Question 1

What is the exact conversion factor between inches and centimeters?

Accepted Answer

1 inch = 2.54 centimeters, exactly. This is not an approximation — it is defined exactly by the 1959 International Yard and Pound Agreement between the U.S., U.K., Canada, Australia, New Zealand, and South Africa. In the United States it is codified in NIST Special Publication 811 and the Metric Conversion Act of 1975 (15 U.S.C. §§ 205a–205l). To convert cm → in, divide by 2.54 (or multiply by 0.3937007874...). Examples: 12 in = 30.48 cm; 5′10″ = 70 in = 177.8 cm.

Question 2

Why did the U.S. survey inch differ, and when was it retired?

Accepted Answer

Before 1959, the U.S. used the “U.S. survey inch” based on the 1893 Mendenhall Order relating the yard to the meter as 1 yard = 3600/3937 m (making 1 survey inch ≈ 2.540 005 08 cm). This differed from the international inch by about 2 parts per million — negligible for household use but material for long-baseline surveying. NIST and the National Geodetic Survey formally retired the survey foot and survey inch effective January 1, 2023 (86 Fed. Reg. 60107, October 2021; 87 Fed. Reg. 20775, April 2022). All new U.S. survey work uses the international inch and international foot.

Question 3

How do the derived units (feet, yards, miles) convert to metric?

Accepted Answer

1 foot = 12 inches = 30.48 cm exactly. 1 yard = 3 feet = 36 inches = 91.44 cm exactly. 1 statute mile = 5,280 feet = 1,609.344 m exactly. 1 nautical mile = 1,852 m exactly (defined by the International Hydrographic Organization in 1929, adopted by the U.S. in 1954 and codified by ICAO for aviation). 1 furlong = 660 feet = 201.168 m exactly. Reverse: 1 meter = 3.280 839 895... ft = 39.370 078 74... inches.

Question 4

How does paper size differ between the U.S. and metric systems?

Accepted Answer

U.S. Letter is 8.5 × 11 inches = 21.59 × 27.94 cm. ISO 216 A4 (used everywhere except the U.S., Canada, and parts of Latin America) is 210 × 297 mm = 8.27 × 11.69 inches. The two are not interchangeable: A4 is slightly narrower and taller than Letter. ISO 216 A-series sizes share an aspect ratio of √2 : 1, so folding A3 in half produces two A4 sheets with identical proportions. U.S. Legal is 8.5 × 14 inches (21.59 × 35.56 cm); the ISO-equivalent size Foolscap Folio is 8.5 × 13 inches, a historical intermediate rarely used today.

Question 5

Where else do inches survive in international specifications?

Accepted Answer

Screen sizes (13.3-inch laptop, 65-inch TV) are measured diagonally in inches worldwide, even in fully metric countries. Tire sizes use a hybrid: P225/60R16 means 225 mm section width, 60% aspect ratio, and a 16-inch rim diameter. Bicycle wheel sizes are quoted in inches (26″, 27.5″, 29″) for mountain bikes and in millimeters (700c ≈ 622 mm bead-seat diameter, per ISO 5775) for road bikes. Plumbing pipe nominal sizes (½″, ¾″, 1″) are inch-derived globally under IEC/ANSI standards, even though the actual pipe wall dimensions are millimeter-defined in most manufacturing.

Inches to CM Converter