LED resistor calculator
Series resistor for an LED, with the power it will dissipate.
Fill in the fields and the result will appear here automatically.
Find the nominal series resistor for one LED on a DC supply. Forward voltage Uf is assumed known at the chosen current and temperature. The page shows voltage drop, resistor dissipation and LED electrical input power; it does not select a standard resistor value or thermal margin.
How it works
Formula and logic
ΔU=Us−Uf>0, R=ΔU/I, PR=ΔU·I, PLED=Uf·I. Convert mA to A by dividing by 1000; an A input needs no conversion. Us, Uf, I must be positive. A fixed Uf model does not solve the actual nonlinear I–V curve.
Example
5 V, Uf=2 V and 20 mA give 150 Ω, 0.06 W resistor loss and 0.04 W LED electrical power. Entering 0.02 A gives the same result. When Us=Uf, this model cannot determine a positive resistor for the requested current.
Fields and units
- Supply voltage — V
- Forward voltage at chosen current — V
- Operating current — in the selected current unit
- Current unit — list option
How to use
- — Use datasheet Uf at the relevant current and temperature, not colour alone.
- — Choose mA or A before entering current.
- — For an actual resistor value, recalculate current with supply, LED and resistance tolerances.
- — PR is not a ready-made component rating; apply datasheet derating and mounting conditions.
Method and limitations
- Calculation method
- Formula and logic
- Data or methodology source
- OpenStax: common series current and parallel-branch voltage TI SLVA325: LED I–V variation and ballast loss — separate parallel-LED circuit
- Limitation
- One LED, DC and assumed Uf; no automatic preferred value, thermal design or guaranteed operating current.
FAQ
Why must the forward voltage be below the supply?
The resistor exists to drop the difference. With no difference there is nothing to drop and no operating point to set.
Which resistor should I actually buy?
The next standard value at or above the result lowers nominal current at unchanged Us, Uf. It does not guarantee maximum current: check Us, max, Uf, min and R, min, then dissipation and thermal derating.
Does the current unit matter?
Only for entry. Milliamps and amps give the same answer once converted, and the calculator converts for you.
Is the LED power the same as the resistor power?
The same current flows through both: PR=(Us−Uf)I and PLED=UfI, whose sum is UsI. PLED is electrical input power; some may become light, so it is not all heat.