Current Limiting Resistor Calculator
Enter supply voltage, load voltage and target current to size the series resistor.
Result
How it is calculated
The resistor must drop the voltage the load does not use, at the current you want. Vdrop = Vsupply − Vload, R = Vdrop / I and P = Vdrop × I. Vsupply is the source voltage in volts, Vload is the forward voltage of the LED or other load in volts, and I is the target current in milliamps (converted to amps internally). Results are the resistance in ohms (two decimals), the voltage across the resistor, and its dissipated power in watts (four decimals). The current must be above zero and the supply must be higher than the load voltage, otherwise there is no result. The tool gives the exact value: it does not round to a standard resistor value or to a standard wattage, so pick the next higher stock value and a rating with a safety margin yourself.
Worked example
An LED with a forward voltage of 2 V is run from a 5 V supply at 15 mA. Voltage across the resistor: 5 − 2 = 3.00 V. Current in amps: 15 / 1000 = 0.015 A. Resistance: 3 / 0.015 = 200.00 Ω. Power: 3 × 0.015 = 0.0450 W, so a small quarter-watt resistor is far enough above that figure to run cool.
Common mistakes
- Entering the current in amps. The field is milliamps, so 0.015 instead of 15 would give a resistor a thousand times too large and a dark LED.
- Using the LED's maximum voltage or leaving out the forward voltage. The load voltage is the typical forward voltage at your chosen current, which depends on the LED colour.
- Rounding the resistor down to the nearest stock value. A slightly lower resistance raises the current above target; round up to the next available value.
- Ignoring power. Small LED currents give tiny wattage, but a larger drop or current can overheat a quarter-watt part.
- Putting several LEDs in series and entering only one forward voltage. For a series string, add their voltages together.
About this calculator
A current-limiting resistor is placed in series with a component like an LED to keep the current at a safe, specified level, since LEDs and many other loads have a low, non-linear resistance that would otherwise let current spike and destroy them. The resistor value is found with a rearranged Ohm's law: R = (supply voltage − load's forward voltage) ÷ target current, and the power the resistor dissipates is found with P = I² × R (or I × V across the resistor). The result is the exact figure, so pick a standard resistor wattage comfortably above it to leave a safety margin.
This calculation is one of the first things taught in basic electronics, and it's used constantly by hobbyists wiring up LEDs on breadboards, Arduino and Raspberry Pi project builders, and electronics technicians repairing or prototyping circuits, since picking too small a resistor burns out the LED while picking too large a one leaves it dim.
This calculator takes your supply voltage, the load's forward voltage, and your target current, then returns the required resistor value, the voltage across it, and the power it dissipates; choose a resistor rated above that power so it does not overheat.
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