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Head Circumference Percentile Calculator

Work out head circumference percentile instantly with clear inputs, formula shown and shareable results.

About Head Circumference Percentile Calculator

Occipitofrontal circumference is standardised as a z-score against the reference mean and standard deviation for age, then read off the normal curve as a centile. Head circumference is charted in the first years because it tracks brain growth, and the plus or minus 2 SD band is the usual trigger for a closer look.

Formulas

Z-score

z = (measured circumference - reference mean) / reference standard deviation

Centile

centile = normal cumulative distribution of z x 100

Worked example

With the default values below, Head Circumference Percentile Calculator returns Head circumference percentile of 64.0%. Change any input above and every figure here is recalculated in your browser.

Inputs used

Head circumferenceminimum 20 cm
44 cm
Reference mean for ageminimum 20 cm
43.5 cm
Reference standard deviationminimum 0.1 cm
1.4 cm

Results

Head circumference percentile
64.0%
Z-score
0.357
Difference from the reference mean
0.50 cm
Interpretation
Within +/- 2 SD of the reference

Step by step

  1. 1Values used
  2. 2Z-score
  3. 3Centile
  4. 4Head circumference percentile= 64.0
  5. 5Z-score= 0.357
  6. 6Difference from the reference mean= 0.50 cm
  7. 7Interpretation= Within +/- 2 SD of the reference

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the Head Circumference Percentile figure worked out?

z = (measured circumference - reference mean) / reference standard deviation. Occipitofrontal circumference is standardised as a z-score against the reference mean and standard deviation for age, then read off the normal curve as a centile.

Why does this matter?

Head circumference is charted in the first years because it tracks brain growth, and the plus or minus 2 SD band is the usual trigger for a closer look.

What do I need to enter?

Enter head circumference, reference mean for age, reference standard deviation. The defaults shown are a realistic worked example — swap in your own figures to get a result you can use.

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