The height "18" by itself is ambiguous without units. Here are two common interpretations:

• If "18" refers to 18 years old (a person's age), the average height for an 18-year-old boy is about 5 feet 9.6 inches (176.5 cm) according to CDC data.

• If "18" is a measure in centimeters, 18 cm is approximately 7.1 inches, or 0 feet 7.1 inches tall.

Please specify if this "18" means something else or has specific units for a more precise answer.