LED Resistor Calculator
Calculate the series resistor an LED needs: required ohms, nearest E12/E24 value, wattage and colour bands.
- Required resistance
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- Nearest standard value
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- Actual LED current
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- Power in the resistor
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- Minimum resistor rating
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- Voltage across resistor
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Resistor colour code
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Everything is calculated in your browser — nothing is uploaded, and there are no ads or sign-ups. Wiring several LEDs in parallel? Give each branch its own resistor instead of sharing one: small differences in forward voltage make a shared resistor push far more current through one LED than the others.
How it works
A series resistor turns the voltage your supply has left over into heat so the LED sees the
current it was designed for. The calculator uses Ohm's law applied to that leftover voltage:
R = (Vsupply − n × Vf) ÷ I, where n is the number
of LEDs wired in series and I is the forward current in amps. The power the resistor
must get rid of is P = (Vsupply − n × Vf) × I, which is the
same as I² × R. Example: a 12 V supply, one red LED at Vf = 2.0 V and
20 mA needs (12 − 2) ÷ 0.02 = 500 Ω dissipating 10 × 0.02 = 0.2 W.
Exact values like 500 Ω are rarely on the shelf, so the tool also picks the nearest standard value at or above the calculated resistance from the E24 (5%) or E12 (10%) series — rounding up keeps the current at or below your target, which is the safe direction. It then recalculates the real current through the LED, shows the resistor's colour bands, and recommends a power rating with roughly 2× headroom over the calculated dissipation. Forward voltage varies between parts, so check the datasheet of your LED when you can; the colour presets are typical values only.
Frequently asked questions
How do I calculate the resistor value for an LED?
Subtract the LED forward voltage from your supply voltage and divide by the forward current in amps: R = (Vsupply − n × Vf) ÷ I, where n is the number of LEDs wired in series. A 12 V supply driving one red LED (Vf = 2.0 V) at 20 mA needs (12 − 2) ÷ 0.02 = 500 Ω. The resistor dissipates P = (Vsupply − n × Vf) × I = 10 × 0.02 = 0.2 W, so a 0.5 W part gives comfortable headroom.
Should I round the resistor up or down to a standard value?
Round up. Standard E24 (5%) and E12 (10%) values rarely land on the exact number, and a slightly larger resistor means slightly less current through the LED — the safe direction. This calculator picks the smallest standard value at or above the calculated resistance and then shows the real current you will get. For the 500 Ω example the next E24 value is 510 Ω, giving 19.6 mA instead of 20 mA, a difference you cannot see.
Can several LEDs share one resistor?
Only if they are wired in series, which is what the LEDs in series field assumes — then the same current flows through all of them and their forward voltages add up. LEDs wired in parallel should each get their own resistor: small differences in forward voltage make the lowest-Vf LED hog the current, so it runs bright and hot while the others stay dim. Also check that your supply voltage exceeds the total forward voltage of the string, or almost no current flows at all.