calculator can use it normally. Adding 14 days gives an estimated conception date of 2024-03-14, and adding 280 days gives an estimated due date of 2024-12-05. This example matters because it shows the date logic is based on actual calendar days, not a simplified month count.
LMP on August 31, 2026
Month-end dates can make mental math harder, so they are useful to test. Starting from 2026-08-31, the estimated conception date is 2026-09-14 after adding 14 days. The estimated due date is 2027-06-07 after adding 280 days. If you were counting by hand, this is the kind of example where a date-based calculator helps prevent off-by-one mistakes.
Key date assumptions used by this estimate
These checkpoints explain what the calculator is doing with your LMP date.
| Checkpoint | Offset from LMP | What it means |
|---|---|---|
| Entered LMP date | 0 days | The first day of the last menstrual period you type into the field |
| Estimated conception date | 14 days | A planning estimate based on ovulation around day 14 of a 28-day cycle |
| Estimated due date | 280 days | The standard 40-week due date estimate counted from LMP |
| Gestational age at due date | 40 weeks | The conventional length used for LMP-based pregnancy dating |
| Possible real-world variation | Earlier or later | Actual birth timing and clinical dating can differ from this model |
The table describes the dating assumptions in this tool. It does not predict labor timing, fetal growth, or pregnancy health.
When an LMP-based due date is most useful
An LMP-based estimate is usually most helpful at the very start of pregnancy, especially before a formal dating visit. It gives you a working timeline for appointment scheduling, pregnancy milestones, and general planning without asking for more information than one remembered calendar date.
This method is strongest when the last menstrual period date is known and the cycle pattern is fairly regular. If ovulation tends to happen much earlier or later than day 14, the estimated conception date can shift, and that shift can make the due date estimate feel off when compared with later clinical dating.
Irregular cycles, recent birth control changes, breastfeeding-related cycle changes, fertility treatment, spotting that is mistaken for a period, and uncertain recall can all reduce confidence in an LMP-based result. In those cases, the estimate still gives a rough anchor, but it should not be treated as the final word.
If you already have a clinician-provided due date from ultrasound or embryo transfer timing, use that medical dating for care decisions and documentation. This tool remains useful as a quick educational reference, but it should not override a personalized assessment.
Common mistakes
- Entering the last day of the period instead of the first day, which shifts every estimated date later than it should be.
- Using the date of a positive pregnancy test as the starting point even though the formula only works from LMP.
- Treating the estimated conception date as exact even though ovulation can happen earlier or later than day 14.
- Assuming the due date is the day birth will happen rather than a midpoint estimate around a broader real-world range.
- Ignoring later clinical dating information from ultrasound or fertility treatment records when those sources are more precise for an individual pregnancy.
Limitations
This estimate assumes you know the first day of your last menstrual period, ovulation happened about 14 days later, and pregnancy length is measured as 280 days from LMP. It does not adjust for shorter or longer cycles, uncertain bleeding dates, conception through fertility treatment, pregnancy dating based on embryo transfer, or clinician-led redating after ultrasound. It also does not estimate gestational age today, labor onset, birth timing, fetal growth, or pregnancy health. Use it as a calendar estimate for planning, not as a substitute for personalized pregnancy dating.
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