Energy & Workshop Math
LED Resistor Calculator
Calculate the series resistor for one LED string from supply and forward voltage.
Enter your measurements
Results update as you type.
About this calculator
For an isolated low-voltage DC indicator-LED circuit, estimate the nominal resistor for one series string from supply voltage, datasheet forward voltage, target current and series count. The preferred resistor and a 2× nominal-power allowance are calculation examples, not a complete thermal or worst-case design. Never connect this circuit model directly to mains.
How to use this calculator
- Enter the supply voltage.
- Pick the LED colour to pre-fill a typical forward voltage, or enter the datasheet value.
- Enter the target current and the number of LEDs in series.
- Choose the next preferred resistor value and a wattage at least double the dissipation.
Formula
R = (supply voltage − LEDs in series × forward voltage) ÷ LED current
Power = current² × R
Choose the next E12/E24 preferred value at or above R
Suggested wattage ≥ 2 × power
Assumptions
- The supply is a stable DC voltage and the LED current is the design target.
- Forward voltage is a typical value at the design current; real LEDs vary.
- One series string only — parallel strings need ballast per string or a driver.
Common mistakes
- Entering 20 A when the LED target is 20 mA.
- Using one shared resistor to balance multiple parallel strings.
- Ignoring resistor tolerance, derating and LED datasheet limits.
Worked example
Try it: press “Load example” above the form.
12 V, one red LED at 2.0 V and 20 mA: (12 − 2) ÷ 0.020 = 500 Ω; the next E24 value is 510 Ω and the resistor dissipates 0.2 W.
Limitations
One isolated low-voltage DC series string only. No mains, isolation, thermal, PWM, worst-case supply/tolerance or driver design. Use the actual LED and resistor datasheets; high-power applications require an appropriate current-controlled driver design. Accepting a number does not establish that a supply is isolated or that voltage/current values are safe; this form does not validate electrical safety.
Frequently asked questions
Do I need a resistor for each parallel LED string?
Yes, each independent string needs appropriate current control. This calculator models one series string with a stable DC supply and does not balance current between parallel LEDs.
Why choose a preferred resistance above the exact value?
A larger nominal series resistor reduces the ideal LED current. The tool selects the next E12 or E24 value at or above the calculated resistance and shows a power allowance. Tolerance, temperature and datasheet limits still apply.
What does the worked example show?
12 V, one red LED at 2.0 V and 20 mA: (12 − 2) ÷ 0.020 = 500 Ω; the next E24 value is 510 Ω and the resistor dissipates 0.2 W.
Which assumptions should I check before using the result?
No thermal, dimming, PWM or driver design. High-power LEDs need constant-current drivers, not a series resistor. Check LED and resistor datasheets.
Sources
Sources explain the methods and any named product presets. Check your product label where coverage, yield or density differs.
- Wikipedia: E series of preferred numbers
E12 and E24 preferred resistor values across decades.
- OpenStax: electrical power and energy
DC power and energy relationships. AC/runtime assumptions are stated separately and require appropriate domain review.
- Analog Devices: LED circuit design and nominal series resistance
Nominal series-resistor model, forward-voltage variation and current-control context. Use actual part datasheets.
- Vishay: preferred resistance decade table
E12/E24 nominal resistor values; not a component thermal-rating or worst-case tolerance guarantee.
Calculation documentation · Updated 2026-11-11 · Version 1.6.4. Estimates are planning aids. Confirm quantities, coverage and prices with your supplier before purchasing. Found an error? Report it.
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