Crown Rump Length (CRL) Calculator

Fetal Length by Gestational Week Calculator

Crown Rump Length (CRL) Formula

Formula

\[i=\left\lfloor 2(w-6)\right\rfloor,\qquad \mathrm{CRL}_{\mathrm{cm}}=c_i,\qquad \mathrm{CRL}_{\mathrm{mm}}=10c_i\]

Where:

\(w\) — gestational age in weeks, from 6 to 14.
\(i\) — the index of the most recent half-week entry, starting at 0 for 6 weeks.
\(c_i\) — the crown–rump length in centimeters at that entry, as listed in the reference table below.

The calculator uses a fixed lookup table rather than a continuous growth formula. For an age between two half-week entries, it uses the earlier entry; it does not interpolate.

Crown Rump Length (CRL) – Calculation Example

Example: enter 10 for Number of weeks and select mm for Unit.

At 10 weeks, the lookup table gives \(c_8=3.1\) cm.

\[i=\left\lfloor 2(10-6)\right\rfloor=8,\qquad \mathrm{CRL}_{\mathrm{mm}}=10(3.1)=31\ \mathrm{mm}\]

Result: the estimated Crown Rump Length is 31 mm. Selecting cm instead gives 3.1 cm.

How to use the calculator

Enter the gestational age in Number of weeks. The accepted range is 6 through 14 weeks. You can enter a decimal, such as 10.5 for 10 weeks and 3½ days.
Choose Unit: mm for millimeters or cm for centimeters.
Read the estimated Crown Rump Length. The result is a table-based estimate, not a measurement of an individual pregnancy.

If you need to work out how far along a pregnancy is before entering a value, use the Gestational Age Calculator or How Many Weeks Pregnant Are You?. This calculator only works from weeks to length: it does not accept a measured CRL to calculate gestational age.

What crown–rump length means

Crown–rump length (CRL) is the distance from the top of an embryo’s or fetus’s head to the bottom of its buttocks. It excludes the legs. A clinician measures CRL on an ultrasound image, particularly in early pregnancy, when it can help establish gestational age and an estimated due date.

This calculator answers a narrower question: what length does its reference table associate with a gestational age you already know? Gestational age is generally counted from the first day of the last menstrual period, not from the date of conception. The result can provide a rough point of comparison, but it cannot tell you whether a particular ultrasound measurement is expected for your pregnancy.

An ultrasound report may use its own dating method and measurement standards. If a scan-based age differs from an age calculated from menstrual dates, ask your healthcare professional which date to use. For other ways to explore pregnancy timing, see the Ultrasound Gestation Period and Pregnancy Due Date calculators.

Reference values used by this calculator

These are the calculator’s fixed values. The millimeter column is the centimeter value multiplied by 10; it is not a separate set of ultrasound measurements.

Gestational age (weeks)	CRL (cm)	CRL (mm)
6	0.45	4.5
6.5	0.65	6.5
7	0.92	9.2
7.5	1.3	13
8	1.5	15
8.5	1.9	19
9	2.2	22
9.5	2.7	27
10	3.1	31
10.5	3.6	36
11	4.1	41
11.5	4.7	47
12	5.2	52
12.5	6.0	60
13	6.5	65
13.5	7.4	74
14	7.9	79

Assumptions and limitations

The table changes only at half-week intervals. For example, entering 10.9 weeks returns the 10.5-week value of 3.6 cm, or 36 mm; it does not estimate a value between 10.5 and 11 weeks. Entering less than 6 or more than 14 weeks produces an error rather than an extrapolated length.

Actual CRL measurements vary, and ultrasound results depend on factors such as fetal position and how the measurement is taken. A difference between this estimate and a scan result is not, by itself, evidence of a problem. Use the ultrasound report and your clinician’s interpretation for decisions about pregnancy dating or fetal development.

Frequently Asked Questions

How can I estimate crown–rump length by hand?

Find the latest half-week mark that does not exceed your gestational age, then read its CRL from the reference table. For example, 11 weeks and 2 days falls after 11 weeks but before 11½ weeks, so you would use the 11-week entry. This gives the same stepwise estimate as the calculator, not a prediction of an individual ultrasound measurement.

What is a normal CRL at 12 weeks?

The calculator’s 12-week reference value is 5.2 cm, or 52 mm, but a single table value is not a normal range. Ultrasound measurements can differ because of dating uncertainty, fetal position, and measurement technique. Your clinician can interpret a measured CRL alongside the scan findings and the dating method used for your pregnancy.

How do I enter weeks and days as a decimal?

Divide the extra days by seven and add the result to the completed weeks. For 10 weeks and 3 days, enter approximately \(10+3/7=10.43\) weeks. Do not enter 10.3: that means 10.3 weeks, not 10 weeks and 3 days. Because this calculator uses half-week steps, several nearby decimal inputs may produce the same result.

How do I convert CRL between centimeters and millimeters?

Multiply centimeters by 10 to get millimeters, or divide millimeters by 10 to get centimeters. For example, \(4.7\text{ cm}=47\text{ mm}\). Check the unit printed on an ultrasound report before comparing values: confusing 4.7 cm with 4.7 mm creates a tenfold difference.

Can a measured CRL be used to work out gestational age?

Yes, clinicians can estimate gestational age from an early-pregnancy CRL measurement using a validated ultrasound dating standard. This calculator does not perform that reverse calculation: its small table cannot uniquely assign an age to every possible measurement. If ultrasound dating differs from menstrual dating, ask your healthcare professional which estimate should guide your care.

Why might my ultrasound CRL differ from the calculator’s estimate?

The calculator gives a reference-table value for a stated age, while an ultrasound measures an individual embryo or fetus. Uncertainty in pregnancy dates, image quality, fetal position, and the measurement method can all contribute to a difference. A mismatch alone cannot establish whether growth is healthy or concerning; discuss your scan report with your clinician.

Is CRL the same as a baby’s total length?

No. CRL stops at the rump, so it is shorter than a head-to-heel measurement that includes the legs. Later in pregnancy, clinicians generally rely on other ultrasound measurements, such as head circumference and femur length, rather than CRL for dating and growth assessment. Do not compare a CRL value directly with a chart of total fetal length.