Length conversion: correct units without false precision
Converting a length does not measure the object again. It expresses the same measurement in a different unit, which helps compare a product specification or drawing with the units on your measuring tool.
The number changes; the length does not
The international inch is exactly 2.54 centimetres. A foot contains 12 inches, giving 30.48 centimetres. Moving to a smaller unit usually makes the number larger while leaving the length unchanged.
Keep a unit beside each value. The number 180 on its own does not tell you whether the measurement is millimetres, centimetres or metres.
References: NIST — SI units: length
5.5 feet is not 5 feet 5 inches
A decimal fraction of a foot is a fraction of 12 inches: half a foot is 6 inches. Five feet five inches means 5 + 5 ÷ 12 feet, not 5.5 feet.
For a feet-and-inches measurement, first turn the feet into inches, add the remaining inches and then convert the total. Do not interpret the separator between units as a decimal point.
Do not give the measurement extra precision
A conversion may produce many digits without improving the original measurement. Measuring a board to the nearest centimetre does not become more precise by displaying five decimal places in inches.
Leave an appropriate measurement and fitting margin when checking a tight space. Also, an ordinary length factor is not the factor for an area or volume: square and cubic units need separate treatment.
Worked example: feet and inches
Five feet ten inches equals 5 × 12 + 10 = 70 inches. Multiplying 70 by 2.54 gives 177.8 centimetres. Entering 5.10 as decimal feet does not represent that measurement.
For 1.8 metres, first use 180 centimetres, then calculate 180 ÷ 2.54 ≈ 70.87 inches. That rounded display is convenient, but it does not prove that the original measurement is known to a hundredth of an inch.