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DIY Cost Tools

DIY Cost Tools

LED Series Resistor Calculator

Estimate a current-limiting resistor and its power dissipation for LEDs connected in series to a DC source.

Use the units shown beside each field. Decimal and scientific notation are accepted for non-integer fields; blanks, commas, fractions and non-finite entries are rejected.

Total LED forward voltage6 V
Required resistance300 Ω
Resistor power dissipation0.12 W
Minimum resistor rating0.24 W

The displayed results apply to the default example. Enable JavaScript to calculate other inputs.

How to use this calculator

  1. Replace the example values with measurements or ratings for the exact vehicle, circuit, component or system.
  2. Keep every value in the labeled unit and use manufacturer data where available.
  3. Select Calculate, then review the result together with the limitations and safety notes.

Formula and method

Resistance = (supply voltage − total LED forward voltage) ÷ current in amperes. Resistor power = current² × resistance.

Calculations run in your browser. Results are hidden whenever an input changes or validation fails, preventing an older result from being mistaken for a new calculation.

Worked example

Supply voltage = 12 V; LED forward voltage each = 2 V; LEDs in series = 3; Target current = 20 mA; Power safety factor = 2 ×. The calculator returns Total LED forward voltage = 6 V; Required resistance = 300 Ω; Resistor power dissipation = 0.12 W; Minimum resistor rating = 0.24 W.

The worked example uses the same defaults shown in the interface, allowing the formula and displayed result to be checked together.

Limitations and safety

LED forward voltage changes with current, temperature and part variation. Use manufacturer limits, a standard resistor value, adequate thermal margin and safe wiring.

This is an educational planning model. Confirm ratings, tolerances, code requirements and safe procedures with manufacturers and qualified professionals before purchasing, modifying, wiring or operating equipment.

Frequently asked questions

Should I choose the exact calculated resistance?

Usually choose the next higher standard resistance to avoid exceeding target current, then verify brightness and dissipation.

Why does the calculator reject my entry?

Each field shows its supported range. Whole-number fields reject decimals, and all fields reject blanks, words, comma-formatted numbers, fractions and non-finite values.

How should I use the displayed precision?

Keep extra digits while checking the calculation, then round to the precision supported by the measurements, component tolerances and published ratings.

Review notes and reference

Reviewed October 4, 2026. The formula and default example were checked together. The script was tested for defaults, input boundaries, invalid formats, condition-specific errors and recovery after an error. Reference: OSHA electrical safe-work requirements. Explanations and examples are original.

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