# Crown-Rump Length Calculator (CRL)

URL: https://speedycalculator.com/calculators/health/crown-rump-length/
Last updated: 2026-10-30

> The crown-rump length calculator estimates gestational age from the CRL measured on an early ultrasound (Robinson and Fleming formula), and works out the due date from the date of the scan.

## Inputs

- Crown-rump length (mm)
- Date of the scan

## Results

- Gestational age
- Gestational age – weeks
- …plus days
- Due date

## Formula

```
Gestational age (days) = 8.052 × √CRL (mm) + 23.73
Due date = scan date − gestational age + 280 days
```

## Explanation

Between about 7 and 13+6 weeks, the length of the embryo from head to rump is the most accurate way to date a pregnancy – accurate to within about 5 days. Major guidelines recommend changing the due date if the CRL date differs from the period date by more than 5–7 days.

### How to use the Crown-Rump Length Calculator

- **Crown-rump length**
- **Date of the scan**

Results update as you type, and every field accepts the units shown in its menu.

### How it works

Enter the CRL in millimetres and the date of the scan.

### Inputs explained

FieldWhat to enterCrown-rump lengthIn mmDate of the scanA date

### What each result means

- **Gestational age** = 8.052×sqrt([Crown-rump length]) + 23.73
- **Gestational age – weeks** = floor([Gestational age]÷7)
- **…plus days** = mod(round([Gestational age]), 7)
- **Due date** = [Date of the scan] - round([Gestational age]) + 280

### Example values

How the main result changes when **Crown-rump length** changes and everything else stays at the example values above:

Crown-rump length (mm)Gestational age – weeks (weeks)22.5833.7510451156.251267.5129014
*Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.*

## Worked example

Inputs — Crown-rump length: 45 mm; Date of the scan: 2026-10-25.

Results — Gestational age = 78 days; **Gestational age – weeks = 11 weeks**; …plus days = 1 days; Due date = Sat, May 15, 2027.

## Key terms

- **CRL**: Crown-rump length.

## FAQ

**What is a normal CRL at 12 weeks?**
About 45–60 mm.

**How accurate is CRL dating?**
Within about ±5 days in the first trimester.

**Why is CRL not used after 14 weeks?**
The fetus curls, so head circumference is more accurate later.


## Sources

- [Robinson HP, Fleming JE. A critical evaluation of sonar crown-rump length measurements. Br J Obstet Gynaecol, 1975](https://pubmed.ncbi.nlm.nih.gov/1182090/)
- [ACOG Committee Opinion 700 – Methods for estimating the due date](https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/05/methods-for-estimating-the-due-date)