Resistance calculator

Resistance is V ÷ I, and the number the calculation gives you is rarely a resistor you can buy. Resistors come in E-series preferred values — E12 gives twelve values per decade, E24 twenty-four — so a calculated 187 Ω means fitting the nearest standard value, usually 180 Ω or 220 Ω, and checking that the difference does not matter. For an LED it does not; for a precision divider it might.

The power figure is the one people skip and it is what destroys resistors. A resistor dissipates V × I as heat, and a quarter-watt part asked to handle half a watt discolours, drifts and eventually fails open. The rule of thumb is to fit a part rated at least twice the calculated dissipation — the derating gives headroom for ambient temperature and for the fact that a resistor at its rated power is genuinely too hot to touch.

The classic use is an LED series resistor, and it is worth doing properly: subtract the LED’s forward voltage from the supply first, then divide by the desired current. A 12 V supply with a 2 V red LED at 20 mA needs (12 − 2) ÷ 0.02 = 500 Ω, not 12 ÷ 0.02 = 600 Ω. Using the full supply voltage runs the LED dim; forgetting the resistor entirely destroys it, because an LED is not ohmic and will draw whatever current it takes to fail.