This calculator does not automatically reject insufficient voltage headroom. If supply voltage is less than or equal to the total LED forward voltage, disregard all resistance, current and power results. A positive power result can still be invalid when headroom is negative.
Enter LED count as a whole number. Use the fixed units stated in the input labels: volts, amperes and ohms. The calculator treats inputs as numeric magnitudes and does not check electrical dimensions or convert units.
Use one resistor in series with one string of identical LEDs. Enter supply voltage and forward voltage per LED in volts, target current in amperes, and the resistor you intend to fit in ohms. For example, 20 mA is 0.020 A. LED count must be a whole number. No preferred resistor value is selected automatically.
Check voltage headroom first: it must be strictly positive. If it is zero or negative, disregard every resistance, current and power result. This calculator does not automatically suppress those results; even a positive power result can be invalid when headroom is negative.
Subtract the combined LED voltage drop from the supply, then apply Ohm’s law to the remaining voltage across the resistor. See SparkFun’s LED tutorial. Resistor dissipation is voltage across the resistor multiplied by branch current; see SparkFun’s Electric Power tutorial.
This is a constant-forward-voltage estimate for a DC circuit. Actual LED voltage varies with current, temperature and device tolerance. The entered resistor value, rather than the target current, determines the estimated actual current. Check component datasheets and resistor power derating before choosing parts.
| Inputs | Headroom | Required resistance | Estimated actual current | Actual resistor dissipation |
|---|---|---|---|---|
| 5 V supply; 2 V per LED; 1 LED; target 0.020 A; actual resistor 180 Ω | 5 − 1 × 2 = 3 V | 3 ÷ 0.020 = 150 Ω | 3 ÷ 180 = 0.016667 A (16.667 mA) | 3² ÷ 180 = 0.050 W |
| 12 V supply; 2 V per LED; 3 LEDs; target 0.020 A; actual resistor 330 Ω | 12 − 3 × 2 = 6 V | 6 ÷ 0.020 = 300 Ω | 6 ÷ 330 = 0.018182 A (18.182 mA) | 6² ÷ 330 = 0.109091 W |
| 6 V supply; 2 V per LED; 3 LEDs; target 0.020 A; actual resistor 330 Ω | 0 V: insufficient | Invalid design; do not interpret as a 0 Ω recommendation | Invalid result; disregard | Invalid result; disregard |
| 5 V supply; 2 V per LED; 3 LEDs; target 0.020 A; actual resistor 330 Ω | −1 V: insufficient | Invalid design | Discard the negative arithmetic result | Discard the positive arithmetic result |
This calculator accepts supply voltage from 0 to 48 V, forward voltage from 0.10 to 6 V per LED, 1 to 100 series LEDs, target current from 0.0001 to 0.1000 A, and actual resistance from 0.1 to 1,000,000 Ω. These are tool input limits, not component ratings. Positive current and resistance bounds prevent zero denominators. An unavailable result must not be interpreted as zero.
Use a whole-number LED count and keep inputs within the stated ranges. Input bounds alone do not ensure positive voltage headroom: check that supply voltage exceeds the combined LED forward voltage before using the results.