Enter electrical values
Add the regulated supply voltage, LED forward voltage, and desired operating current.
Electrical & Energy Tool
LED Resistor Calculator
Calculate the current-limiting resistor for a single LED, a series LED string, or multiple parallel branches. Get the next safe standard resistor value, estimated current, resistor wattage, and complete circuit demand.
Series and parallel layouts E24 standard resistor Power rating guidance
How to use the LED resistor calculator
Use values from the LED datasheet and your actual supply. The calculator subtracts the total LED voltage from the supply, applies Ohm's law, then rounds up to a readily available standard resistor.
Describe the layout
Set the LEDs in each series string and the number of identical parallel branches.
Choose the safe part
Use the recommended standard resistance and a resistor rated above the calculated heat dissipation.
LED resistor formulas and practical design guidance
An LED is not normally connected directly to a fixed-voltage source. A series resistor limits current as the LED's forward voltage and temperature change.
Core formulas
Required resistance
R = (VS − N × VF) ÷ IResistor power
PR = I² × RTotal parallel current
IT = Ibranch × branchesSeries versus parallel
Series LEDs share the same current, so one resistor can control the complete string. Parallel strings must not share a single current-limiting resistor because small forward-voltage differences can cause uneven current sharing.
Resistor power margin
A resistor should not continuously operate at its absolute maximum rating. A 2× margin generally keeps it cooler, improves reliability, and allows for supply and component tolerances.
Common LED forward-voltage ranges
| LED type or colour | Typical forward voltage | Common indicator current | Design note |
|---|---|---|---|
| Infrared | 1.1–1.5 V | 5–20 mA | Check wavelength-specific datasheet |
| Red / orange | 1.8–2.2 V | 2–20 mA | Modern indicators may be bright below 10 mA |
| Yellow / green | 2.0–3.2 V | 2–20 mA | Voltage varies substantially by LED chemistry |
| Blue / white | 2.8–3.6 V | 5–20 mA | Use the datasheet value at the chosen current |
| High-power LED | Varies | 350 mA and above | Use a constant-current driver, not only a resistor |
Do not use a resistor-only design for mains voltage or high-power LEDs. Use a properly isolated, regulated constant-current LED driver with suitable thermal management, protection, and compliance for those applications.
LED resistor calculator FAQs
How do I calculate the resistor for an LED?
Subtract the total LED forward voltage from the supply voltage, then divide the remaining voltage by the desired current in amperes. For multiple LEDs in series, multiply the forward voltage by the number of LEDs first.
Should I choose the nearest higher or lower resistor?
Choose the nearest standard value at or above the calculated resistance. A higher resistor slightly reduces current and is normally safer for the LED than a lower value.
Does every parallel LED need its own resistor?
Yes. Each LED or identical series branch should normally have its own current-limiting resistor. One shared resistor can allow unequal current because LED forward voltages are never perfectly matched.
What wattage resistor should I use?
Calculate resistor power using I²R or voltage drop multiplied by current. Choose a physical resistor with a rating comfortably above that value; around twice the calculated dissipation is a practical general target.
Can I use the typical LED forward voltage?
It is acceptable for an estimate, but final designs should consider the datasheet minimum and maximum forward voltage, supply tolerance, temperature, resistor tolerance, and the required brightness.
When should I use a constant-current LED driver?
Use a proper current driver for high-power LEDs, long strings, battery systems with wide voltage variation, precision brightness, or applications where efficiency and thermal performance matter.
Electrical safety reminderThis tool is for low-voltage DC design estimates. Verify LED and resistor ratings against manufacturer data and test the final circuit safely before continuous operation.