How the conversion works
Every length is first converted to meters, then from meters to the unit you asked for. The metric side is simple powers of ten: 1 km = 1,000 m, 1 m = 100 cm = 1,000 mm. The US customary side has been tied to the meter since the International Yard and Pound Agreement of 1959, so the factors are exact rather than rounded: 1 inch = 25.4 mm, 1 foot = 12 inches = 0.3048 m, 1 yard = 3 feet = 0.9144 m, and 1 mile = 5,280 feet = 1,760 yards = 1,609.344 m. A nautical mile, used at sea and in aviation, is defined as exactly 1,852 m, roughly one minute of latitude; one knot is one nautical mile per hour.
Because the US definitions are exact, converting metric to US and back again loses nothing. What you may see is rounding in the last displayed digit, since the converter shows eight significant figures. For day-to-day work, two or three decimals are plenty.
Worked examples
Height: 1.75 m is 1.75 / 0.0254 = 68.9 inches, which is 5 feet 8.9 inches. Going the other way, someone who is 6 feet 2 inches is 74 inches, and 74 x 0.0254 = 1.8796 m, so about 188 cm. Running: a 5K is 5,000 / 1,609.344 = 3.107 miles, a 10K is 6.21 miles, and a marathon of 42.195 km is 26.22 miles. Driving: a 65 mph speed limit is about 105 km/h, and 100 km is 62.14 miles.
Around the house, US lumber, pipe and hardware are sold in inches and fractions of an inch. A 3/8-inch bolt is 9.525 mm, a 1/2-inch pipe is 12.7 mm, and a 2x4 stud is nominal; the actual cross-section is 1.5 by 3.5 inches (38 by 89 mm). TV and phone screens are measured diagonally in inches, so a 55-inch TV has a 139.7 cm diagonal. Tire sizes such as 225/60R17 mix units: the 17 is the rim diameter in inches, while 225 is the tread width in millimeters.
Feet and inches, and other tips
Americans state height and short lengths as feet plus inches rather than decimal feet. The converter shows both: 5.74 ft is not 5 feet 7 inches, it is 5 feet 8.9 inches, because 0.74 ft x 12 = 8.9 in. When a plan or a listing says 12 ft 6 in, enter 12.5 ft, or enter 150 inches.
A mental shortcut for miles to kilometers is to multiply by 1.6 (or use consecutive Fibonacci numbers: 5 miles is about 8 km, 8 miles is about 13 km). For inches to centimeters, multiply by 2.5 and add a little. For meters to feet, multiply by 3.28. These get you within a couple of percent; use the converter when the number has to be right, such as a shipping dimension, a fence line or a race distance.