LED Resistor Calculator
Series resistor for an LED or LED string: required ohms, the nearest E12 part, the current you actually get, power dissipated and the wattage to buy.
LED Resistor Calculator: with the default inputs, required resistance (ω) is 500.
The voltage feeding the LED and its resistor.
From the datasheet: red/amber ≈ 1.8–2.2 V, green ≈ 2.1–3.0 V, blue/white ≈ 2.8–3.4 V.
20 mA is the classic indicator LED figure; modern high-efficiency parts are bright at 2–5 mA.
Parallel branches each need their own resistor; sharing one is the classic mistake.
Ohms, per resistor. Exact value before rounding to a stock part.
- Nearest E12 standard value (Ω)
- 470
- Current with that E12 part (mA)
- 21.28
- Power in the resistor (mW)
- 200
- Resistor wattage to buy
- 1/2 W
- Resistors needed
- 1
- Total supply current (mA)
- 20
- Total power from the supply (mW)
- 240
- Power in the LEDs (mW)
- 40
- Share of power reaching the LEDs
- 16.7%
Assumptions
- Forward voltage is treated as constant at the current you enter; a real LED's Vf rises slightly with current and falls with temperature.
- The supply is stiff (its voltage does not sag under load).
- Parallel mode assumes one resistor per LED, which is the correct way to build it.
- Wattage recommendation derates to 50% of the resistor's rating.
| Colour / type | Vf at 20 mA | Resistor from this supply |
|---|---|---|
| Infrared | 1.2–1.6 V | 560 Ω |
| Red | 1.8–2.2 V | 470 Ω |
| Amber / yellow | 2.0–2.2 V | 470 Ω |
| Green | 2.1–3.0 V | 470 Ω |
| Blue | 2.8–3.4 V | 470 Ω |
| White | 2.9–3.4 V | 470 Ω |
Forward voltage is a range, not a constant — it varies part to part and drops as the LED heats. The right-hand column is the nearest E12 resistor for 1 LED(s) of that colour at 20 mA from your 12 V supply.
How this is worked out
The formula
Series string: R = (Vsupply − n × Vf) ÷ I Parallel: R = (Vsupply − Vf) ÷ I, one resistor per LED P_resistor = I² × R = (Vsupply − n × Vf) × I Recommended rating = smallest standard value ≥ 2 × P (50% derating) I is in amperes: 20 mA = 0.02 A
Open How it’s calculated above to see this worked through with your own numbers.
What you enter
- Supply voltage
- The voltage feeding the LED and its resistor.0 or more · defaults to 12
- LED forward voltage (Vf)
- From the datasheet: red/amber ≈ 1.8–2.2 V, green ≈ 2.1–3.0 V, blue/white ≈ 2.8–3.4 V.0 or more · defaults to 2
- Desired LED current
- 20 mA is the classic indicator LED figure; modern high-efficiency parts are bright at 2–5 mA.0 or more · defaults to 20
- Number of LEDs
- A number.1 or more · whole numbers only · defaults to 1
- Arrangement
- Parallel branches each need their own resistor; sharing one is the classic mistake.Series — all LEDs in one string, one resistor · Parallel — each LED gets its own resistor
What you get back
- Required resistance (Ω)main answer
- Ohms, per resistor. Exact value before rounding to a stock part.
- Nearest E12 standard value (Ω)
- Current with that E12 part (mA)
- Power in the resistor (mW)
- Resistor wattage to buy
- Resistors needed
- Total supply current (mA)
- Total power from the supply (mW)
- Power in the LEDs (mW)
- Share of power reaching the LEDs
What this assumes
- Forward voltage is treated as constant at the current you enter; a real LED's Vf rises slightly with current and falls with temperature.
- The supply is stiff (its voltage does not sag under load).
- Parallel mode assumes one resistor per LED, which is the correct way to build it.
- Wattage recommendation derates to 50% of the resistor's rating.
About this calculator
An LED is a diode, not a resistor: past its forward voltage the current rises almost vertically with voltage, so connecting one straight across a supply destroys it. The fix is a series resistor that eats the leftover voltage and sets the current. This works out the resistor value, the nearest part you can actually buy, the current you will really get with it, and the wattage rating you need.
The formula and why it looks like that
The supply voltage splits between the LED and the resistor. The LED takes its forward voltage Vf (roughly fixed — that is what a diode does), so the resistor gets whatever is left, and Ohm's law turns that into a resistance for your chosen current: R = (Vsupply − Vf) ÷ I. For LEDs in series all of them drop their Vf from the same supply, so subtract n × Vf; the current through the string is the same everywhere, so one resistor does the whole job. For LEDs in parallel, each branch needs its own resistor.
The parallel-LED mistake
Wiring several LEDs in parallel behind one shared resistor looks economical and fails badly. Forward voltage varies part to part, even in the same batch; the LED with the lowest Vf takes more than its share of the current, heats up, which lowers its Vf further, which takes more current still. You end up with one blazing LED, several dim ones, and eventually a dead one. Give each branch its own resistor, or drive them in series.
Headroom, and when a resistor is the wrong answer
A resistor sets current only in proportion to the voltage across the resistor. If the supply is 3.4 V and a white LED drops 3.2 V, the resistor has 0.2 V to work with — and a part whose Vf is 0.1 V lower than nominal instantly gets 50% more current. As a rule of thumb keep at least 15–20% of the supply across the resistor. Below that, or for anything above roughly 100 mA where the wasted heat matters, use a constant-current driver instead: it regulates current directly rather than inferring it from a voltage difference.
Reading the results
The exact resistance almost never matches a stock part, so the calculator gives you the nearest E12 value (the ±10% series most through-hole resistors come in) and, honestly, the current that value will actually pass. If it comes out above your target and the LED is near its rating, go one step up. The wattage output applies a 50% derating — a resistor run at its full rated power is running hot enough to discolour the board.
Vf is the number people get wrong most often. It is not 2 V for everything: blue and white LEDs sit near 3.1 V, and a "12 V LED strip" already has resistors built in and needs none of this.
Frequently asked questions
▸What resistor do I need for an LED on 12 V?
For a 2 V red LED at 20 mA, (12 − 2) ÷ 0.02 = 500 Ω, so a 470 Ω or 560 Ω part. For a 3.2 V white LED it is (12 − 3.2) ÷ 0.02 = 440 Ω.
▸What happens if the resistor is too big?
Nothing breaks — the LED just runs dimmer, because it passes less current. Going one E12 step up (470 Ω instead of 430 Ω) costs a few percent of brightness you will not see.
▸Can several LEDs share one resistor?
Only in series, where the same current necessarily flows through all of them. In parallel they do not share current equally and one LED ends up taking most of it, so give each branch its own resistor.
▸How many LEDs can I put in series?
As many as fit under the supply with headroom to spare: floor((Vsupply × 0.85) ÷ Vf). On 12 V that is five red LEDs or three white ones.
▸What wattage resistor should I buy?
At least twice the calculated dissipation. A 500 Ω resistor at 20 mA burns 200 mW, so a 1/4 W part is running at 80% of rating and will be hot — use 1/2 W.
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