Standard resistance, current, and resistor heat
The selected standard value produces current at or below the entered target in the fixed-voltage model. The tolerance row uses the resistor’s lower resistance limit. The rating output is twice that row’s dissipation, leaving a stated 50% loading allowance; choose an available rated part and check its datasheet derating. Electrical quantities in the table use up to eight significant digits, with scientific notation for very small or large values.
To check voltage, current, or power for a separate ohmic load, use the Ohm’s Law Calculator .
For the resistance of the wiring feeding a circuit, use the Wire Resistance Calculator .
The formula
Vs and Vf are in volts, n is a whole-number count of LEDs in one series branch, and the entered current is in milliamperes. The rounded resistance comes from the selected preferred-value series. LED forward voltage is treated as fixed, including in the tolerance case; supply variation and LED voltage spread are not included.
One LED on a 5 V supply
With supply 5 V, forward voltage 2 V, and target 20 mA, the ideal resistance is 150 Ω. E12 already contains 150 Ω, giving nominal current 20 mA and dissipation 0.06 W. At −5% resistor tolerance, resistance is 142.5 Ω and current about 21.053 mA. The 50% loading calculation requires a rating of at least 0.126316 W before any additional derating.
How to use this calculator
- Enter supply and forward voltages, the number of LEDs in a single series string, and the target current in mA.
- Choose E12 or E24 values. In advanced options, enter the tolerance of the actual resistor you intend to use.
- Calculate, compare the tolerance row with the LED’s permitted operating current, and choose a suitable resistor rating from its datasheet.
Use forward voltage at the planned current
The same LED can have different forward-voltage specifications at different test currents and temperatures. Use the operating information for the actual part. If the sum of series forward voltages reaches or exceeds the supply, there is no positive voltage left for a current-limiting resistor and this model rejects the combination.
Standard value spacing does not set tolerance
E12 and E24 describe available nominal resistance steps, while the tolerance control describes uncertainty in an actual component. Selecting a value series does not automatically set tolerance. Upward rounding reduces nominal current, but a resistor at its lower tolerance boundary can raise it again. Compare that case against your design limit, rather than treating the nominal value as a guarantee.
Each parallel string needs its own current limiting
The LED count describes one series branch, not several LEDs connected directly in parallel. For repeated branches, calculate the resistor for each branch and account for their combined supply current separately. A single resistor shared by unequal parallel LEDs can produce uneven currents. The table helps compare resistor choices; it does not model a constant-current driver or high-power lighting thermal design.
Assumptions & limitations
What this calculation assumes
- Steady DC supply and identical fixed forward voltages within one series branch.
- The reported rating uses an explicit 50% loading allowance, not a universal component rule.
What to keep in mind
- No supply extremes, LED forward-voltage tolerance, temperature model, transistor drop, pulsed operation, or resistor thermal derating.
Common questions
Why choose the next higher resistor instead of the nearest value?
A smaller nominal resistance increases current above the entered target in this model. Upward selection avoids that nominal increase.
Is a 0.125 W resistor enough in the example?
The selected 50% loading allowance needs at least 0.126316 W at the lower tolerance limit, so 0.125 W falls just below it. Check an available larger part and its operating-temperature derating.
Can I enter the maximum LED current from a datasheet?
Use your intended operating current. An absolute maximum is a boundary, not automatically an appropriate operating target.