Solve for Resistance: 9V Battery, 20mA Draw
Enter any two of voltage, current, resistance, and power, and the page fills in the other two.
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Result
About this tool
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.
Example. Voltage 12 V and resistance 4 Ω return current 3 A and power 36 W. Voltage 9 V and current 20 mA return resistance 450 Ω and power 180 mW.
Limit. At least two of voltage, current, resistance, and power must be strictly positive. A later power-and-resistance step overwrites current and voltage when power is also filled in.