Standard Resistor Values

Why there is a 470 Ω resistor but no 450: the values are spaced by ratio, so each one starts where the last one's tolerance ends.

The values you can actually buy

Every through-hole and chip resistor sold comes from one of the E series. Each series divides a decade of resistance into equal ratio steps, then repeats the same figures at 10x, 100x, 1000x and so on. The 22 in E12 gives you 22 Ω, 220 Ω, 2.2 kΩ, 22 kΩ and 220 kΩ, and nothing in between.

E12, ±10%, twelve per decade

10 · 12 · 15 · 18 · 22 · 27 · 33 · 39 · 47 · 56 · 68 · 82

E24, ±5%, twenty-four per decade

10 · 11 · 12 · 13 · 15 · 16 · 18 · 20 · 22 · 24 · 27 · 30 · 33 · 36 · 39 · 43 · 47 · 51 · 56 · 62 · 68 · 75 · 82 · 91

E96, ±1%, ninety-six per decade

100 · 102 · 105 · 107 · 110 · 113 · 115 · 118 · 121 · 124 · 127 · 130 · and 84 more

E24 contains every E12 value plus one between each pair; E96 subdivides again. That nesting is why an assortment kit of E12 values still covers most of what a schematic asks for.

Why the numbers look arbitrary

Each E12 step is about 21% above the one before, which is exactly two 10% tolerances meeting. A resistor marked 47 could measure anywhere from 42 to 52; the next value, 56, covers 50 to 62. No gaps worth worrying about, minimal overlap, twelve values per decade. Once you see that, 4.7 stops looking like a strange number to standardise on.

The kinds you will meet

Same value, different construction. What changes is tolerance, stability, noise and how much power it can shed.

Carbon film resistor — **Carbon film**5%, cents each The default hobby resistor: cream or beige body, four color bands, a gold tolerance band. Cheap, everywhere, and good enough for LEDs, pull-ups and anything that does not care about a percent either way. It gets noisy and drifts with temperature, which only matters in audio and precision work.

Metal film resistor — **Metal film**1%, blue body Blue or pale green body with five bands, because three digits need an extra ring. Quieter and far more stable with temperature than carbon film, which is what you want in a divider feeding an ADC, a voltage reference, or an audio signal path.

Wirewound resistor — **Wirewound**whole watts Resistance wire wound on a ceramic core, potted in a white cement block, rated in watts rather than fractions. Built for dummy loads, current sensing and bleeder duty. It is slightly inductive, so keep it out of fast switching or RF paths.

Chip (SMD) resistor — **Chip (SMD)**printed, not banded A ceramic brick with the value printed on top instead of banded: 1002 means 100 x 10^2, so 10 kΩ. Sizes are named by footprint in hundredths of an inch, 0805 being 0.08 by 0.05. Decode the print in the SMD code lookup.

Potentiometer resistor — **Potentiometer**three legs, a shaft A resistive track with a wiper riding on it. The outer legs are the ends of the track, the middle leg is wherever the knob has reached. Marked with a code like 103 for 10 kΩ, plus a letter for the taper: B is linear, A is logarithmic.

Trimmer resistor — **Trimmer**set once, with a screwdriver A potentiometer with a screw slot instead of a knob, meant to be set at build time and left alone. Multi-turn types such as the 3296 take about 25 turns end to end, which is what makes fine calibration possible.

Tolerance bands

Band	Tolerance	Series	Where you meet it
Silver	±10%	E12	old stock and cheap kits
Gold	±5%	E24	the everyday resistor
Brown	±1%	E96, five bands	dividers, references
Red	±2%	E48	occasional metal film
Green	±0.5%	E192	instrumentation
Blue	±0.25%	E192	precision references

Reading the bands on a part already in your hand is the color code calculator's job.

Power rating and body size

Through-hole resistors come in standard wattages and the body grows with the rating. The bench rule: pick one rated at least twice the power it will really dissipate, because the printed rating assumes free air and a cool room.

Rating	Body length	Where it shows up
1/8 W	3.2 mm	dense signal boards
1/4 W	6.3 mm	the default hobby resistor
1/2 W	9 mm	LED strings, higher voltage
1 W	11 mm	power dividers, bleeders
5 W	22 mm, ceramic block	dummy loads, current sense

Chip sizes

SMD resistors are named by footprint in hundredths of an inch: an 0805 is 0.08 by 0.05 inches. Below 0603 they stop being comfortable to hand solder, and below 0402 they stop being easy to see.

Code	Size	Typical rating	Hand soldering
1206	3.2 × 1.6 mm	0.25 W	easy
0805	2.0 × 1.25 mm	0.125 W	comfortable
0603	1.6 × 0.8 mm	0.1 W	fiddly but fine
0402	1.0 × 0.5 mm	0.063 W	tweezers and patience
0201	0.6 × 0.3 mm	0.05 W	machines only

Hitting a value you do not stock

Two standard parts in parallel reach almost anything: 5.1 kΩ beside 5.1 kΩ gives 2.55 kΩ, closer to 2.5 k than any E12 value gets. In series they simply add, which is usually the tidier answer. When the exact figure genuinely matters, buy 1% parts instead of chasing combinations, because tolerance stacks faster than intuition suggests.