LED resistor calculator
Enter the supply voltage, the LED's forward voltage (presets per color), the target current and how many LEDs share one string in series — get the exact minimum resistance, the next E24 standard value up, the real current that value gives, and the wattage the resistor must handle. To read the bands on the part you should buy, use the resistor color code calculator.
510 Ω
next E24 value up from the 500 Ω minimum
| Minimum resistance (exact math) | 500 Ω |
| E24 value to fit (rounded up) | 510 Ω |
| Actual current with the E24 fitted | 19.6 mA |
| Total LED forward drop | 2 V |
| Resistor power dissipation | 196 mW |
| Suggested resistor rating | 1/2 W |
Current at or below target
Forward-voltage presets are typical values for standard 5mm LEDs at ~20 mA. Real parts vary by manufacturer, bin and temperature — check your datasheet and use the custom Vf field for an exact result.
How it works
The resistor drops the voltage the LEDs do not use:R = (Vs − n × Vf) ÷ I, with I in amps, and dissipates that dropped voltage times the current as heat. All of it is one Ohm's-law step — the Ohm's law calculator solves the same four quantities from any two.
The exact minimum is then rounded UP to the next E24 standard value — never down, because a smaller resistor would pass more current than the target. The rating suggestion applies the usual 2× derating: run a resistor at no more than half its rated power, so a 0.2 W dissipation already calls for a 1/2 W part.
A worked example: 12 V and one red LED
Twelve volts, one red LED, 20 mA target: the resistor has to eat the difference between supply and forward voltage, (12 − 2.0) ÷ 0.02 = 500 Ω. That is the minimum. Fit exactly 500 and the LED runs at its target; the nearest stock value below, 470 Ω, pushes 21.3 mA through the same LED, past the rating of a standard 5mm part. The next E24 value up is 510 Ω, and 10 V across 510 Ω gives 19.6 mA — close enough that the brightness difference is invisible, with the current safely under target. The resistor burns 10 V × 19.6 mA, about 0.2 W, which is why the wattage note suggests a 1/2 W part once the usual 2× derating margin is applied; a bare 1/4 W resistor would run hot.
The same arithmetic covers strings. Three red LEDs on 12 V drop 6 V between them, leaving 6 V for the resistor, so 300 Ω sets the same 20 mA. Once the total forward voltage reaches the supply there is nothing left to drop: four white LEDs at 3.2 V each need more than 12.8 V, so they cannot share one string on a 12 V rail — the calculator shows the too-low-voltage warning instead of a value.
After the value comes the marking: 510 Ω reads green-brown-brown-gold on a 4-band part, and the resistor color code calculator decodes it both ways. All of the math above is one Ohm's-law step, and the Ohm's law calculator solves V, I, R and P from any two knowns. If the goal is a lower voltage rather than a current limit, that is a different circuit — the voltage divider calculator covers it, load included.
Frequently asked questions
What is the formula for an LED series resistor?
R = (Vs − Vf) ÷ I, with the current I in amps. For several LEDs in series, subtract every LED forward voltage first: R = (Vs − n × Vf) ÷ I. A 12 V supply with one 2.0 V red LED at 20 mA gives (12 − 2) ÷ 0.02 = 500 Ω.
Why should I never round the resistor value down?
A smaller resistor passes more current — Ohm's law cuts both ways. Rounding 500 Ω down to the nearest stock value, 470 Ω, raises a 20 mA target to 21.3 mA, over the rating of a standard 5mm LED. Rounding up to 510 Ω gives 19.6 mA instead: slightly dimmer on paper, invisible in practice, and safely under the limit. This calculator always picks the next E24 value up.
How accurate are the forward-voltage presets?
They are planning values for standard 5mm LEDs at about 20 mA, nothing more. Forward voltage depends on color, manufacturer, bin, current and temperature: red sits near 2.0 V, blue and white near 3.0–3.6 V, and two LEDs of the same color can differ by a few tenths of a volt. Check the datasheet of the actual part and enter its Vf with the custom option.
Do LEDs always need 20 mA?
No. 20 mA is the traditional maximum rating for 5mm LEDs, not a brightness requirement. Modern high-efficiency parts are clearly visible at 2–5 mA, and for a simple power indicator 1–2 mA is often enough. Lower current means a larger, cooler resistor and longer battery life — try 5 mA in the calculator and compare.
Should I wire multiple LEDs in series or in parallel?
Series, as long as the supply has the headroom: one resistor then sets the current for the whole string, and the supply only has to exceed the sum of the forward voltages. In parallel, give every string its own resistor — never share one resistor between parallel LEDs, because small Vf differences make one branch hog the current and age faster.