How to use it
- Type the Supply voltage and choose the LED colour. Leave Forward voltage per LED empty to use the typical value, or type the value from your LED's datasheet.
- Type the LED current (mA) you want and the number of LEDs in series.
- Read Resistor to fit (nearest E24 value above) and check the Current with the E24 value, which is a little below the set current.
- Buy a resistor with the Recommended power rating or a higher one. Open Working, step by step to see every figure.
How the resistor is calculated
An LED has an almost fixed voltage across it once it conducts, the forward voltage Vf. The series resistor takes the rest of the supply voltage, so the current is set by the resistor. With N LEDs in series the forward voltages add up.
R = (Vs − N × Vf) ÷ I
P = I² × R
Vs supply voltage N number of LEDs in series
Vf forward voltage per LED I LED current (A)- Typical forward voltages at 20 mA, from the Kingbright catalogue (typical values, not maximum): red 2.0 V, orange 2.0 V, yellow 2.1 V, green 2.2 V (GaP) and blue 3.3 V (InGaN).
- The resistor is rounded up to the next E24 value, so the LED never gets more current than you asked for. The E24 values are from IEC 60063, as listed on Wikipedia.
- The power is worked out with the current the E24 value actually gives, not the calculated one.
Power rating
The resistor must dissipate P = I² × R. The calculator suggests a rating of at least twice that power. This is a common design margin that keeps a resistor cool and stable; it is not a figure taken from a standard, so choose a rating your supplier sells that is above the suggestion.
What this calculator does not cover
- White, purple and other LEDs whose forward voltage is not in the list. Type the value from their datasheet.
- Tolerance of the forward voltage between LEDs of the same type, and the change of forward voltage with temperature.
- Constant-current drivers, PWM dimming and LEDs that flash: these need a different circuit.
Worked example: one red LED on 5 V at 20 mA
With N = 1, Vf = 2.0 V and I = 20 mA, the resistor is R = (5 − 1 × 2.0) ÷ 0.02 = 150 Ω, which is already an E24 value.
The current with 150 Ω is 3 V ÷ 150 Ω = 20 mA, and the power is P = (0.02 A)² × 150 Ω = 60 mW.
The recommended rating is at least 2 × 60 mW = 120 mW, so choose a 150 Ω resistor whose rating is above that figure.
Frequently asked questions
Why is the nearest E24 value rounded up?
A resistor value above the calculated one gives a slightly lower current, which protects the LED. Rounding down would push the current above the value you chose.
What happens if the supply is lower than the LED voltage?
No current can flow through a resistor that would need negative voltage across it. Use a higher supply, fewer LEDs in series, or an LED with a lower forward voltage.
How much current should I give an LED?
Indicator LEDs are often run at 20 mA, but the maximum current is on the datasheet. A lower current makes the LED dimmer and lasts longer.
Can I put several LEDs in series?
Yes, if the supply voltage is above the sum of their forward voltages. Each extra LED takes its forward voltage from the supply, so the resistor gets smaller and the LEDs share one current.
Why does the current differ slightly from what I entered?
The resistor is an E24 value, which is rarely exactly the calculated one. The calculator shows the current the standard value gives, so you can judge the difference.
Sources
- Kingbright catalogue, forward voltage table (red and orange series)
- Kingbright catalogue, forward voltage table (orange and green series)
- Kingbright catalogue, forward voltage table (yellow and blue series)
- Wikipedia: E series of preferred numbers (E24 values, IEC 60063)
Figures and text last checked on Tuesday, November 10, 2026.