Calculators · Current limiting
LED series resistor
LivePick the right resistor for LEDs from supply voltage, forward voltage, and current.
Size an LED resistor with practical power-rating checks.
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Advanced: identical parallel branches
Increase this only when you are building multiple identical LED strings in parallel. Each branch needs its own resistor with the calculated value.
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Calculator guide
LED series resistor formulas and practical checks
Formulas, examples, and practical notes for using the result with confidence.
Overview
An LED is eager to glow, but it is not good at limiting its own current. A series resistor takes the leftover supply voltage and turns it into a predictable, LED-friendly current instead of a very short light show.
Use this LED resistor calculator for indicators, small lighting strings, and breadboard experiments. It calculates the resistor for each branch, checks resistor power, and helps you decide when a simple resistor is enough and when a constant-current driver is the smarter part.
Current example
Enter valid values above and this example will update with the calculator state.
Start with voltage headroom
The resistor can only control current if the supply voltage is higher than the total LED forward voltage.
Headroom is the supply voltage minus the voltage dropped by the LEDs in series. That remaining voltage appears across the resistor.
If headroom is zero or negative, the resistor cannot set current properly. Use fewer LEDs in series, a higher supply voltage, or a driver designed for the LED string.
Leave a little practical margin for supply sag and LED forward-voltage spread. A design with only a sliver of resistor voltage can change brightness noticeably as the battery or LED warms up.
LED resistor headroom
Turn the leftover volts into a resistor
Once headroom is known, Ohm’s law sets the resistor value.
The target current is usually taken from the LED datasheet. For modern indicator LEDs, full rated current may be brighter than needed, so 2 mA to 10 mA is often enough.
After calculating the resistance, choose a standard value equal to or higher than the result if you want to keep current below the target.
Series resistor value
5 V status LED
With a 2 V LED at 10 mA, the resistor sees 3 V. R = 3 V ÷ 0.01 A = 300 Ω, so 330 Ω is a friendly standard choice for a slightly gentler current.
Check the heat before soldering
The resistor turns headroom voltage into heat, so wattage matters.
The resistor power is the voltage across the resistor multiplied by LED current. This is the power in one resistor for one LED string.
If multiple identical strings are placed in parallel, each string needs its own resistor. The resistor value and resistor wattage stay the same per branch, while total current and total power increase with the number of branches.
For small indicators the power is often tiny. For high-current LEDs or high supply voltages, resistor heat can become the limiting design issue.
Resistor dissipation
Where a simple resistor works well
Series resistors are best for simple, low-current LED circuits.
Use this approach for panel indicators, microcontroller status LEDs, test fixtures, and one-off prototypes.
For high-power LEDs, changing battery voltage, or strings that need matched brightness, use a proper constant-current LED driver instead.
Microcontroller indicators
Limit current from GPIO pins and keep port current under the microcontroller rating.
Panel LEDs
Choose a resistor that keeps brightness comfortable and resistor heat low.
Prototype checks
Estimate current and wattage before grabbing the nearest resistor.
Easy LED traps to avoid
The risky errors are usually forward-voltage assumptions and shared resistors.
Forward voltage varies by LED colour, current, temperature, and manufacturing spread. A red LED and blue LED are not interchangeable in the calculation.
Avoid putting parallel LEDs behind one shared resistor. Small forward-voltage differences can make one LED or branch take more current than the others.
Assumptions and limits
- Results are design estimates, not a substitute for datasheets, measurements, safety approvals, or engineering review.
- Component tolerance, temperature, supply variation, and real loading can move the final circuit away from the ideal calculation.
- Calculator results are estimates for design and learning. Verify values against datasheets, tolerances, temperature, load behaviour, and safety requirements before using them in a real circuit.
Licensing
Calculator copy, equations, and generated visuals are provided for learning and design-reference use on Kobee unless a specific licence is shown.
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