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LED Resistor Calculator

Find the resistor that keeps one LED or a string of LEDs at a safe current, with the nearest standard value, its heat and a safe rating.

Level: Grades 9–10

Printable worksheet

LED Resistor Calculator

Know the answer you need? Pick an input to work backward.

Inputs

The voltage of the battery or power supply feeding the LED string.

From the LED's data sheet. Typical values: red about 1.8–2.2 V, yellow and green about 2.0–2.2 V, blue, white and true green about 2.8–3.4 V.

The forward current you want. Many small indicator LEDs are rated for 20 mA; running them at 5–15 mA is gentler and still bright.

LEDs wired one after another in a single string, sharing one resistor.

Standard resistor range

Result

Resistance needed

150 Ω

Use 150 Ω (exactly 150 Ω needed), rated 1/8 W; the string draws about 20 mA.

Nearest E12 value at or above it
150 Ω
Current with the standard resistor
20 mA
Resistor power (standard value)
0.06 W
Suggested resistor rating (at least 2× the power)
1/8 W
Power in each LED
0.04 W
Share of supply power reaching the LEDs
40%

See the step-by-step solution ↓

Step-by-step solution

  1. The resistor takes whatever voltage the LED doesn't: subtract the forward voltage from the supply

    V_R=5 V−2 V=3 V

  2. Convert the current to amperes (1 mA = 0.001 A)

    20 mA1,000=0.02 A

  3. Ohm's law (resistance = voltage ÷ current) gives the resistance that lets exactly this current through: R = V_R ÷ I

    R=3 V0.02 A=150 Ω

  4. Work out the heat the resistor must handle, so you can pick a part that won't overheat: P = V_R × I

    P=3×0.02=0.06 W

  5. Round up to the next E12 value you can buy, so the current stays at or below the target. Then recheck the current

    150 Ω: I = 3150 = 20 mA

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

What this calculator does

Find the right resistor to keep your LED from burning out. Once an LED starts to light, it has almost no resistance of its own. Wired straight to a battery, it draws far too much current (flow of electricity) and burns out. A resistor wired in line with it (in series) fixes that. It takes up the voltage the LED doesn't need, and it sets the current to a safe level.

This calculator finds that resistor for a single LED or several LEDs in series. Enter the supply voltage, the LED's forward voltage and the current you want. The forward voltage is the voltage an LED uses up while it's lit. You'll get the exact resistance, and the next standard value you can actually buy. You'll also see the current that value really gives, the heat the resistor must handle and a sensible power rating.

Where it's used

Why this math matters outside the classroom:

  • First circuits. A student wiring a red LED to a 5 V board finds (5 − 2) ÷ 0.02 = 150 Ω for 20 mA.
  • Battery projects. A maker powering a 2.1 V LED from a 9 V battery at 15 mA needs 460 Ω and fits a 470 Ω part.
  • Car accent lights. A hobbyist running three 3.2 V white LEDs from 12 V at 20 mA chooses a 120 Ω resistor.
  • Indicator panels. Technicians run status LEDs at 10 mA instead of 20 mA to cut heat and extend their life.
  • Power ratings. A designer sees 22 V across a resistor at 0.1 A is 2.2 W and picks a 5 W part to stay cool.

Grade level and standards: Grades 9–10; Common Core State Standards for Mathematics HSA.CED.A.4. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.

How to use it

  1. Enter the supply voltage, such as 5 V from USB, 9 V from a battery or 12 V from a car or adapter.
  2. Enter the LED's forward voltage and the current you want. Use its data sheet if you have one.
  3. Enter how many LEDs share the resistor in one series string. Then choose the E12 or E24 range of standard values.
  4. Read the resistance needed, then use the standard value and rating shown.

