Free Online Due Date Calculator
Calculate your estimated due date (EDD) and track your current week of pregnancy using your last period, conception date, or IVF transfer details. This accurate due date calculator supports both US Imperial units and UK/EU Metric units, in alignment with standards from the CDC, NHS, and WHO.
How Your Due Date is Calculated
A standard human pregnancy typically spans 280 days (40 weeks) from the first day of the last menstrual period (LMP), or 266 days (38 weeks) from conception. The exact date of delivery can be calculated using several standard obstetric methods:
1. Naegele's Rule (Last Menstrual Period)
Naegele's rule is the traditional formula for establishing due dates. It is based on a standard 28-day menstrual cycle. The rule calculates the due date by adding 280 days (or subtracting 3 months and adding 7 days, then adding 1 year) to the start of the last menstrual period.
If your menstrual cycle is shorter or longer than the standard 28 days, ovulation (and therefore conception) takes place earlier or later. This calculator adjusts for cycle variations with the following formula:
2. Conception Date Method
If you tracked your ovulation or are certain of the date conception occurred (e.g. through temperature charting or ovulation test kits), the due date is calculated by adding 266 days to that specific date.
3. IVF (In Vitro Fertilization) Transfer Method
IVF pregnancy dating uses the exact day of the embryo transfer to calculate the due date, bypassing cycle length variations. The calculation differs slightly depending on the age of the embryo when transferred:
- Day 3 Embryo Transfer: The embryo is 3 days old, meaning the due date is 263 days from the transfer date.
- Day 5 Embryo Transfer: The embryo is 5 days old (blastocyst stage), meaning the due date is 261 days from the transfer date.
Day 5 Transfer: Due Date = Transfer Date + 261 Days
Menstrual Cycle & Due Date Adjustments
Menstrual cycles can range from 20 to 45 days. The table below outlines how these cycle length variations adjust your calculated pregnancy due date relative to the first day of your last period:
| Menstrual Cycle Length | Difference from Average (28 days) | Due Date Formula Adjustment | Pregnancy Length from LMP |
|---|---|---|---|
| 20 Days | -8 Days | LMP + 272 Days | 38 weeks 6 days |
| 22 Days | -6 Days | LMP + 274 Days | 39 weeks 1 day |
| 25 Days | -3 Days | LMP + 277 Days | 39 weeks 4 days |
| 28 Days (Standard) | 0 Days | LMP + 280 Days | 40 weeks 0 days |
| 30 Days | +2 Days | LMP + 282 Days | 40 weeks 2 days |
| 32 Days | +4 Days | LMP + 284 Days | 40 weeks 4 days |
| 35 Days | +7 Days | LMP + 287 Days | 41 weeks 0 days |
| 40 Days | +12 Days | LMP + 292 Days | 41 weeks 5 days |
| 45 Days | +17 Days | LMP + 297 Days | 42 weeks 3 days |
What to Expect on Your Due Date
It is important to remember that a due date is only an **estimation**. Only about 4% to 5% of babies are born precisely on their estimated due date. Most babies arrive between 37 and 42 weeks of pregnancy. Deliveries within this window are considered healthy and normal.
Frequently Asked Questions
How accurate is a free online due date calculator?
A free online due date calculator estimates your due date with high accuracy based on Naegele's rule (280 days from the start of your last menstrual period, adjusted for cycle duration). However, since ovulation timing and fertilization rates vary, a dating ultrasound performed by a healthcare professional (usually between 10 and 14 weeks) remains the clinical gold standard under NHS and CDC guidelines.
Can my estimated due date (EDD) change during pregnancy?
Yes. While your initial EDD is calculated from your last period, your obstetrician or midwife may adjust the date if an early ultrasound scan measures the fetus as larger or smaller than expected for that gestational week. Health authorities standardly update the due date if the ultrasound dating differs from the LMP date by more than 7 days.
How do cycle length variations affect the due date calculation?
The standard calculation assumes a 28-day cycle where ovulation happens on Day 14. If your average cycle length is 32 days, ovulation typically occurs 4 days later (around Day 18). Our accurate calculator accounts for this by adding 4 days to the Naegele's rule calculation, ensuring a customized estimate for UK/EU Metric and US Imperial users.
How is a due date calculated for IVF (In Vitro Fertilization) transfers?
For IVF, the due date is extremely precise because the fertilization or transfer date is known. It is calculated by adding 263 days from a Day 3 embryo transfer, or adding 261 days from a Day 5 blastocyst transfer. This bypasses the uncertainty of LMP-based dating.