Apprentice path · Stage 6: The quantities behind every calculation · Lesson 22

Units and prefixes

A letter in front of a unit multiplies or divides it, and missing that letter changes your answer by a factor of a thousand.

Why this matters on the job

This is the single most common way a correct calculation produces a wrong answer. The arithmetic was right. The unit was not.

How to think about it

  1. Milli, written m, means one thousandth.
  2. Kilo, written k, means one thousand.
  3. Write the unit beside every answer, every time.

Worked through

0.5 amps and 500 milliamps are the same current written two ways: 0.5 A x 1000 = 500 mA. Going the other way, 2 kilowatts is 2 x 1000 = 2000 watts. Nothing changed but the way it is written -- which is exactly why dropping the prefix is so easy and so expensive.

Check yourself: A meter reads 500 mA. How many amps is that?

Answer: 0.5 A. Milli means one thousandth, so 500 thousandths of an amp is half an amp.

The prefix is a multiplier, not decoration. 500 mA is 500 / 1000 A, which is 0.5 A.

Watch out for: Writing a bare number. An answer of "12" is not an answer. 12 V and 12 mV are a thousand times apart, and the only thing that told you which one it was, was the letter you did not write down.
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Written from the owner's poster for this lesson. Nothing added beyond what the poster states.

Written for Texas apprentices as free study material. The Texas apprentice licence is an application, not an exam (Occupations Code 1305.161), and nothing here counts as apprenticeship hours or continuing-education credit. VoltMark is not affiliated with, endorsed by, or sponsored by NFPA, PSI, TDLR, or any state or local licensing authority. Always follow your employer's safety programme and your supervising electrician.