LED Resistor Calculator
The little resistor that keeps the diode honest — ohms, dissipation and headroom for a series LED chain on any supply.
LED Resistor Calculator
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What this result does not account for
- Ballast-resistor model — no driver efficiency
- Single forward voltage per diode, no curves
In short: A single 2-volt LED on a 5-volt rail at 20 milliamps: the resistor eats the spare 3 volts and so wants 150 ohms, burning 0.06 W — an eighth-watt part carries it with double the margin, a quarter-watt with four. Add a second identical LED in series and the pair eats 4 volts, the resistor's share halves to 50 ohms, and the heat halves with it. The chain ends where the LEDs' drops meet the rail — at that line the resistor runs out of volts to eat.
Formula
R = (Vs − n·Vf) ÷ I — P = I²R
An LED is a diode, not a lamp: past its forward voltage it conducts ferociously, so something must set the current. The series resistor eats the difference between supply and chain drop, and Ohm's law sets its value. Its dissipation is the square of the current times the resistance — usually the quarter-watt class. The resistor, not the diode's datasheet, is what turns a voltage source into a current source; for big chains a constant-current driver takes over.
Worked Example
- Sum the chain's forward drops.
- The rail's spare volts over the current is R.
- Size the resistor's body for I²R with margin.
Defaults: one 2 V LED on 5 V at 20 mA — 150 ohms eating 3 volts, 0.06 W of heat. The 4-LED chain on the same rail hits the wall: 8 volts of drop, no spare, no answer that is not a driver.
Strengths & Limits Of This Model
Where this engine is strong
- Chain, heat and headroom in one answer
- Honest wall when the drops meet the rail
Where it stops
- Resistor ballast wastes the spare volts as heat
Practical Use Cases
Panel indicators
the classic 5 V, 150 ohm job
Light strips
chain maths before the cut
Bench prototypes
keep the magic smoke inside
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
LED Resistor Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
Why does an LED need a resistor at all?
Its current climbs steeply once the forward volt age is crossed, and small manufacturing differences become large current differences. The resistor adds a ballast drop that absorbs those differences — crude, cheap and effective for indicator currents.
Can I parallel LEDs on one resistor?
No — the diode with the lowest drop hogs the current and the rest stay dark or die. One resistor per diode, or a proper driver, always.
Why does the chain stop at the rail?
Drops in series add; when they equal the supply there is nothing left for the ballast, and beyond it there is no solution at all. This is the honest wall: parallel strings or a boost driver are the ways onward.
How much margin should the resistor body have?
Double at minimum — the computed dissipation against a part rated twice it, so ventilation and tolerance cannot cook it. The heat line quotes the exact figure; pick the next standard rating up with room to spare.
Does the resistor set the brightness?
It sets the current, and current sets the brightness within a diode generation. Modern LEDs blind at a few milliamps — the 20 mA of the classics is often more indicator than eye needs.
Why is my LED dim at the computed value?
The forward voltage was guessed high, or the supply sags under load. Measure the drop at current, retype it, and the ohms re-solve honestly.
When does a constant-current driver win?
When the chain is long, the watts are real or efficiency matters: a driver recovers the ballast volts as light instead of heat. The resistor is honest up to perhaps half a watt of loss; past that it is a small heater.
What about pulsing for more brightness?
Peak currents multiply while average stays within rating — the eye integrates. The computed resistor governs the average; pulse ratings live in the diode's sheet, not in this arithmetic.