Femur Length to Gestational Age Calculator (FL mm → weeks+days & Percentile)

Use a single femur length (FL) measurement to estimate gestational age (weeks+days) and see where that length sits by percentile for this stage.

Femur length (FL) in mm
Enter the measured femur length from ultrasound.
Known GA (optional)
If provided, we compute percentile at this GA. If left empty, we estimate GA from FL.
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Educational ranges; ultrasound reports and clinician judgment come first.

What this tool calculates

  • Estimated gestational age from a single femur length (FL) measurement in millimetres.
  • Percentile of the entered FL at that gestational age using a normal model.
  • Typical FL range at the estimated or provided gestational age, shown as an approximate 5th to 95th percentile band.

Measurement assumptions

  • Standard fetal femur length on ultrasound, measured along the ossified diaphysis with the beam as close to perpendicular as practical.
  • Out-of-plane or partial-foreshortening views can bias FL low; use consistent technique.

Computation

  • Reference table: mean and standard deviation for FL by completed gestational week (14–40). Values are smoothed and linearly interpolated at 0.1-week steps.
  • GA estimate from FL: invert the smoothed mean curve to the week where the mean FL is closest to the entered FL.
  • Percentile: z = (FL − mean) / SD at that GA; percentile = normal CDF(z) × 100.
  • Typical range: mean ± 1.645 SD (approximate 5th–95th percentile).

Scope and limits

  • Single-parameter estimates are coarse. Clinicians interpret FL with other biometry and clinical dates.
  • Skeletal dysplasias, constitutional small/large fetuses, and measurement error can shift FL relative to population means.
  • Educational tool only. Not a diagnostic device.

Last updated: 6 Oct 2025

Next step
Date your pregnancy from ultrasound

Have GA from the scan? Confirm your estimated due date instantly.

Use the Ultrasound Due Date Calculator →

How to use this calculator

  1. Enter femur length (mm): type the FL from your ultrasound report (outer-to-outer, standard plane).
  2. Optionally enter GA: add known weeks+days to get the percentile at that exact age. If you leave it blank, the tool estimates GA from FL.
  3. Calculate: view estimated GA (and weeks-decimal), the FL percentile, and the typical range for this stage.

How to read the result

Estimated GA is derived from published FL growth curves using linear interpolation.

Percentile compares your FL with peers at the same GA (e.g., 30th means longer than 30% of fetuses at this age).

Typical range shows an approximate band (~5th–95th). Values below/within/above the band are flagged for quick context.