LED Resistor Calculator
This LED Resistor Calculator determines the current-limiting resistance required between a voltage source and an LED string. It also selects the next higher E24 value, estimates resistor dissipation, and recommends a practical power rating.
Use it for indicator LEDs, embedded hardware, panel controls, prototypes, and low-power lighting circuits. Series LED count and parallel branch count are handled separately so voltage headroom and total supply current can be reviewed together.
Each parallel LED string is assumed to use its own resistor. Results are based on the entered forward voltage and target current, so the selected LED datasheet remains the primary source for device limits.
LED Resistor Calculator
Calculate one current-limiting resistor per LED branch, select the next higher E24 value, and check current and power requirements.
Voltage applied to each resistor and series LED branch.
Typical forward voltage of one LED at the target current.
Target current through one LED string and its resistor.
LEDs sharing one branch resistor.
Identical LED strings, each with its own resistor.
Recommended component
150 Ω
Next higher E24 resistance
Recommended power rating
0.25 W
Component selection
- Calculated resistor value
- 150Ω
- Nearest higher E24 resistor
- 150Ω
- Voltage across resistor
- 3V
- Resistor power dissipation
- 60mW
- Minimum derated power rating
- 0.125W
- Recommended power rating
- 0.25W
Circuit summary
- Estimated current per branch
- 20mA
- Total target LED current
- 20mA
- Total circuit current
- 20mA
- Total power estimate
- 100mW
Total target LED current uses the requested current. Total circuit current estimates the slightly lower current produced by the selected E24 resistor.
LED branch topology
One current-limiting resistor is used for each parallel LED string.
- Series string
- 1 LED
- Parallel paths
- 1 branch
- Current control
- One resistor per branch
LED Current-Limiting Resistor Formulas
Use base SI units in the equations: volts, amperes, ohms, and watts. The calculator converts V/mV and A/mA inputs before applying the formulas.
Voltage across resistor: Vr = Vs - (Vf × Nseries)Resistor value: R = Vr / IfResistor power: P = If² × RTotal current: Itotal = If × Nparallel- Vs = Supply voltage in volts (V)
- Vf = Forward voltage of one LED in volts (V)
- Nseries = Number of LEDs connected in each series string
- If = Target current through one branch in amperes (A)
- Nparallel = Number of identical parallel LED branches
- R = Current-limiting resistance in ohms (Ω)
How to Use This Calculator
- Enter the supply voltage and the typical LED forward voltage from the datasheet at the intended operating current.
- Enter the target current through one LED string in amperes or milliamperes.
- Set the number of LEDs connected in series and the number of parallel branches. Each branch is modeled with a separate resistor.
- Press Calculate and review the exact resistance, next higher E24 value, estimated current, resistor power, and total supply power.
- Confirm the resistor's tolerance, voltage rating, pulse rating, and thermal derating before finalizing the circuit.
Worked Example
A 5 V indicator circuit uses one LED with a 2 V forward voltage and a target current of 20 mA.
Design inputs
- Supply voltage
- 5 V
- LED forward voltage
- 2 V
- Target LED current
- 20 mA
- Series LEDs
- 1
- Parallel branches
- 1
Calculation sequence
- 1Vr = 5 V - (2 V × 1) = 3 V.
- 2R = 3 V / 0.02 A = 150 Ω, which is an E24 value.
- 3P = (0.02 A)² × 150 Ω = 0.06 W, or 60 mW.
Final selection: 150 Ω, 0.25 W
A 0.125 W part meets basic 50% derating; 0.25 W provides more practical thermal margin. Input power is approximately 0.1 W.
Engineering Notes
Treat the calculated value as a design starting point, then confirm device variation, thermal margin, and the behavior of every branch.
Forward voltage varies
LED forward voltage depends on color, semiconductor chemistry, current, temperature, and part number. Use the datasheet range rather than a generic color assumption for critical designs.
Round resistance upward
The next higher E24 value keeps nominal current at or below the target. Rounding down can increase LED current and reduce operating margin.
Check power and derating
Resistor dissipation rises with current squared. Select a rating with margin for ambient temperature, PCB copper area, enclosure heating, and manufacturer derating curves.
Use one resistor per branch
Parallel LED strings should generally have separate resistors. Small forward-voltage differences can otherwise create unequal current sharing and thermal imbalance.
Never omit current limiting
Do not connect an LED directly to a stiff voltage source. LED current changes sharply with voltage and can exceed the device rating without a resistor or regulated current driver.
Common Mistakes
- Entering milliamperes as amperes and calculating a resistor that is far too small.
- Using a typical forward voltage without checking its minimum and maximum range.
- Allowing the total series LED voltage to equal or exceed the supply voltage.
- Sharing one resistor across parallel LED branches.
- Selecting resistor wattage without thermal or reliability margin.
FAQ
How do you calculate the resistor for an LED?
Subtract the total LED forward voltage from the supply voltage, then divide the remaining voltage by the target LED current: R = (Vs - Vf total) / If. Use amperes and volts to obtain resistance in ohms.
What resistor do I need for a 5V LED circuit?
For one LED with a 2 V forward voltage at 20 mA, the calculated resistance is (5 V - 2 V) / 0.02 A = 150 Ω. Check the actual LED datasheet because forward voltage and safe current vary.
Why should I use the next higher standard resistor value?
A higher standard resistance keeps current at or below the design target when the exact calculated value is unavailable. This provides safer current limiting than rounding down to a smaller resistor.
How much power should the LED resistor handle?
Calculate P = I² × R for the selected resistor, then choose a rating with thermal margin. A resistor should normally operate well below its maximum rating, with ambient temperature and PCB cooling considered.
Can I connect multiple LEDs in series with one resistor?
Yes, if the supply voltage is greater than the sum of all LED forward voltages with enough remaining headroom for current regulation. LEDs in a series string carry the same current.
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Reference Links
Browse the Resistors calculator category for related component tools, visit the LEDs calculator category, or review supporting information in the Engineering Reference Center.
Disclaimer
This calculator uses an ideal constant forward-voltage model. Verify LED current limits, forward-voltage range, resistor ratings, thermal conditions, supply tolerance, and measured hardware performance before using the result in production equipment.