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LED Resistor Calculator

Series resistor for LEDs from supply voltage, LED forward voltage, and target current. Exact ohms, next E12 value, and resistor wattage hint.

LED series resistor

An LED behaves like a diode with a nearly fixed forward voltage, while current sets brightness and heat. A series resistor limits that current. This tool computes that resistor from your supply voltage, estimated Vf, and the current you want through the loop.

01

What you're feeding it

Volts (DC). Same node as the resistor top.

Color sets a ballpark Vf. The reference card below adds typical max continuous current, which the shortcuts do not include. Prefer values from the datasheet when you have one; otherwise measure and adjust if the brightness or temperature is off.

Results
Voltage across resistor
3.00 V
Exact resistor value
150.00 ฮฉ
Next E12 standard (โ‰ฅ exact)

E12 is a common set of preferred resistor values available from most suppliers. This value is the next one in that set that is still at or above your exact ohms.

150.00 ฮฉ

That rounding slightly lowers current versus the exact calculation, which is usually fine for indicator LEDs.

Current with E12 resistor
20.00 mA
Power in the resistor
60.00 mW Shelf-friendly rating: 1/8 W or higher
Power at E12 value
60.00 mW

Work shown

V across resistor = 5.00 โˆ’ 2.00 = 3.00 V. Then R = 3.00 V รท 0.02 A โ‰ˆ 150.00 ฮฉ.

Assumes one LED in series with one resistor on DC, and a supply stiff enough to ignore sag here. Always confirm against your component datasheet and applicable electrical safety rules for your installation.

Typical indicator LED cheat sheet

Same color rows as the shortcuts, with an extra column for typical maximum continuous forward current from common small epoxy through-hole parts. High-power and specialized LEDs need their own datasheets; treat these figures as starting points.

LED color, typical forward voltage range, typical maximum continuous forward current
LED colorTypical Vf (V) Typ. max IF continuous
Red1.8โ€“2.130 mA
Amber2โ€“2.230 mA
Orange1.9โ€“2.230 mA
Yellow1.9โ€“2.230 mA
Green2โ€“3.125โ€“30 mA
Blue3โ€“3.720โ€“30 mA
White3โ€“3.420โ€“30 mA

* Approximate absolute-maximum DC forward current for generic 5ย mm epoxy indicators at room temperature; chemistries vary, which is why some rows show a range. Operating well below the maximum (often around 10โ€“20ย mA for indicators) usually improves lifetime. Confirm on the datasheet for your specific part.

LED series resistor quick reference

Enter supply voltage, estimated LED forward voltage, and target current. Color shortcuts match common distributor charts (red through white), each filling the midpoint of a typical Vf band. Outputs include exact resistance, the next E12 value at or above it, resulting current, and resistor dissipation. For general Vโ€“Iโ€“Rโ€“P relationships without LED framing, use the Ohm's law calculator linked from this page.

Before you wire it

Voltage across the resistor

Subtract LED forward voltage from your DC supply. The remainder is the voltage across the series resistor. If it is zero or negative, increase the supply or change the LED string before picking R.

Target LED current

This is the loop current through LED and resistor in series. It sets brightness and heat. Start with a conservative value and increase only if the application allows.

Why we round up on E12

Standard values come in discrete steps. Choosing the next resistor at or above your calculated value lowers current slightly compared with the exact value, which is usually acceptable.

Diode reality check

An LED still follows a diode Iโ€“V curve: forward voltage varies with current and temperature. Leave margin in current and wattage, then verify with a meter if the application needs it.

LED Resistor Calculator: Series Resistor for Forward Voltage and Current

Turn supply voltage, LED forward voltage, and target LED current into a resistor value. Includes E12 rounding, wattage hints, and the algebra spelled out.

What This LED Resistor Calculator Does

This LED resistor calculator sizes a series resistor so you get roughly the LED current you intend. Enter DC supply voltage, an estimate of LED forward voltage (VfV_f), and target loop current. It returns the exact resistance from Ohm's law on the resistor, the next E12 standard value at or above that value (for parts you can buy off common reels), the resulting current with that E12 value, and power dissipated in the resistor so you can pick an adequate wattage rating. It assumes one LED and one resistor in series on DC with a stiff supply. It does not replace constant-current drivers, PWM dimming analysis, or detailed thermal modeling.
  • Inputs:
    Supply voltage, LED forward voltage, target series current (with unit selector).
  • Outputs:
    Voltage across the resistor, exact resistance, suggested E12 value, updated current, resistor power, clipboard-friendly summary.

How the Math Works

Model the LED as a nearly fixed forward voltage in series with your resistor. The resistor drops the supply minus that forward voltage. Current is the same everywhere in a simple series loop, which links voltage across R to resistance via Ohm's law.
  • Voltage across the resistor:
    Subtract forward voltage from supply voltage: VR=Vsupplyโˆ’VfV_R = V_{\text{supply}} - V_f

    If VRV_R is zero or negative, fix the supply voltage or LED configuration before choosing R.

  • Resistor value:
    Ohm's law on the resistor only: R=VRI=Vsupplyโˆ’VfIR = \frac{V_R}{I} = \frac{V_{\text{supply}} - V_f}{I}

    Here II is the LED current you asked for, because series components share that current.

