One inch is exactly 2.54 centimeters — not approximately, exactly. That definition was legally fixed by the International Yard and Pound Agreement of 1959, and it's what makes today's inch a precisely known metric quantity rather than a fuzzy historical unit.
Which raises the obvious question: if the conversion has been exact and settled for over sixty years, why does the US still measure everything in inches and feet instead of just switching to centimeters and meters?
Why Hasn't the US Switched to Metric?
The real reasons are economic and industrial, not ideological — and they mostly come down to the absence of any government mandate.
The Metric Conversion Act of 1975 declared metric the US's "preferred system" for trade and commerce, but it was entirely voluntary — no mandatory conversion schedule, no deadline, no penalty for staying customary. In a market with no legal pressure to switch, industries did what markets do: they chose whatever was cheapest, which was almost always the status quo.
And the status quo was expensive to leave. The US had (and still has) a massive construction and manufacturing base built entirely around customary dimensions. Standard home-building lumber is nominal 2×4 boards, framed with 16-inch spacing between wall studs specifically so a 4×8-foot sheet of drywall lands cleanly on the frame. Converting that entire supply chain and skillset to metric would have meant re-equipping factories and retraining workers across an industry that had no legal reason to bother.
The US Is Already Mostly Metric
It's a common misconception that America rejected the metric system outright. In practice, large parts of the US already run on it: science and medicine (drug dosages, lab measurements), the military (much of its equipment and munitions specs), and the auto industry (metric fasteners have been standard in US-built cars since the late 1970s) all use metric units as a matter of course.
What's left on the customary side is everyday, consumer-facing measurement: construction (as above), retail and grocery shopping, body weight and height, cooking, temperature, and road signage. The split isn't "science vs. the public" so much as "wherever switching was cheap and useful" vs. "wherever switching would have meant redoing an entire built environment for no practical gain."
A Timeline of the US and the Metric System
1866 — The Metric Act
Congress made it legal to use the metric system in trade and contracts. Entirely optional — nothing required anyone to actually use it.
1893 — The Mendenhall Order
The Superintendent of Weights and Measures, Thomas Mendenhall, formally redefined US customary units in terms of the metric standards — the yard and pound were tied to the meter and kilogram rather than physical US prototypes, since the metric standards were more precisely reproducible. The ratio used at the time was slightly different from today's (off from the modern figure by roughly two parts per million) — the clean, internationally uniform values of exactly 0.9144 meters per yard and 0.45359237 kilograms per pound weren't set until the 1959 agreement mentioned above.
1975 — The Metric Conversion Act
President Gerald Ford signed legislation directing federal programs to promote voluntary metric adoption, and created the US Metric Board to coordinate the transition.
1982 — The US Metric Board Is Defunded
With no enforcement power and limited public appetite for a mandatory switch, the board lost its funding during the Reagan administration's budget cuts and was effectively shut down.
Both systems have coexisted in the US for a century and a half, cycling through phases of official promotion and quiet abandonment — but never a hard mandate in either direction.