Crown-rump Length Calculator

How to use the crown-rump length calculator

What this calculator does

This calculator converts a first-trimester crown-rump length (CRL) from an ultrasound report into an estimated gestational age. CRL is the straight-line distance from the fetal crown to the rump in a suitable midsagittal ultrasound view. The calculation uses the corrected Robinson – Fleming relationship and reports both the continuous formula result and a rounded clinical-style weeks-and-days value. It is a dating estimate, not a diagnosis, growth assessment, viability assessment, or substitute for the interpretation of the clinician who performed or reviewed the scan. The ISUOG fetal biometry guideline explains why early ultrasound CRL is used for pregnancy dating and why other measurements are preferred once CRL exceeds 84 mm.

When to use it

Use the calculator to translate a CRL written on an early ultrasound report into an estimated gestational age, to check a manual weeks-and-days conversion, or to understand how a small change in the measured CRL affects the dating estimate. It can also help when comparing a CRL-based estimate with a date already documented by a healthcare professional. A discrepancy should be discussed with that professional rather than used to change pregnancy dates independently.

How to calculate

1. The calculator opens with a ready-to-use demonstration: 55 mm. That sample immediately produces a valid estimate and a validated Excel workbook, so Download Excel is available on first open.
2. Replace Crown-rump length with the CRL from the ultrasound report. Enter ordinary decimal notation such as 55 or 55.4; commas, scientific notation, words, and mixed-unit text are rejected rather than silently reinterpreted.
3. Choose the Crown-rump length unit: millimeters, centimeters, or inches. Changing this unit converts the current value so the physical measurement remains unchanged.
4. Choose Gestational age display. Weeks + days shows the nearest whole-day result in the familiar mixed format; Days shows that rounded whole-day age; Decimal weeks shows the continuous formula estimate divided by seven; Hours shows the continuous estimate multiplied by 24.
5. Read the primary estimate and supporting values, then use Download Excel to export the current validated model. Reset clears the demonstration/input data, returns the selectors to their neutral settings, removes calculated output, and disables Excel until a complete valid CRL is entered again.

Input guide

Crown-rump length is required and must be a finite positive measurement equivalent to 5 – 85 mm. The startup example is 55 mm. A larger CRL produces a larger estimated gestational age through a square-root relationship, so the increase in estimated age is not perfectly linear. Enter only the measured CRL, not a gestational-age number or a label copied from the report. The Crown-rump length unit is also required: mm is the standard calculation unit, while cm and in are converted internally to millimeters. For example, 5.5 cm and about 2.165 in represent the same 55 mm CRL. The Gestational age display control changes formatting only and never changes the underlying age estimate; its available formats are weeks + days, days, decimal weeks, and hours.

Output guide

Primary estimate is the gestational age formatted according to the display selector. Formula estimate is the unrounded result in days, shown to two decimals so you can see the value before whole-day rounding. Completed weeks and Remaining days come from the nearest whole day: remaining days therefore run from 0 to 6. Normalized CRL shows the exact physical input converted to millimeters, which is the unit used by the equation. The summary pills repeat normalized CRL, rounded age in days, and the named method. In the CRL reference table, “CRL (mm)” is the lookup measurement and “Estimated gestational age (weeks + days)” is the equation rounded to a whole day at 5 mm intervals. These outputs are estimates tied to this mathematical dating model; they do not establish fetal health or future growth.

Worked example

With the startup value of 55 mm, the calculator evaluates 8.052 × √(55 × 1.037) + 23.73 = 84.54 days. Rounding 84.54 to the nearest whole day gives 85 days. Eighty-five days contains 12 completed weeks (84 days) plus 1 remaining day, so the first-open primary result is 12 weeks 1 day. The same values are used in the on-page result cards and the startup Excel export.

Learn more

Public screening guidance from the UK Fetal Anomaly Screening Programme defines CRL as the crown-to-rump ultrasound measurement and explains that gestational age is calculated from it. The NHS guide to the 12-week dating scan explains how early ultrasound is used to estimate how far along a pregnancy is.