Solve for Resistance: 9V Battery, 20mA Draw

This page opens the Ohm's law calculator with a 9V supply and a 20mA current already filled in — resistance and power are solved live on load from those two values.

This is the classic current-limiting question: a 9V battery and a component (an LED, a sensor, a relay coil) rated for 20mA — what resistor keeps it there? R = V / I gives 9V / 0.02A = 450Ω, and the power that resistor must dissipate is P = VI = 0.18W, comfortably inside a standard quarter-watt part.

This is a simplified version of the same math the LED resistor calculator does with the LED's own forward-voltage drop subtracted first — if you are sizing a resistor for an actual LED rather than a generic 20mA load, that dedicated tool accounts for the diode drop for you.