Electronics · LED circuits
LED Resistor Calculator
Pick your supply and LED — get the ballast resistor, wattage rating, and a live schematic before you touch the iron.
R = (V_s − V_f_total) / I
What to solve
Forward: pick a target current. Reverse: enter a resistor you have.
How it is wired
Topology changes how Vf adds up and how many resistors you need.
Supply & LED
Values
Wattage of the resistor in your parts box — used for “will it melt?” checks.
Live schematic
Current flow animates when values are valid — focus a field to highlight it on the diagram.
Bill of materials
Pick supply, LED, and target current — get the ballast resistor and wattage.
160 Ω
E24 standard
Circuit OK
Current and power are within safe limits for your selected part.
Will it melt?
OK64 mW
Your ¼ W part · 26% loaded
Total: 100 mW
Resistor: 64 mW (64%) · LED: 36 mW (36%)
Minimum safe rating: ¼ W
Exact R
160 Ω
→ 20 mA with E24
E12 nearest
150 Ω
Supply draw
20 mA
Standard E-series value rounds up so actual current stays at or below target.
Size the series resistor so your LED runs at a safe current without overheating the part.
An LED behaves roughly like a fixed forward voltage drop Vf in series with a small dynamic resistance. The remaining supply voltage appears across the ballast resistor, which sets the current.
Vs is supply voltage, Vf,total is the sum of LED drops in series (or one Vf for parallel banks), and I is the desired current. Resistor power is P = I²R — pick a wattage rating with at least 2× headroom.
Red LED Vf ≈ 1.8 V, target 20 mA on a 5 V USB supply:
Headroom = 5 − 1.8 = 3.2 V
R = 3.2 / 0.02 = 160 Ω → nearest E24 = 160 Ω
P = 0.02² × 160 = 64 mW → use ⅛ W or ¼ W
- Series string: Vf,total = n × Vf. One resistor limits current through the whole chain.
- Parallel (each R): Each branch is independent — best for matched brightness.
- Parallel (shared R): One resistor feeds matched LEDs. Mismatched Vf causes current hogging — hobby OK, production not recommended.
Why round resistor values up?
Rounding up keeps current at or below your target, protecting the LED. Rounding down would drive harder than intended.
Can I use one resistor for parallel LEDs?
Only if the LEDs are well matched. For reliable brightness, give each LED its own series resistor.
What wattage resistor do I need?
Calculate P = I²R and choose the next standard rating (⅛ W, ¼ W, ½ W…) with at least 2× safety margin.
- Voltage divider — split a supply into reference levels
- Resistor network — equivalent resistance for series/parallel combinations
- RC time constant — timing with resistors and capacitors
- DC or steady current only — PWM dimming needs average-current analysis.
- Fixed Vf model — actual drop varies with current and temperature.
- Shared parallel mode assumes matched LEDs.
- Does not size constant-current drivers or switching regulators.