Electrical Units and Prefixes — V, A, Ω, W and Why m vs M Are 10 Apart

1 mΩ and 1 MΩ. Look almost identical. Differ in value by 10⁹ — a billion times. Get that wrong, and your part is dust.

Welcome to Episode 7 of the Electric Circuits Textbook series. This is the "boring, but you absolutely need it" episode. We'll lock in the four electrical units (V, A, Ω, W) and the SI prefixes (k, M, m, μ, n, p) so that for the rest of the series — and the rest of your time touching electronics — you can read a 220 µF capacitor or a 3.3 kΩ resistor instantly, without second-guessing.

If you missed Episode 6 (DC vs AC), that's the previous post. This one stands alone.

Today's Goal

Four things to take away:

1. Why this matters — misreading one prefix shifts your value by 1000×
2. The four units of electricity — V (volt), A (ampere), Ω (ohm), W (watt)
3. The prefix system — k, M, m, μ, n, p — and the case-sensitivity that kills
4. The conversion drill — going 3300Ω ⇄ 3.3 kΩ fluently

There's a 3-question quick check, and a bonus on why these unit symbols are uppercase (it's a person-name thing).

Why Units & Prefixes Matter — One Letter = 1000× Off

In a recipe, mistaking "a teaspoon of salt" for "a *table*spoon of salt" makes things saltier. Inconvenient, but recoverable.

In electronics, the analogous mistake is lethal to components.

Say the datasheet says a part can handle up to 100 mA (100 milli*amperes). You read it as *100 A** (no prefix). You're now planning to drive 1000× the rated current. The next thing that happens is smoke.

A prefix is a 1000× decision packed into a single character. Get the character right or you destroy hardware.

This is the foundation of everything you do with circuits from now on. If you internalize the prefix system here, you'll save yourself painful debugging — and dead components — forever.

The Four Electrical Units — V, A, Ω, W

Just like length uses centimeters and weight uses grams, each electrical quantity has its own dedicated unit. The four you'll see constantly:

Unit	Symbol	Measures	We covered this in…
Volt	V (uppercase)	Voltage — potential difference between two points	Episode 5
Ampere	A (uppercase)	Current — charge per second crossing a point	Episode 4
Ohm	Ω (uppercase Greek omega)	Resistance — how hard it is for current to flow	Episode 2
Watt	W (uppercase)	Power — energy delivered or dissipated per second	(coming later in the series)

Why are all four symbols uppercase? Because they're named after people. There's an SI convention: unit symbols named after a person are written with an uppercase letter (or uppercase-equivalent for Greek letters, like Ω). Ohm uses an uppercase Greek omega for the same reason — it honors Georg Ohm. More on this in the bonus section.

The internals (what resistance actually is, etc.) you've already met for most of these. Power (W) we'll deal with later in the series.

Prefixes — k, M, m, μ, n, p

Now today's other big topic: SI prefixes.

A prefix is a single-character abbreviation you stick in front of a unit symbol to multiply it by a power of 10. Instead of writing 3,300 Ω you write 3.3 kΩ. Instead of 0.000022 F you write 22 µF. They make big and small numbers human-readable.

The ones you'll use 95% of the time in electronics:

Prefix	Symbol	Multiplier	Power of 10	Example
Mega	M (uppercase)	× 1,000,000	10⁶	1 MΩ = 1,000,000 Ω
Kilo	k (lowercase)	× 1,000	10³	1 kΩ = 1,000 Ω
(none)	—	× 1	10⁰	1 Ω
Milli	m (lowercase)	× 0.001	10⁻³	1 mA = 0.001 A
Micro	μ (Greek mu)	× 0.000001	10⁻⁶	1 µA = 0.000001 A
Nano	n (lowercase)	× 0.000000001	10⁻⁹	1 nA = 0.000000001 A
Pico	p (lowercase)	× 10⁻¹²	10⁻¹²	1 pA = 0.000000000001 A

Two patterns to notice:

1. Each step is a 1000× jump. That's why "milli" → "micro" → "nano" → "pico" don't feel evenly spaced if you think in 10×. They jump 3 decimal places at a time.
2. The big ones (M, k) are huge. The small ones (m, μ, n, p) are tiny.

The case-sensitivity that gets people killed

Here's where almost everyone slips:

m and M differ by a factor of 10⁹ — a billion.

m (lowercase) = milli = 10⁻³.
M (uppercase) = mega = 10⁶.

Same letter. Opposite size. Different by a billion.

Trap	What's right	What goes wrong if you confuse them
k vs K	k (lowercase) = kilo. K (uppercase) = kelvin (temperature unit).	1 KΩ is technically not a valid SI prefix; K is reserved for kelvin, so the symbol doesn't mean kilo-ohm even when context says it "should."
m vs M	m (lowercase) = milli. M (uppercase) = mega.	1 mΩ ≠ 1 MΩ — they differ by 10⁹. This one frequently destroys components in practice.

Important caveat: don't try to infer "case = size" as a general rule. k (kilo, lowercase) is on the big side. μ (micro, lowercase Greek mu) is on the small side. There's no universal pattern — you have to memorize each prefix's case individually. The one that bites most often is just m vs M: lowercase m = milli, uppercase M = mega. Burn that pair into your eyes.

If you're handwriting, write m and M distinctly enough that someone else can tell them apart at a glance.

