LED resistor calculator for a series string
Use this LED resistor calculator to estimate the current-limiting resistor between a DC source and one LED or several identical LEDs in series. It calculates the ideal resistance and, optionally, the next nominal E12 or E24 resistance at or above that ideal. It then recomputes string current and resistor dissipation from the selected resistance.
The existing Ohm’s Law Calculator solves general voltage, current and resistance relationships. This page adds the LED string’s combined forward-voltage drop, preferred resistor selection and LED-specific interpretation. It does not replace the LED datasheet or a constant-current driver design.
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Optional digital multimeters (paid link) can help check DC voltage and resistance within their stated measurement limits. No purchase is required.
Enter supply voltage, LED forward voltage and current
- Enter the positive DC supply voltage in volts, up to 60 V.
- Enter one LED’s forward voltage at the intended operating current, using the relevant datasheet conditions.
- Enter the target current in milliamperes and the whole number of identical LEDs in this series string.
- Choose the ideal resistance, or an upward-rounded E12 or E24 nominal value.
- Calculate and review current, power and the input-based steps.
A number such as 20 mA is an example, not a universal LED operating current. Forward voltage depends on current, temperature and manufacturing variation. The combined LED voltage must be below the supply voltage; otherwise the tool reports that positive resistor headroom is required.
Optional solderless breadboards (paid link) are useful for low-voltage prototype assembly. This calculation models one current path: it does not divide current among bare parallel LEDs or model an AC waveform.
Series LED resistor formula and preferred values
VR = Vs − VLED
Rideal = VR / Itarget
Iactual = VR / Rselected
Presistor = VR × Iactual
Convert milliamperes to amperes by dividing by 1,000. E12 and E24 are preferred nominal resistance sequences repeated by powers of ten. The calculator searches the chosen sequence for the smallest nominal value greater than or equal to the computed ideal resistance. It never deliberately rounds down. Exact mode retains the calculated resistance without choosing a stock part.
E12 and E24 selection does not apply a tolerance band or guarantee availability. Optional resistor assortments (paid link) should be checked for the actual value, tolerance, rated power and maximum working voltage. The estimate still uses the entered fixed LED voltage; the real LED’s nonlinear characteristic can change its current.
LED resistor calculator examples for 5V and 12V
5 V example: one LED with an entered 2 V forward voltage at a target 20 mA. The resistor has 5 − 2 = 3 V across it. Ideal R = 3 / 0.020 = 150 Ω. This is already an E24 value, so nominal current is 20 mA and resistor dissipation is 3 × 0.020 = 0.060 W. The displayed 2× reference is 0.120 W; it is not a temperature-tested component recommendation.
12 V series example: three identical LEDs at an entered 2 V each and a 20 mA target. Total LED drop = 6 V, leaving 6 V across the resistor. Ideal resistance = 300 Ω. E24 includes 300 Ω, giving 20 mA and 0.120 W. E12 rounds upward to 330 Ω, giving about 18.18182 mA and 0.1090909 W. The steps change when preferred-value selection is enabled.
Current and power assumptions to check
Twice the calculated resistor dissipation is shown only as a 2× planning reference. Choose and verify a real component using its datasheet derating curve, ambient temperature, ventilation, pulse behavior, package and working-voltage rating. A fixed multiplier alone cannot guarantee an acceptable resistor temperature.
Supply variation, LED forward-voltage spread, resistor tolerance, thermal behavior, source current limits and wiring resistance are not included. With little voltage headroom, small voltage changes can cause a large relative current change. Separate parallel strings need separate current limiting; high-power LED systems often need a suitable driver instead of this simple resistor model.
The supported numeric scope is 0.001–60 V supply, 0.001–60 V forward voltage per LED, 0.001–1,000 mA target current, 1–100 identical series LEDs and 0.001–1 billion ohms calculated ideal resistance. These are computational limits, not approved operating conditions. Results display up to seven significant digits; calculations retain floating-point precision.
Optional jumper wires (paid link) can connect a compatible breadboard prototype. Use the Electrical Power Calculator for a separate general power calculation and the Voltage Divider Calculator for a two-resistor signal ratio. A divider is not a substitute for LED current limiting.
LED Resistor Calculator FAQs
Subtract the total series LED forward voltage from the supply voltage, then divide the remaining voltage by the target current in amperes. The supply must exceed the LED string voltage.
Yes. Enter 5 or 12 as the supply voltage, along with forward voltage and a suitable target current from your LED data. No default LED voltage or current is assumed.
Enter the number of identical series LEDs. The model multiplies one LED’s forward voltage by that count. The same current flows through every LED in that one string.
Rounding the ideal resistance upward to a preferred nominal value keeps the calculated current at or below the entered target under the fixed-voltage model. Real component tolerances and LED voltage variation are not modeled.
The tool calculates resistor dissipation and shows twice that value as a planning reference. A real part must meet its datasheet power derating, voltage and thermal conditions; the reference is not a guaranteed rating.
No. It models one DC series string with one resistor, up to 60 V input. Separate parallel strings need their own current limiting, and AC or high-power driver design is outside scope.
Sources and methodology
Independent circuit equations and implementation, reviewed October 24, 2026. Manufacturer datasheets control real component ratings. The linked organizations do not endorse this tool.