What the inputs mean

Supply voltage
The voltage of the battery or power supply feeding the LED string.
LED forward voltage (each)
Forward voltage depends on the LED's color and what it's made of. Red LEDs use about 1.8 to 2.2 V. White, blue and true-green LEDs use about 2.8 to 3.4 V. A data sheet value is best.
LED current
Small 5 mm and 3 mm indicator LEDs are commonly rated for 20 mA (milliamps) at most. Running them at 5 to 15 mA makes them last longer and is often bright enough. High-power LEDs need far more current, and usually a dedicated driver.
LEDs in series
For LEDs side by side (in parallel), give each one its own resistor, and work it out as a string of one. If parallel LEDs share one resistor, one of them can take most of the current.

Formula and method

V_R = V_supply − n × V_f
R = V_R ÷ I
P = V_R × I
actual I = V_R ÷ R_standard

V_supply is the supply voltage, n is the number of LEDs in the string and V_f is each LED's forward voltage. V_R is the voltage left over for the resistor. R is the resistance in ohms (Ω), I is the current in amps and P is the power in watts. R_standard is the standard resistor value you'd actually buy.

Each LED in a string uses up roughly its forward voltage. Whatever is left of the supply ends up across the resistor. Ohm's law (resistance = voltage ÷ current) then gives the resistance that lets the chosen current through.

The resistor turns its share of the power into heat, P = V_R × I. The calculator rounds up to the next standard value, so the current comes out at or slightly below your target. It also suggests a rating at least twice the actual heat, so the part runs cool.

Worked example

Three white LEDs, each with a 3.2 V forward voltage, run in series from a 12 V supply at 20 mA. What resistor do they need?

  1. Voltage left for the resistor: 12 − 3 × 3.2 = 2.4 V.
  2. Current in amperes: 20 mA ÷ 1,000 = 0.02 A.
  3. Resistance: 2.4 ÷ 0.02 = 120 Ω, which is itself an E12 value.
  4. Power: 2.4 × 0.02 = 0.048 W, so a 1/8 W resistor (at least twice 0.048 W) is enough.

Result: Use a 120 Ω resistor rated 1/8 W or more. The string then draws 20 mA, and 80% of the supply power reaches the LEDs.

Understanding the result

If the standard value gives a current a little below the target, that's on purpose. The LED will be very slightly dimmer, and it'll run cooler.

The efficiency figure shows how much supply power is wasted in the resistor. If the resistor takes most of the voltage, you'll waste less by adding LEDs to the string or lowering the supply voltage.

Assumptions and limitations

  • Each LED uses its stated forward voltage at the chosen current. Real parts vary by a few tenths of a volt.
  • The supply voltage is steady. Batteries lose voltage as they run down, which lowers the current.
  • The resistor's actual value is the value printed on it. A ±5% part can move the current by about the same amount.
  • Not for high-power LEDs and LED strips. They need constant-current drivers, or have resistors built in.
  • It doesn't account for temperature. Forward voltage falls as an LED warms up, so the current rises slightly.
  • This is a design aid, not safety guidance for circuits powered from household mains electricity.

Frequently asked questions

What happens if I use an LED without a resistor?

Unless the supply is already a current-limited driver, the current rises until something stops it. Often that's the LED failing. Even a small coin cell can overheat an LED without a resistor, which is why one is always used.

Should I round the resistor up or down?

Up. A higher resistance gives a lower current, which is safe. Rounding down gives a current above your target, which may be more than the LED is rated for.

Can several LEDs share one resistor?

Yes, if they're in series. They carry the same current, so one resistor sets it for all of them. LEDs in parallel should each get their own resistor, because their forward voltages never match exactly.

Why does the calculator say the supply must be higher?

LEDs need at least their combined forward voltage to light. Say three 3.2 V LEDs need 9.6 V, and the supply is 9 V. Then nothing is left for the resistor, and the string won't light properly. Use fewer LEDs per string.

How this calculator is checked

Method: Kirchhoff's voltage law and Ohm's law (R = V ÷ I, P = V × I); IEC 60063 E12 and E24 preferred values. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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