Inputs
Vf and If come from the LED's datasheet (typ. ~2 V / 20 mA for indicator LEDs). The standard value is rounded up so the current never exceeds your target. Everything is computed in your browser — nothing is uploaded.
Result
Enter a supply voltage, forward voltage and current greater than 0.
- Exact resistance
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- Standard value
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- Actual current
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- Power in resistor
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- Min. power rating
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A calculator for the current-limiting resistor you put in series with an LED. Enter the supply voltage, the LED's forward voltage Vf and the forward current If (mA) you want, and it solves Ohm's law R = (supply − Vf) / If. Pick a color preset (red, orange, yellow, green, blue, white, UV, infrared) to fill in a typical Vf, or type the value from the datasheet. Since the exact computed resistance isn't sold, it rounds up to the nearest E6, E12 or E24 standard value (rounding up so the current never exceeds your target), then shows the actual current that flows (mA), the power dissipated in the resistor (W / mW) and a suggested minimum power rating (1/8 W, 1/4 W, 1/2 W…) with a 2× safety margin. Set how many LEDs are in series and it subtracts the total Vf (and warns when the supply is too low). Handy for electronics and microcontrollers (5 V / 3.3 V), 12 V / 24 V LED strips, car LED swaps, or any time you need the resistor value and rating fast. Everything is computed in your browser; the values you enter are never sent to any server or API.
How to use
- Enter the supply voltage (e.g. 5 V / 12 V), a color preset or forward voltage Vf, and the forward current If (mA).
- Choose the standard series (E6 / E12 / E24) and how many LEDs are in series.
- Read off the recommended resistor (standard value), the exact resistance, the actual current, the power and the minimum rating (nothing is sent).
FAQ
Are the values I enter sent anywhere?
No. The resistor calculation runs entirely in your browser. The values you enter are never sent to any server or API — everything stays on your device.
What formula is used?
Ohm's law: R = (supply voltage − LED Vf) / If. For LEDs in series it subtracts Vf × count. For example, 5 V supply, Vf 2 V, If 20 mA, one LED gives R = (5 − 2) / 0.02 = 150 Ω. Because real resistors come in standard values, it also shows the recommended standard value and the actual current.
Does it work for 12 V or 24 V supplies and LED strips?
Yes — enter 12 or 24 as the supply voltage. Increase the number of LEDs in series and it subtracts the total Vf. If you run several LEDs in parallel off one supply, give each branch its own resistor (use the single-LED result for each branch).
What are E6 / E12 / E24, and which should I pick?
They are the standard value series that off-the-shelf resistors come in; E24 is the finest (24 values per decade), with E12 and E6 coarser. Pick the series your resistors use. The tool rounds up to the nearest standard value at or above the computed value so the current stays at or below your target.
How do I choose the resistor's power rating?
It computes the power dissipated, P = (supply − Vf) × If, applies a 2× safety margin, and shows the smallest standard rating that covers it (1/8 W, 1/4 W, 1/2 W…). Indicator LEDs are usually fine with 1/4 W, but a large voltage drop or high current needs a bigger rating.