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

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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.

* For the percentile comparison, an independently known gestational age gives the clearest context. If age is estimated from the same FL, the result naturally falls near the middle of the curve. The published table covers 14–40 weeks; outside it, the nearest reference age is identified and the dating result shows “or earlier/later.”

Peer-Reviewed Science
Updated September 2026
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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.

Methodology and sources

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: published INTERGROWTH-21st FL 5th, 50th and 95th percentiles at exact weeks 14–40. Values are linearly interpolated between adjacent weeks. An approximate standard deviation is derived from the 5th–95th span divided by 3.2897.
  • GA estimate from FL: invert the interpolated 50th-percentile curve to the week where the midpoint FL is closest to the entered FL.
  • Percentile: z = (FL − mean) / SD at that GA; percentile = normal CDF(z) × 100.
  • Typical range: the interpolated published 5th–95th percentile values.

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: September 2026

Source: INTERGROWTH-21st international fetal growth standard: femur length, Papageorghiou et al., Lancet 2014. At 40 weeks the published midpoint is 72.1 mm and the 5th–95th range is 66.8–77.4 mm.