Relationship of head circumference to length in the first 400 days of life: a mnemonic.

: The relationship between length and head circumference can be expressed by the simplified formula: head circumference (centimeters) = 0.5 length +9.5 +/- 2.5; head circumference (inches) = 0.5 length + 3.75 +/- 1. This formula is accurate for 95% of measurements in the first 400 days of life. The correlation coefficient of length to head circumference is .94 (P = .0001).