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06 Numbers and Powers

LED Voltage Calculator: Find the Right Series Resistor

Card No. 06·13631 · Updated · Standard formula, shown below

Series resistance 150 ohm
Resistor power
0.060 W
Result
Supply voltage
5 V
You entered
LED forward voltage
2 V
You entered
You enter
Supply voltage, LED forward voltage, LED current
You get
Series resistance, Resistor power
Rounding
0 dp · 3 dp

An LED voltage calculator works out the series-resistor value needed to limit current through an LED, using the supply voltage, LED forward voltage, and desired current. Enter those three values to get the resistance in ohms and the resistor’s approximate power dissipation; the result is a starting point for selecting a suitable standard resistor, not a substitute for checking component ratings.

Formula

For one LED in series with a resistor, the resistor drops the voltage left over after the LED’s forward voltage is accounted for. Ohm’s law gives the resistance:

R = (VS − VF) / I

  • R is the series resistance in ohms (Ω).
  • VS is the supply voltage in volts (V).
  • VF is the LED’s forward voltage in volts at the intended operating current.
  • I is the LED current in amperes (A). The calculator accepts milliamperes (mA), so it divides the entered current by 1,000.

It also estimates resistor power with P = (VS − VF)² / R, where P is in watts (W). This is the electrical power dissipated by the resistor under the entered conditions.

Worked Example

Using the calculator’s defaults: supply voltage is 5 V, LED forward voltage is 2 V, and current is 20 mA.

  1. Convert current to amperes: 20 mA ÷ 1,000 = 0.020 A.
  2. Find the voltage across the resistor: 5 V − 2 V = 3 V.
  3. Calculate resistance: 3 V ÷ 0.020 A = 150 Ω.
  4. Calculate resistor power: 3² ÷ 150 = 0.060 W.

The calculator therefore returns a required resistance of 150 Ω and an estimated dissipation of 0.060 W.

How to Use the Result

Use a supply voltage that represents the circuit’s actual operating condition, not merely the nominal label if the source can vary substantially. Look up the LED’s forward voltage and permitted current in its datasheet. Forward voltage is not a perfectly fixed value: it varies among LEDs and changes with current, temperature, and device type. If the datasheet gives a range, consider the full range when checking how the circuit behaves.

Choose a standard resistor value near the calculated result. A larger resistance generally reduces current; a smaller one increases it. For a simple indicator circuit, it is common to choose the next standard value above the calculated resistance to avoid exceeding the target current under ordinary component variation. Recalculate the approximate current using I = (VS − VF) / R after choosing the actual resistor.

Check the resistor’s rated power as well as its resistance. The calculated power is not the resistor rating; it is the estimated heat generated in the stated circuit. Select a component with an appropriate rating and operating margin, taking account of its datasheet and the environment. This calculator does not determine a safe design for every enclosure, temperature, or high-power application.

What the Calculator Assumes

The calculation assumes a steady DC supply, one LED, and one series resistor. In a series circuit, the same current passes through the LED and resistor. The resistor takes the difference between supply voltage and LED forward voltage, making the basic Ohm’s law calculation possible.

The model does not account for supply ripple, startup behavior, resistor tolerance, LED-to-LED differences, wiring losses, or temperature changes. It also does not model a constant-current driver. For a battery-powered circuit, current can change as the battery voltage falls; calculate or measure at the relevant supply extremes rather than relying on a single nominal value.

Common Mistakes and Limits

  • Mixing units: Enter current in milliamperes, not amperes. For example, 20 mA is 0.020 A. Entering 0.020 as though it were mA produces a vastly different result.
  • Using the wrong voltage: The LED’s forward voltage is not the voltage of the whole circuit. Subtract it from the supply voltage to find the resistor’s voltage drop.
  • Ignoring an impossible input: If the supply voltage is equal to or below the LED forward voltage, this simple model cannot provide the requested positive current through a resistor. Raising the supply or using a different circuit may be necessary.
  • Assuming a nominal LED voltage is exact: A datasheet’s specified range and test conditions matter. A resistor calculated from a single typical value does not guarantee identical current in every unit.
  • Rounding too early: Keep the calculated value until choosing a real resistor, then recompute current and power using that resistor’s actual nominal value.

The calculator accepts zero forward voltage as a mathematical input, but real LEDs have a forward voltage that should be taken from a datasheet or measurement. A zero current input is also not a useful operating target; the minimum entry only prevents division by zero. Use positive, realistic values.

One LED, Multiple LEDs, and Alternatives

For several LEDs in series, the supply voltage must cover the sum of their forward voltages. The corresponding resistor calculation is R = (VS − ΣVF) / I, where ΣVF is the sum of the forward voltages of all LEDs in the string. The same current flows through each series LED. Ensure the supply has enough voltage headroom, and calculate resistor power from the remaining voltage and current.

Do not assume that one resistor will divide current evenly among LEDs connected in parallel. Small differences in forward voltage can cause uneven current sharing. Separate resistors for parallel LED branches, or a suitable constant-current driver, are common ways to control current more predictably. A driver is often preferable where supply voltage varies widely, current must be regulated accurately, or the LED load is substantial; its requirements depend on the specific circuit and component datasheets.

Frequently Asked Questions

What resistor do I need for a 5 V LED?
There is no single value: it depends on the LED’s forward voltage and intended current. For example, the calculator’s 5 V, 2 V, and 20 mA inputs give 150 Ω before selecting a standard component.

How do I calculate LED current from a resistor?
Use I = (VS − VF) / R. Convert the result from amperes to milliamperes by multiplying by 1,000.

Can I connect an LED directly to a voltage source?
Usually, an LED needs a current-limiting method, such as a series resistor or a constant-current driver. A voltage source alone does not reliably set the LED’s current.

What happens if the resistor is too large?
Current falls, so the LED may be dimmer or may not visibly light. If the resistor is too small, current can exceed the LED’s permitted rating or the resistor’s power rating.

Can I use this calculator for AC power?
No. It models a steady DC supply and a simple series circuit. AC circuits and mains-powered designs need additional analysis and appropriate components.

Related

Formula used
in vs | Supply voltage | V | 5 | 0 | 1000
in vf | LED forward voltage | V | 2 | 0 | 1000
in i | LED current | mA | 20 | 0.001 | 1000
out r | Series resistance | ohm | (vs - vf) / (i / 1000) | 0
out p | Resistor power | W | ((vs - vf) ^ 2) / r | 3

Each line is one input (in, sel, list) or one result (out: label, unit, formula, decimals). How the calculators work

Method: the standard formula for this calculation, rounded as each result states. See how the calculators work or report an error.

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