How to Pick a Resistor Value (Without Guessing)

Almost every resistor in a hobby circuit is doing one of four jobs, and each job has its own short calculation. Once you know which job you're solving, the arithmetic is one line.

Start here: what is this resistor for?

Job	Typical value	Set by
*Limiting current (LED)	220–330Ω	Ohm's law
*Pull-up / pull-down (button)	10kΩ	Noise vs wasted current
*Driving a transistor base	220Ω–1kΩ	Required base current
*Voltage divider	Ratio matters	The two values' proportion

Pick the row, then read that section. Everything else is detail.

Job 1: limiting current

The classic LED case. One formula:

R = (V_supply − V_component) ÷ I_target

The resistor's job is to absorb the leftover voltage. A red LED on 5V drops about 2V and wants around 20mA, so the resistor drops the remaining 3V: 3 ÷ 0.02 = 150Ω.

In practice everyone uses 220Ω or 330Ω, which run the LED slightly dimmer and further from its limit. Full worked version here, including values for other LED colours.

Job 2: pull-up and pull-down

Here you're not solving an equation so much as picking a sensible middle.

Too large (1MΩ): the pull is weak and electrical noise can overcome it. Unreliable readings.
Too small (100Ω): works, but wastes current continuously whenever the button is held. Bad on battery.

10kΩ sits comfortably between and is right almost always. I2C is the exception — it wants lower, for reasons in the I2C guide. And on Arduino you can often skip the component entirely with INPUT_PULLUP.

Job 3: driving a transistor base

Work backwards from the current you need to switch:

Load current ÷ a conservative gain figure = base current needed
R = (pin voltage − 0.7V) ÷ base current

Switching 200mA with a conservative gain of 10 needs 20mA of base current, so (5 − 0.7) ÷ 0.02 = 215Ω — use 220Ω. The 0.7V is the base-emitter drop of a silicon transistor. The transistor datasheet guide covers where the gain figure comes from.

Job 4: voltage divider

Two resistors in series; you tap the voltage between them.

V_out = V_in × R2 ÷ (R1 + R2)

Only the ratio sets the output voltage, so 1kΩ+2kΩ and 10kΩ+20kΩ both turn 5V into 3.33V. The absolute size decides how much current the divider itself wastes, and how easily whatever you connect disturbs it.

Common use: dropping a 5V signal to something a 3.3V input can accept — but never on an I2C line. See the level-shifting guide.

Then round to a value that exists

Your calculation will give something like 215Ω, which nobody sells. Real resistors come in the E12 series:

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

...and those ×10, ×100, ×1k and so on. So 215Ω becomes 220Ω.

Which way to round? For current limiting, round up — more resistance means less current, which is the safe direction. For a pull-up, either way is fine.

Two things people forget

Power rating

Resistors turn the voltage they drop into heat. P = V × I. Most hobby resistors are quarter-watt, which is plenty for signal work — the LED example above dissipates 0.06W. But drop 10V at 100mA and that's 1W, which will cook a quarter-watt part.

Tolerance

A 5% resistor marked 220Ω might actually be 209Ω or 231Ω. Fine for LEDs and pull-ups. If your circuit needs better than that, buy 1% parts — and measure them.

The habit that beats all of this

Measure before you fit. Colour bands are genuinely hard to read under bench lighting, and red/orange/brown confusion has cost everyone an evening at some point. A multimeter takes two seconds and removes the doubt — see the bench guide.