An inch is exactly 25.4 mm

Many “round” imperial–metric pairs are exact by definition (1 in = 25.4 mm, 1 lb = 0.453 592 37 kg). Others are measured.

Why it works

The converter uses definitional factors where they exist, not rounded teaching approximations — so repeated round-trips stay consistent.

Do it exactly

Estimate with the rule, then check it against the calculator that models it properly.

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More in Mechanical

• The bolt-torque shortcutTightening torque ≈ 0.2 × preload × bolt diameter (dry steel).
• Torque mostly fights frictionRoughly 90% of tightening torque is lost to friction; only ~10% actually stretches the bolt.
• Deflection limits: span/250Keep beam deflection under span/250 generally, span/360 where people will notice a bouncy floor.
• Depth beats width, cubedDouble a beam’s depth and its deflection drops 8-fold. Double its width and it only halves.

How many millimetres in an inch?

Many “round” imperial–metric pairs are exact by definition (1 in = 25.4 mm, 1 lb = 0.453 592 37 kg). Others are measured. The converter uses definitional factors where they exist, not rounded teaching approximations — so repeated round-trips stay consistent.

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