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LED Resistor Calculator

Enter supply voltage, LED forward voltage, desired current, and LED count to estimate a safe series resistor value.

Supply voltage (V)

LED forward voltage (V)

LED current (mA)

LEDs in series

—Ideal series resistance

—Nearest E12 value

—Resistor dissipation

—Resistor voltage drop

—Total LED Vf

LED forward voltage varies by device, current, and temperature. Use the LED manufacturer’s datasheet and stay within component ratings. Intended for low-voltage electronics only.

Electrical model check

Verify the circuit relation, units, and result

simple low-voltage DC LED string with a series resistor

R = (Vs − nVf) / I · PR = I²R

Reading the current calculator inputs…

Forward voltage varies by part, temperature and current; verify the actual LED datasheet.

Low-voltage educational scope. This review does not provide mains wiring, live-work, conductor-sizing, protection-device, or safety-certification instructions.

Standard resistor & power audit

Compare the ideal resistor with a real preferred value, actual current and practical resistor power headroom.

E-series design

Preferred resistor series

Parallel branches

Power headroom

Use the calculator above, then run this verification.

Verification uses the same visible inputs plus the assumptions shown here.

Input uncertainty & sensitivity envelope audit

Propagate editable input/component uncertainty through the current low-voltage calculation, then rank which input most changes the primary result. The review threshold is a user-selected triage signal, not a component, wiring, protection or safety certification.

Supply uncertainty (%)LED Vf uncertainty (%)Resistor tolerance (%)Review if result span exceeds (%)

Envelope layer	Min / base / max	Evidence

Change the calculator inputs above, then refresh this uncertainty audit.

Use component datasheets, measured values and qualified design review where safety or compliance matters.

Electrical uncertainty cluster

Calculate the nominal circuit, then expose what input spread can change

Nominal equations are only the center point. Use component tolerances and measurement uncertainty to inspect a min/base/max operating envelope, then verify real component ratings and datasheets separately.

Ohm's Law SMD resistor codes Battery runtime

All 24 electrical tools

LED ResistorSupply, Vf and resistor spread → current envelope mAh → WhBattery rating spread → energy envelope Voltage DropCurrent, run and conductor spread → delivered voltage Voltage DividerVin and resistor spread → output envelope Capacitor NetworkPart tolerance → equivalent-capacitance sensitivity Resistor BandsDecoded tolerance → current and power consequences Parallel ResistorsBranch tolerance → Req/current/power envelope W / V / AKnown-input spread → solved-value envelope

Practical guide and verification

The resistor controls current approximately

A basic series-resistor calculation uses supply voltage, LED forward voltage and target current. The resistor drops the remaining voltage, but real LED forward voltage varies with device, current and temperature.

Power matters as well as resistance

After finding resistance, calculate resistor dissipation from current and voltage drop. A resistor should have suitable power margin rather than being selected only by its ohmic value.

Common mistake

Do not connect multiple parallel LEDs to one simple current-limiting resistor and assume they will share current evenly. Device variations can cause unequal current unless the circuit is designed for parallel operation.

Match the calculator to the circuit model

Ohm’s law, dividers, RC/RL timing, reactance and battery estimates answer different questions. Keep units and assumptions visible instead of combining unrelated quantities.

Low-voltage educational scope

WebToolArc does not certify wiring, component safety, mains installations or high-energy systems. Use component datasheets and qualified guidance where safety matters.

Governing electrical relation

R = (Vs − nVf) / I · PR = I²R — simple low-voltage DC LED string with a series resistor.

How to cross-check the result

Check that resistor drop equals Vs − nVf, then verify R = Vdrop/I and resistor dissipation = I²R.

Real-component boundary

Forward voltage varies by part, temperature and current; verify the actual LED datasheet. Component tolerance, temperature, parasitics, datasheet limits, installation rules, and hazardous-energy safety are not inferred by this idealized calculator.