SI Units & Prefixes#

The metric prefix system is one of those things that seems trivially simple until micro and milli get mixed up and the wrong part gets ordered or a circuit gets blown up. Getting units right — especially the scaled forms used constantly in electronics — is a prerequisite for everything else.

The Units That Matter in Electronics#

SI Prefixes Used in Electronics#

The Common Range#

Most bench electronics lives in the milli-to-mega range:

• Voltages: millivolts to hundreds of volts
• Currents: microamps to amps
• Resistances: milliohms to megaohms
• Capacitances: picofarads to millifarads
• Inductances: nanohenries to henries
• Frequencies: hertz to gigahertz

Prefix Mistakes That Kill Hardware#

Milli vs. Micro#

The most dangerous confusion. 1 mA (milliamp) is 1000× larger than 1 µA (microamp). Setting a current limit to 100 mA when 100 µA was intended delivers 1000× more current than expected.

On schematics and in datasheets:

• m = milli (10⁻³)
• µ or u = micro (10⁻⁶)

Some older schematics use “M” for both mega and milli depending on context. This is terrible practice but does occur. A “100M” resistor could mean 100 MΩ or 100 mΩ — context is everything.

Kilo (k) vs. Mega (M) Capitalization#

• k (lowercase) = kilo = 10³
• M (uppercase) = mega = 10⁶

“K” (uppercase) for kilo is technically wrong but widely used. “m” (lowercase) for mega would be confused with milli. Care is needed when reading handwritten or informal notes.

The R/K/M Notation on Schematics#

European-style component value notation puts the multiplier where the decimal point would be:

• 4R7 = 4.7 Ω
• 47R = 47 Ω
• 4K7 = 4.7 kΩ
• 47K = 47 kΩ
• 4M7 = 4.7 MΩ

This avoids ambiguity from decimal points that might be missed on a photocopy or screen. This notation is common on PCB silkscreens and some schematics.

Converting Between Prefixed Units#

The mechanical process: count how many factors of 10³ are being moved.

• 0.001 µF = 1 nF (multiply by 1000, move one prefix down)
• 2200 pF = 2.2 nF (divide by 1000, move one prefix up)
• 0.1 µF = 100 nF (multiply by 1000)

For capacitors especially, the same value gets expressed in different prefixes on different datasheets:

• 0.1 µF = 100 nF = 100,000 pF — all the same capacitor

Train to convert fluently. Mixing up prefixes during calculation is a common source of off-by-1000 errors.

Tips#

• When in doubt, convert to base units (volts, amps, ohms) before calculating, then convert back
• Use the R/K/M notation on silkscreens and schematics to avoid decimal point ambiguity
• Double-check prefix conversions whenever the result seems surprising

Caveats#

• Farads are huge — A 1 F capacitor is enormous. Most capacitors are microfarads, nanofarads, or picofarads. Supercapacitors are the exception, with values in farads, but they’re a special case
• Henries are large — Most inductors are microhenries or millihenries. A 1 H inductor is physically large. Nanohenries matter at RF
• dB is not a unit — It’s a ratio. dBm is referenced to 1 mW. dBV is referenced to 1 V. dBu is referenced to 0.775 V. Don’t mix them up
• Percent vs. ppm — 1% = 10,000 ppm. Temperature coefficients in ppm/°C and tolerance in percent are measuring the same kind of thing at different scales
• Angular frequency vs. frequency — ω = 2πf. Forgetting the 2π gives an answer that’s off by a factor of 6.28. Some formulas use ω, some use f. Know which one is expected

In Practice#

• A measurement that’s off by exactly 1000× usually indicates a prefix conversion error
• Component values that seem wrong often trace back to milli/micro confusion on the label or schematic
• Oscilloscope and DMM readings in unexpected ranges suggest the wrong scale was assumed
• When ordering parts, verify the prefix in the part number matches the intended value