  • Power dissipated in the resistor:
    PR=VRร—I=I2RP_R = V_R \times I = I^2 R

    Compare PRP_R to the resistor power rating and choose a part with adequate margin.

  • Worked example:
    Say Vsupply=9ย VV_{\text{supply}} = 9\ \text{V}, a red indicator with Vfโ‰ˆ1.8ย VV_f \approx 1.8\ \text{V}, and I=20ย mAI = 20\ \text{mA}. Then VR=7.2ย VV_R = 7.2\ \text{V}, so R=7.2/0.02=360ย ฮฉR = 7.2 / 0.02 = 360\ \Omega. The next E12 value at or above 360 ฮฉ is 390 ฮฉ, so actual current ends up slightly below 20 mA.

How to Use This Calculator

Enter supply voltage first, then LED forward voltage from the datasheet or from a color preset as a starting point. Set target LED current and pick milliamps or amps in the dropdown to match how you think about the circuit. Results update as you type: voltage across the resistor, exact ohms, then an E12 suggestion and the current that results from using that standard value. Use Copy summary for a short clipboard note. To decode resistor color bands on the part you buy, open the resistor color code calculator. If you want the full voltage-current-resistance playground without LED framing, use the Ohm's law calculator.

Why LED Forward Voltage Varies

Forward voltage depends on semiconductor chemistry, die layout, temperature, and operating current. Reds and ambers often sit lower than blue and white InGaN parts. The same nominal color can still vary between manufacturers. Because VfV_f moves along the diode curve, treat tabulated color guesses as approximations. When stakes are high, take VfV_f from the datasheet, leave enough supply headroom, and confirm with a voltage or current measurement at your chosen operating point.

From Paper Ohms to Parts You Can Actually Buy

Calculated resistance rarely matches a stocked value exactly. Preferred numbers cluster in decades; E12 is a common twelve-values-per-decade set for through-hole parts. Rounding up to the next available value reduces current slightly versus the exact calculation, which usually absorbs small errors in VfV_f. When ordering, pick both ohms and wattage: carbon-film 1/4 W parts are typical for low-power indicators, but dissipation scales with current and the voltage across the resistor. Power and metal-film versus carbon tradeoffs depend on the part series and your BOM; this calculator addresses resistance and resistor power from the inputs you provide.

When the Series Resistor Is Only Half the Story

PWM dimming, constant-current drivers, multiple LEDs in series, and RGB packages need different analysis than a single resistor on a DC rail. This calculator covers one LED plus one series resistor from a steady DC supply. For long leads or supplies that sag under load, also consider voltage drop on the feed. Verify critical designs with measurement.

FAQ

What resistor do I need for an LED on 12 V?

Subtract the LED forward voltage from 12 V to get the voltage across the resistor, then divide by your target current in amps. Example: 12 V supply, 2 V LED, 20 mA goal gives (12 - 2) / 0.02 = 500 ฮฉ. Use the next standard value at or above that number so current stays at or below what you planned.

What is LED forward voltage?

Forward voltage is the approximate voltage where the diode conducts in the intended direction. Color and chemistry shift it; temperature shifts it too. Prefer numbers from your datasheet when they exist.

How much LED current should I pick?

Common indicator LEDs are often run near 10โ€“20 mA. Modern high-efficiency parts may look bright at lower current. Approach the datasheet maximum continuous current only when thermal design and the application justify it.

Why does everyone mention Ohm's law for LEDs?

Because once you know the voltage across the resistor and the current you want, resistance is voltage divided by current. The LED accounts for VfV_f; the resistor drops the remaining voltage. That is Ohm's law on the resistor branch.

Can I wire LEDs in series?

Yes. Add forward voltages together in your head before you subtract from the supply. You still need enough headroom so the resistor has positive voltage across it. If VsupplyV_{\text{supply}} barely clears โˆ‘Vf\sum V_f, you may not have enough voltage left for a sensible resistor drop.

Is an LED the same as a normal diode for math?

They are both diodes, but typical forward drops differ. A silicon rectifier and an LED both drop voltage when forward biased, but useful VfV_f levels differ. For sizing you often treat the LED as a nearly fixed voltage and size the series resistor around that.

What wattage resistor should I buy?

Compute dissipation as voltage across the resistor times current. Pick a part rated comfortably above that (many designs use roughly 2ร— headroom). Small indicator circuits often use 1/4 W parts; if dissipation rises, move to a higher power rating.

Sources & citations

References used for the calculation method and definitions. Links open in a new tab when available.

[1]
Electronics Tutorials: The Light Emitting Diode

Introduces LED behavior, typical forward voltage bands by construction, and why diode curves matter when picking operating points.

[2]
SparkFun: Light-Emitting Diodes (LEDs)

Hobby-oriented overview of LED behavior, forward voltage and current, and why real parts need margin instead of a single magic Vf.

Electronics Reference Note

Educational Use: These tools use standard electrical formulas (e.g., Ohm's Law, NEC voltage drop) for learning, hobby projects, and general reference, not for licensed electrical work or safety-critical installations.

Verification Recommended: Wire sizing, voltage drop, and circuit design depend on local codes, ambient conditions, and load profiles. For real installations, consult a qualified electrician or engineer.

Not Professional Advice: This site does not provide electrical or engineering advice. All calculations run locally in your browser; no data is stored or transmitted.

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