Series current limit

LED Resistor Calculator: R from Supply, Vf, and Current

This calculator sizes a series LED resistor from supply voltage, forward voltage (Vf), and target current using R = (Vsupply − Vf) / I. It returns exact ohms, the next E12 standard value, loop current with that E12 part, and resistor power with a shelf wattage hint. One LED on DC only; not for constant-current drivers or PWM.

By Jeff Beem

Updated June 29, 2026

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 V_f, and the current you want through the loop.

What you're feeding it

Supply voltageVolts (DC). Same node as the resistor top.

LED forward voltage (V_f)

Color sets a ballpark V_f. 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.

Target LED current

Decimals

Results

Voltage across resistor

3.00 V

Exact resistor value

150.00 Ω

Next E12 standard (≥ 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 Ω

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 − 2.00 = 3.00 V. Then R = 3.00 V ÷ 0.02 A ≈ 150.00 Ω.

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 color	Typical V_f (V)	Typ. max I_F continuous
Red	1.8–2.1	30 mA
Amber	2–2.2	30 mA
Orange	1.9–2.2	30 mA
Yellow	1.9–2.2	30 mA
Green	2–3.1	25–30 mA
Blue	3–3.7	20–30 mA
White	3–3.4	20–30 mA

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

How to use this calculator

Section 01: supply voltage, LED forward voltage (color preset or typed V_f), and target current (default 20 mA). The dark results card shows voltage across the resistor, exact Ω, next E12 ≥ exact, current with that E12 value, and power in mW plus a shelf wattage hint. Work shown lists R = (V_supply − V_f) / I with your numbers. Use the cheat sheet table for typical max continuous current by color; confirm against your datasheet before you finalize current.

Reading your LED resistor result

The dark results card lists voltage across the resistor, exact ohms, next E12, current with that E12 value, and resistor power. Work shown repeats the algebra.

Example: 5 V supply, 2 V Vf, 20 mA (defaults)

Section 01 at defaults → 3 V across the resistor, exact 150 Ω, next E12 150 Ω, 20 mA with that E12 part, 60 mW in the resistor, shelf rating 1/8 W or higher.

Example: 12 V, 2 V Vf, 20 mA

Supply 12 V, V_f 2 V, 20 mA → 10 V across R, exact 500 Ω, next E12 560 Ω. Current with E12 about 17.9 mA; power at E12 about 179 mW. Shelf-friendly rating on the card follows exact-ohm dissipation (200 mW → 1/2 W or higher).

LED resistor calculator: series R from Vf and current

Size one series resistor from DC supply voltage, LED forward voltage, and target current. Exact ohms, next E12 value, resulting current, and resistor power. One LED on stiff DC only.

What this calculator does

The widget sizes a single series resistor for one LED on DC. Enter supply voltage, LED forward voltage (V_f), and target loop current; it returns voltage across the resistor, exact resistance, the next E12 standard value at or above that ohms, current if you use that E12 part, power dissipated in the resistor, and a shelf-friendly wattage hint. Color presets fill typical V_f midpoints; the cheat sheet table lists typical max continuous current by color. It does not model constant-current drivers, PWM, multi-LED strings inside the form, or wire voltage drop.

Resistor value:

$R=\frac{V_{\text{supply}}-V_f}{I}$
Power in the resistor:

$P_R=V_R\times I=I^2 R$
E12 pick:

Smallest preferred value in the E12 series still ≥ calculated R (12 values per decade).

How the math works

Treat the LED as a nearly fixed forward drop in series with R. The resistor sees supply minus V_f; loop current is the same through both. If supply ≤ V_f, the widget blocks the calculation.

Worked example (not the default):

9 V supply, red V_f ≈ 1.8 V, 20 mA target → V_R = 7.2 V, exact R = 360 Ω, next E12 = 390 Ω, current with 390 Ω ≈ 18.5 mA, P_R ≈ 133 mW.

Limits

Forward voltage moves with temperature, current, and part chemistry; color presets are midpoints, not substitutes for a datasheet on critical builds. E12 rounding lowers current versus exact ohms, which is usually acceptable for indicators. Wattage hints use a simple 50% headroom rule on common through-hole ratings and follow exact-ohm dissipation on the results card; confirm temperature in the actual enclosure.

FAQ

What resistor do the default inputs show?

Section 01 defaults: supply 5 V, forward voltage 2 V, target current 20 mA. Results: voltage across resistor 3 V, exact 150 Ω, next E12 150 Ω (already on the E12 scale), current with E12 20 mA, power in resistor 60 mW, shelf-friendly rating 1/8 W or higher.

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

Enter supply 12 V, estimate forward voltage (red preset mid 1.95 V or type 2 V), and target current 20 mA. The widget gives exact 500 Ω and next E12 560 Ω, which runs the loop slightly below 20 mA. Always confirm wattage on the results card before you solder.

How does this calculator compute resistance?

It subtracts LED forward voltage (V_f) from supply voltage, then divides by target current in amperes:

R=\frac{V_{\text{supply}}-V_f}{I}

. The Work shown panel prints the numeric substitution. Supply must stay above V_f or the widget shows a validation message.

What is the next E12 standard row?

E12 is a 12-values-per-decade resistor series stocked on most reels. The widget picks the smallest E12 value still at or above the exact ohms, which usually lowers current a little versus the exact calculation. At defaults, exact and E12 both land on 150 Ω.

How should I pick LED forward voltage (Vf)?

Use the datasheet when you have one. Otherwise tap a color shortcut in section 01 (Red through White); each fills the midpoint of a typical band. The Typical indicator LED cheat sheet table adds max continuous current per color that the shortcuts do not set.

What wattage resistor should I buy?

Results show power in the resistor in mW and a Shelf-friendly rating line using a rough 50% headroom rule on common carbon-film steps (1/8 W, 1/4 W, 1/2 W, 1 W, 2 W). At defaults, 60 mW dissipation suggests 1/8 W or higher. Use a higher rating if the part runs hot in your enclosure.

Can this size multiple LEDs in series?

No. The widget assumes one LED and one series resistor on stiff DC. For a string, add forward voltages and subtract the sum from supply before using

R=(V_{\text{supply}}-\sum V_f)/I

by hand, or verify headroom so the resistor still has positive voltage to drop.

Does this replace a constant-current driver?

No. It sizes a single series resistor from your typed supply, V_f, and target current. It does not model PWM dimming, constant-current modules, RGB packages, or supply sag under load.

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

LED behavior, typical forward voltage bands by construction, and diode I–V curves for operating-point margin.

[2]

SparkFun: Light-Emitting Diodes (LEDs)

Hobby-oriented overview of forward voltage, current, and why real parts need margin instead of a single V<sub>f</sub>.

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.