A.0.P—SI unit prefixes

SI prefixes change the scale of a unit: milli is 10⁻³, micro 10⁻⁶, nano 10⁻⁹ and kilo 10³. Convert to one common unit before comparing or substituting.

Write the prefix as a power of ten, cancel units algebraically and check whether the direction of the conversion makes sense.

250 μm = 250 × 10⁻⁶ m = 2.50 × 10⁻⁴ m; it is smaller than 250 mm, not larger.

The prefix belongs to the unit, not to the numerical value; mixing μm and mm can create a thousand-fold error.