Converting — Get Fluent at Going Back and Forth

Now the drill: going from a "raw" number to a prefixed one, and back.

It's the same skill as converting 1 m = 100 cm. You just need to know which prefix matches the magnitude.

Example 1 — Big number → kilo

A 3,300 Ω resistor. That's "1000 ohms, 3.3 times." 1000 ohms is 1 kΩ. So:

3,300 Ω = 3.3 kΩ

Way more readable.

Example 2 — Small number → milli

A current of 0.001 A. That's "1 A, divided by 1000." 1/1000 is "milli." So:

0.001 A = 1 mA

And one step further:

1 A = 1,000 mA = 1,000,000 µA

Three decimal places per step. Always.

Example 3 — The mental trap test

Which is bigger: 10,000 mΩ or 1 kΩ?

If you go by the size of the number alone, "10,000" looks much bigger than "1," so the first one wins, right?

Wrong. Let's actually convert both:

• 10,000 mΩ = 10,000 × 0.001 Ω = 10 Ω
• 1 kΩ = 1,000 Ω

So 1 kΩ is actually 100× bigger than 10,000 mΩ. The big number with a m lost to the small number with a k. Always convert before comparing.

Writing convention

Two rules:

1. The prefix always comes immediately attached to the unit symbol. 3.3 kΩ — k and Ω together, no gap between them.
2. Never drop the unit. "100" by itself tells me nothing. 100 V vs 100 mA are very different things.

(Strict SI also wants a space between the number and the unit, like 3.3 kΩ rather than 3.3kΩ. Datasheets vary. The non-negotiable rule is: prefix and unit stay together.)

Field Note: All Four Units Are Named After People

The four electrical units we use every day are all named after real scientists and engineers:

Unit	Named after
Volt (V)	Alessandro Volta (Italy) — built the voltaic pile, the first true battery
Ampere (A)	André-Marie Ampère (France) — pioneered the study of currents and magnetism
Ohm (Ω)	Georg Ohm (Germany) — discovered Ohm's law (current ∝ voltage / resistance)
Watt (W)	James Watt (Scotland) — radically improved the steam engine, foundational to the industrial revolution

Every time you write 5V or 220Ω, you're invoking the names of the people who built the foundations of electrical engineering. Quietly, daily.

The rule for uppercase

Here's where the uppercase rule comes from:

Unit symbols named after people start with an uppercase letter. That's why V, A, W are uppercase Latin letters. The ohm is written with the capital Greek letter omega, Ω, following the same spirit.

This is separate from the prefix case rules. Don't conflate them:

• Unit symbol case ← named after a person? → uppercase.
• Prefix symbol case ← fixed for each prefix individually (k lowercase, M uppercase, m lowercase, etc.). Determined by the SI standard, not by personal-name logic.

Keep these two case systems separate in your head and you'll stop making errors.

Quick Check — 3 Questions

Three questions. Pause before peeking.

Q1. What's the right way to write 1,000 Ω using a prefix?

1. 1 kΩ (lowercase k)
2. 1 KΩ (uppercase K)
3. 1 mΩ (lowercase m)

Q2. Rewrite 3,300 Ω using a prefix.

1. 3.3 kΩ
2. 33 kΩ
3. 0.33 kΩ

Q3. Which is bigger: 10,000 mΩ or 1 kΩ?

1. 10,000 mΩ (the bigger-looking number)
2. 1 kΩ
3. They're equal

Got your answers?

Quick Check: Answers

Click to reveal the answers

#	Answer	Why
Q1	1. 1 kΩ (lowercase k)	Kilo = 1000×, written with lowercase k. Uppercase K is kelvin (temperature). Lowercase m is milli — opposite direction.
Q2	1. 3.3 kΩ	3,300 Ω = 3.3 × 1000 Ω = 3.3 kΩ.
Q3	2. 1 kΩ	10,000 mΩ = 10 Ω. 1 kΩ = 1,000 Ω. 1 kΩ is 100× bigger. The number with m was tiny; the number with k was huge. Always convert before comparing — never trust raw digit counts when prefixes are different.

Q3 is the trap that's lifelong-relevant. Anytime you compare quantities with different prefixes, convert to the same scale first. Otherwise you will, eventually, blow something up.

Section Summary

The whole story:

• Electricity has four core units: V (volt), A (ampere), Ω (ohm), W (watt)
• Their symbols start with an uppercase letter because they're named after people (Volta, Ampère, Ohm, Watt)
• Prefixes scale a unit by a power of 10. The common ones are M, k, m, μ, n, p
• Each prefix step is a factor of 1000 (3 decimal places)
• m and M differ by 10⁹ — billion-fold. This is the single most expensive mistake in electronics
• Always convert to a common scale before comparing quantities with different prefixes
• Write a prefix attached to the unit symbol (3.3 kΩ, not 3.3 k Ω)

This was the "infrastructure" episode. The rest of the series, every datasheet you'll ever read, every calculator screen full of microamps and picofarads, depends on the muscle memory you just built.

You've now finished Episode 1–7 of the series — the first half of "DC Circuit Basics." Thanks for sticking with it.

Coming up: actual calculations. Episode 8 dives into resistance and conductance in depth; then Ohm's law, then power. The units you just nailed down are about to start doing real work.

See you in Episode 8.