Most unit-conversion mistakes are not difficult mathematics. They are direction mistakes: multiplying when you should divide, mixing square and linear units, or forgetting that temperature scales include an offset.
Write the units next to the number
12 is not enough information. 12 cm and 12 in describe very different lengths. Keeping the unit attached to every value makes it harder to accidentally combine incompatible quantities.
Estimate the direction before calculating
A meter is longer than a centimeter, so a length expressed in centimeters should usually have a larger numeric value than the same length in meters. If 2 meters turns into 0.02 centimeters, you know the calculation is wrong before checking the exact factor.
Area and volume multiply the conversion factor
A square meter is not 100 square centimeters. Because both dimensions change, 1 m² = 10,000 cm². Volume compounds the factor again. This is why blindly reusing a linear conversion for area or volume can produce large errors.
Temperature is different
Celsius and Fahrenheit do not share the same zero point, so you cannot convert them with a single multiplication factor. Temperature formulas include both scaling and an offset. Kelvin introduces another reference point.
Be precise only when the source is precise
If a measurement was taken with a rough household ruler, converting it to six decimal places does not create new accuracy. Keep enough digits for the task, but do not imply more measurement precision than you actually have.
Use dimensional analysis for important work
Write the conversion factor as a fraction so unwanted units cancel. For example, multiplying inches by 2.54 cm / 1 in leaves centimeters. This small habit is especially useful in science, engineering and medication calculations where a silent unit error can be serious.
A converter is fastest when you already know what the answer should roughly look like. Estimate first, convert second, and investigate any result that violates the estimate.