Choose a safe series resistor for one LED row and see the real current after standard resistor rounding.
Table of contents
How to use our LED Resistor Calculator
- Enter Supply voltage for the DC source feeding the LED row, such as 5 V, 9 V, or 12 V.
- Pick an LED color preset, or choose Custom and type the LED forward voltage from the LED datasheet.
- Enter Wanted LED current and LEDs in a row for one series path that uses one resistor.
- Open Advanced options if you want a standard resistor family, a power safety factor, repeated identical rows, or a different number display.
- Click Calculate, then sanity-check that Total LED voltage drop is below Supply voltage and that Heat in each resistor is below the resistor watt rating you plan to buy.
Definitions
Supply voltage: The DC voltage feeding the LED row and resistor.
LED forward voltage: The voltage drop across one LED when it is on.
Wanted LED current: The target current through one LED row, entered in milliamps.
LEDs in a row: The number of LEDs connected end-to-end in one series path with one resistor.
Resistor values to choose from: The choice between the exact math value and common E-series resistor values you can buy.
Actual LED current with that resistor: The current after the calculator rounds the resistor up to the chosen standard value.
Heat in each resistor: The power that one resistor turns into heat, measured in watts.
Identical rows of LEDs: Repeated LED rows that each have their own resistor and use the same values.
Common mistakes and quick fixes
Mistake: Using Supply voltage from an AC wall outlet or an unlabeled charger.
Fix: Use the DC output voltage that actually feeds the LED circuit, such as the regulated 5 V rail.
Mistake: Leaving LED forward voltage at a color preset when your LED datasheet gives a different value.
Fix: Choose Custom under LED color preset and enter the datasheet voltage in LED forward voltage.
Mistake: Typing 0.02 in Wanted LED current because you are thinking in amps.
Fix: Enter milliamps, so 0.02 A becomes 20 in Wanted LED current.
Mistake: Putting the total number of LEDs in the project into LEDs in a row.
Fix: Count only the LEDs connected end-to-end with one resistor, then use Identical rows of LEDs for repeated rows.
Mistake: Sharing one Resistor to use across several parallel LED rows.
Fix: Give each row its own resistor; Identical rows of LEDs only estimates Total supply current and Total resistor heat.
Mistake: Buying a resistor rated exactly equal to Heat in each resistor.
Fix: Use Minimum resistor watt rating as the lower limit, then choose the next common watt rating above it.
Limitations & Key Assumptions / Boundary Conditions
- This calculator is for one simple DC LED row with one series resistor per row.
- It assumes the LED forward voltage you enter is a good estimate at the current you want. Real LEDs vary by part, temperature, and current.
- If Total LED voltage drop is at or above Supply voltage, the calculator cannot choose a series resistor. Raise Supply voltage, use fewer LEDs in a row, or choose LEDs with lower forward voltage.
- If Voltage left for the resistor is very small, small changes in the power supply or LED voltage can cause a large current change.
- High-power LEDs, often above about 100 mA, usually need a constant-current LED driver and thermal design instead of only a resistor.
- Minimum resistor watt rating is a helper based on Power safety factor. Real resistor temperature also depends on package size, airflow, board layout, and nearby heat sources.
- The standard resistor result rounds up to avoid current above the target, so Actual LED current with that resistor may be lower than Wanted LED current.
Methodology
Core calculation
The calculator first finds the voltage used by the LEDs, then the voltage left across the resistor. Ohm's law relates voltage, current, and resistance [1].
V_led_total = N_series * V_forward
V_resistor = V_supply - V_led_total
R_exact = V_resistor / (I_wanted_mA / 1000)
If a standard resistor family is selected, the calculator chooses the next standard value at or above the exact resistance. Rounding up lowers the current instead of raising it.
R_chosen = next_standard_value_at_or_above(R_exact, resistor_series)
Current and heat
After the resistor is chosen, the calculator recomputes the current through the LED row. Resistor power is found from the current and resistance [2].
I_actual_mA = 1000 * V_resistor / R_chosen
P_resistor = (I_actual_mA / 1000)^2 * R_chosen
P_min = P_resistor * power_headroom
For repeated identical rows, each row is treated as a separate copy with its own resistor.
I_total_mA = I_actual_mA * N_branches
P_total = P_resistor * N_branches
Worked example
For a 5 V supply, one red LED at 2.0 V, and a wanted current of 20 mA, the LED uses 2.0 V and the resistor gets 3.0 V. The exact resistor is 3.0 / 0.020 = 150 ohms. With E12 selected, 150 ohms is already a standard value, so the actual current stays 20 mA. The resistor heat is 0.020 * 0.020 * 150 = 0.06 W. With a power safety factor of 2, the minimum watt rating is 0.12 W, so a common 1/4 W resistor is above that minimum.