Series resistance
R = (V_s − V_f)/I_LED
The resistor must drop the voltage remaining after the LED junction.
LED Resistor Calculator
Estimate the series resistor needed to set LED current from a DC supply and calculate resistor and LED power.
Access: Free to use, no installation, and No account required.
Inputs
LED forward voltage varies with device, current, and temperature. Use the actual datasheet range and select a standard resistor with adequate power rating.
Results
Series resistor model
The resistor drops the supply voltage remaining after the LED forward voltage. The ideal resistance is R = (Vs − Vf)/I.
Engineering reference
LED Resistor Calculator: background and worked detail
Use the LED Resistor Calculator to estimate the series resistance required for a target LED current and to calculate resistor and LED power from supply voltage and assumed forward voltage.
Picking a series resistor, step by step
Use a 5 V source, 2 V forward voltage, and 15 mA target current.
- The resistor must drop 3 V.
- R = 3 V / 0.015 A = 200 Ω.
- Resistor power is 45 mW and LED electrical power is about 30 mW.
Result: A practical design would choose a standard resistor value and verify worst-case current and power.
Equations used by this calculator
Resistor power
P_R = (V_s − V_f)I = I²R
The resistor dissipates the dropped voltage times current.
LED power estimate
P_LED ≈ V_f I
A first-order electrical input-power estimate for the LED.
Why an LED needs current limiting
A series resistor sets LED current by dropping the supply voltage that remains after the LED forward voltage. The ideal relationship is R = (Vs−Vf)/I when Vs is greater than Vf.
LED forward voltage varies with current, temperature, device bin, and color. A robust design should verify current over supply and Vf tolerance, select an available resistor value, and check resistor power and LED thermal limits.
Assumptions and model boundaries
Assumptions
- LED forward voltage is represented as a fixed user-entered value.
- The resistor is ideal and linear.
- The supply is treated as a constant voltage source.
Limitations
- Does not solve the LED diode equation or temperature-dependent Vf.
- Does not determine optical output, junction temperature, pulsed-current rating, or current-regulator behavior.
LED Resistor Calculator FAQ
Should I use the exact calculated resistance?
Choose a standard resistor value that keeps worst-case current within the LED's limits, then verify resistor power with margin.
Why does LED forward voltage matter?
The resistor only drops Vs−Vf. A change in Vf directly changes resistor voltage and therefore current.
What if the supply voltage is below the LED forward voltage?
A simple series resistor cannot create the missing voltage. The LED may not reach the target current; a different supply or driver topology is required.
Validation checks
No voltage headroom
Case: Set supply voltage equal to forward voltage.
Expected: The calculator should indicate no valid finite resistor for the requested current.
Current scaling
Case: Halve target current with the same voltage headroom.
Expected: Required resistance should double and resistor power should halve.
Where this calculation comes from
Forward-voltage handling comes from the Introductory Circuits Workbench, which also models diode curves rather than a fixed drop.
Open the source workbench →Read calculation and source methodology →