LED Current Calculator
This LED Current Calculator estimates operating current, resistor voltage, resistor power, LED power, total supply current, and total circuit power for resistor-limited LED circuits.
Use Calculate LED Current when the resistor is already known. Use Calculate Required Resistance when you have a target LED current and need the exact resistance plus the next higher E24 value.
Engineering tool
LED Current Calculator
Calculate LED branch current, resistor voltage, resistor power, LED power, total supply current, and required resistance for LED circuits.
Voltage applied to the LED branch.
Forward voltage of one LED at the expected operating current.
Resistance used in each LED branch.
Number of LEDs sharing the same branch current.
Identical LED branches connected in parallel.
Current per LED
20 mA
Result console
- Voltage across resistor
- 3V
- Current per branch
- 20mA
- Current per LED
- 20mA
- Total supply current
- 20mA
- Resistor power per branch
- 60mW
- Total resistor power
- 60mW
- Total LED power
- 40mW
- Total circuit power
- 100mW
Operating point looks reasonable for a basic resistor-limited LED circuit.
Formula reference
LED Current and Power Formulas
Use volts, amperes, ohms, and watts in the equations. The calculator converts engineering units before applying the formulas.
Vr = Vs - Vf × NseriesIbranch = Vr / RItotal = Ibranch × NparallelPresistor = Ibranch² × RPLED = Vf × Ibranch × Nseries × NparallelPtotal = Vs × ItotalRequired resistance: R = (Vs - Vf × Nseries) / IfVariable definitions
- Vs
- Supply voltage
- Vf
- Forward voltage of one LED
- Nseries
- LEDs connected in series per branch
- Nparallel
- Identical parallel LED branches
- R
- Series resistor per branch
- If
- Target LED current per branch
Variable Definitions
- Supply voltage
- The voltage source feeding each LED branch.
- LED forward voltage
- The voltage drop of one LED at the intended operating current.
- Series resistor
- The current-limiting resistance in each branch.
- LEDs in series
- The number of LEDs carrying the same branch current.
- Parallel branches
- The number of repeated LED strings connected to the supply.
- Voltage headroom
- The voltage left across the resistor after LED forward voltage is subtracted.
Worked Example
A 5 V supply drives one LED with a 2 V forward voltage through a 150 Ω resistor.
Inputs
- Supply voltage
- 5 V
- LED forward voltage
- 2 V
- Resistance
- 150 Ω
- Series LEDs
- 1
- Parallel branches
- 1
Expected results
- Voltage across resistor
- 3 V
- Current
- 20 mA
- Total current
- 20 mA
- Resistor power
- 60 mW
- LED power
- 40 mW
- Total circuit power
- 100 mW
Check LED datasheets
Forward voltage and maximum current vary with LED color, bin, current, and temperature.
Leave voltage headroom
Small resistor voltage makes current sensitive to supply tolerance and LED forward-voltage variation.
Use one resistor per branch
Parallel LEDs or strings should normally have separate resistors to improve current sharing.
Derate resistor power
Choose a resistor wattage above calculated dissipation, especially in warm enclosures.
Support reference
FAQ
How do you calculate LED current?
Subtract the total LED forward voltage from the supply voltage, then divide the remaining resistor voltage by the series resistor: I = (Vs - Vf x Nseries) / R.
What current should I use for an indicator LED?
Many small indicator LEDs are used around 2 mA to 20 mA, but the correct current depends on LED type, brightness target, thermal conditions, and datasheet limits.
Why does an LED need a resistor?
A resistor limits current because LED forward voltage changes with current, temperature, and device variation. Without current limiting, the LED can draw excessive current.
Can multiple LEDs share one resistor?
LEDs in series can share one resistor if the supply voltage provides enough headroom. Parallel LED branches should normally use separate resistors for better current sharing.
How much power should the LED resistor handle?
Calculate resistor power with P = I²R, then choose a resistor power rating with margin for ambient temperature, package size, and PCB cooling.
Documentation
Related Engineering Guides
Design notes, guides, and engineering articles linked to this tool.
Engineering Guide
Understanding LEDs
Learn LED fundamentals including forward voltage, forward current, wavelength, color, luminous intensity, LED types, packages, current limiting, and thermal design.
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Engineering Guide
How to Choose the Right LED
Choose LEDs by application, forward voltage, current, brightness, wavelength, CCT, viewing angle, package, power, thermal behavior, and datasheet limits.
15 min · Intermediate
Engineering Blog
10 Common LED Design Mistakes (and How to Avoid Them)
Avoid LED design mistakes involving current limiting, resistor value, forward-voltage variation, excessive current, resistor power, parallel branches, polarity, brightness, thermal design, and color selection.
14 min · Intermediate
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Engineering Disclaimer
This calculator provides first-order estimates for resistor-limited LED circuits. Verify LED current, resistor rating, thermal behavior, supply tolerance, and LED datasheet limits before production use.