How to Convert Inches to Cm
If you've ever bought a TV outside the United States, you've done this conversion without realizing it. That "55-inch" display on the box? In Europe, the same box says "139 cm." Same panel, same factory, two different numbers. The inch-to-centimeter ratio — 1 inch = 2.54 cm — was locked down in 1959 by the International Yard and Pound Agreement, which defined the inch as exactly 25.4 millimeters. Before that, the US had its own Survey inch (25.400051 mm), and the UK had yet another definition. Today there's one inch, and it's exactly 2.54. No rounding, no approximation — the math is exact down to the last decimal. That precision matters when you're machining a bearing housing; it doesn't matter much when you're buying curtains. Either way, the formula is the same.
The Conversion Formula
cm = inches × 2.54
Worked Examples
Example 1: Convert 12 inches to centimeters
12 × 2.54 = 30.48 cm (exactly one foot)
Example 2: Convert 5.5 inches to centimeters
5.5 × 2.54 = 13.97 cm (typical smartphone screen diagonal)
Example 3: Convert 3/8 inch to centimeters
0.375 × 2.54 = 0.9525 cm (common bolt head size)
Common Inches To Cm Conversions
| inches | cm |
|---|---|
| 1 inch | 2.54 cm |
| 6 inches | 15.24 cm |
| 12 inches (1 ft) | 30.48 cm |
| 18 inches | 45.72 cm |
| 24 inches (2 ft) | 60.96 cm |
| 36 inches (1 yd) | 91.44 cm |
| 48 inches (4 ft) | 121.92 cm |
Related Unit Converters
Frequently Asked Questions
Is 1 inch exactly 2.54 cm, or is that rounded?
It's exact. Since 1959, the international yard and pound agreement defined 1 inch = 25.4 mm precisely. That makes 1 inch = 2.54 cm with no rounding and no approximation. Before 1959, the US used a slightly different "Survey inch" (25.400051 mm), which was formally deprecated on January 1, 2023 by NIST. For all modern engineering, manufacturing, and trade purposes, 2.54 is the one and only number.
How do I convert feet and inches to cm in one step?
Convert everything to inches first: (feet × 12) + inches = total inches. Then multiply by 2.54. Example: 5 feet 8 inches = (5 × 12) + 8 = 68 inches. 68 × 2.54 = 172.72 cm. If you prefer metric thinking: 1 foot = 30.48 cm and 1 inch = 2.54 cm, so you can also compute (feet × 30.48) + (inches × 2.54) directly.
Why do TV and monitor sizes use inches worldwide, even in metric countries?
Tradition and the CRT tube legacy. Early cathode-ray tubes were manufactured in the US and measured diagonally in inches. When LCD panels took over, the inch-based diagonal measurement was so entrenched in consumer expectations that switching to centimeters would have confused buyers comparing old and new models. Even in Japan and Germany — metric manufacturing powerhouses — display sizes are advertised in inches. Only regulatory packaging labels include the cm equivalent.
What's the quick mental math for inches to cm?
Multiply by 2.5 and add 1.6%: double the inches, add half, then add a tiny bit. Example: 40 inches → double (80) + half (20) = 100. Then add about 1.6% (+1.6) = 101.6 cm. The error is roughly 0.4 mm per meter — fine for furniture shopping, not for machining. Common shortcuts: 6" = 15.24 cm, 12" = 30.48 cm, 36" = 91.44 cm.
Engineering Context
The inch-to-centimeter conversion has broken more hardware than anyone wants to admit. The US Survey inch — 25.400051 mm instead of the international 25.4 mm — was used for all US land surveys from 1893 until NIST finally killed it on January 1, 2023. That 0.000051 mm difference sounds laughable until you realize it accumulates to about 3.2 mm per mile. Over the 2,900-mile US-Canada border, that's 9.3 meters of accumulated discrepancy — and that's before you factor in satellite GPS coordinates in meters being overlaid on century-old survey maps in Survey inches. In PCB design, the more common failure mode is simpler: a US engineer specs a component footprint in mils (0.001"), the Chinese fab house reads it as mm, and the board comes back 39.37× too large or too small. The fix is always the same: label your units. Every time.
More: cm to inches · inches to mm · inches to feet · feet to cm · Guide