LED Resistor for a 9V Battery + Standard LED
Find the series resistor for one or more LEDs from the supply, the forward drop, the count, and the current.
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Result
About this tool
This page opens the LED resistor calculator with a 9V supply, a 2V LED forward voltage, one LED and a 20mA target current already filled in — the resistor value below is computed live on load.
This is the textbook LED circuit: a 9V battery, one standard 5mm LED (red, yellow or green LEDs typically drop about 2V), and the widely used 20mA target current that gives a bright, safe brightness without shortening the LED's life. The calculator subtracts the LED's drop from the supply (9V − 2V = 7V left for the resistor) and divides by the target current to get 350Ω, then checks the resistor's own power dissipation — 0.14W here, so a standard quarter-watt resistor is fine.
Change the supply voltage, forward voltage or LED count above and the resistor value, power and nearest E24 preferred values all recompute instantly — useful the moment your battery, LED color or LED count differs from this example.
Example. The defaults, 5 V, a 2 V drop, one LED, and 20 mA, return 150 Ω and 60 mW. The nearest E24 values are 150 Ω, 160 Ω, and 130 Ω. 9 V with the same LED and current returns 350 Ω and 140 mW.
Limit. The LEDs are in series. The calculation stops when the combined forward drop is not strictly below the supply. The current unit menu opens on milliamps.