Lighting Calculator — LED Resistor & Power Planning
Plan LED quantities, calculate resistor values and size the power supply for diorama lighting effects.
How We Calculate This
The calculator uses Ohm's law to size the current-limiting resistor for each LED string in your diorama.
Formula
- R = (Vsupply − n × Vforward) ÷ Iled — where n is the number of LEDs in the series string
- Resistor power P = (Vsupply − n × Vforward) × Iled
- n LEDs in series share one resistor, up to n = floor(Vsupply ÷ Vforward) (e.g. 12 V ÷ 3.2 V → 3 warm-white LEDs per string)
- Each parallel string needs its own resistor and draws the full LED current
Choosing the resistor
The ideal resistance is rounded up to the next E12 standard value (10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82 and their decade multiples). Rounding up keeps the actual current at or just below the rated value, which is the safe direction for an LED. Fit a resistor rated above the calculated wattage — a ¼ W (0.25 W) part is plenty for typical diorama LEDs.
Uneven strings
If the LED count is not an exact multiple of the LEDs-per-string, the last string is shorter. A shorter string drops less LED voltage, so it needs a larger resistor — this is calculated and shown separately so every string stays at the target current.
Frequently Asked Questions
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Last updated: February 2026
All calculations are estimates. Always verify quantities before purchasing materials.