What Is an LMP Calculator?
An LMP calculator estimates your pregnancy due date and current gestational age based on the first day of your last menstrual period (LMP). Because the exact date of conception is rarely known, healthcare providers commonly date a pregnancy from the LMP, which is a reliable and easy reference point. Enter the first day of your last period and the calculator gives your estimated due date (EDD) and how many weeks and days pregnant you are.
How to Use the LMP Calculator
- Enter the first day of your last menstrual period.
- Optional: adjust your average cycle length if it differs from 28 days.
- Calculate — see your estimated due date and current gestational age.
How Due Date Is Calculated (Naegele's Rule)
The standard method is Naegele's rule: add one year, subtract three months, and add seven days to the first day of your LMP — which is equivalent to adding 280 days (40 weeks). For example, an LMP of January 1 gives an estimated due date of October 8.
Pregnancy Timeline from LMP
| Milestone | Weeks from LMP |
|---|---|
| Estimated conception | ~2 weeks |
| First trimester ends | 13 weeks |
| Second trimester ends | 27 weeks |
| Full term | 39–40 weeks |
| Estimated due date | 40 weeks (280 days) |
Why LMP Dating Is Used
Gestational age is measured from the LMP rather than conception because most people know when their last period started but not the precise day of ovulation or fertilization. This LMP-based age is the standard used in prenatal care, ultrasound comparisons, and screening test timing. An early ultrasound may later refine the due date if it differs significantly from the LMP estimate.
Adjusting for Cycle Length
Naegele's rule assumes a 28-day cycle with ovulation around day 14. If your cycles are consistently longer or shorter, ovulation shifts, and the due date can be adjusted accordingly. Longer cycles push the due date slightly later; shorter cycles bring it earlier.
Note: This calculator provides an estimate for general information and does not replace professional prenatal care.
Frequently Asked Questions
How do you calculate a due date from LMP?
Add 280 days (40 weeks) to the first day of your last menstrual period, or use Naegele's rule: add one year, subtract three months, and add seven days.
What does LMP stand for?
LMP stands for Last Menstrual Period — specifically the first day of your most recent period, which is the reference point for dating a pregnancy.
How accurate is the LMP due date?
It is a good estimate for those with regular 28-day cycles. Only about 1 in 20 babies arrive on the exact due date; an early ultrasound can refine the estimate.
What if my cycle is not 28 days?
If your cycle is longer or shorter, ovulation timing changes and the due date shifts accordingly. Enter your average cycle length so the calculator can adjust.
Is this LMP calculator free?
Yes — it is completely free, requires no signup, and gives your due date and gestational age instantly.