LED Resistor Calculator
Pick the right current-limiting resistor for a single LED or a series string. See nearest E24 / E96 values, dissipated power, and the resulting LED current.
Supply voltage
LED preset
Forward voltage Vf
Forward current If
LEDs in series
Required resistance
150 Ω
R = (Vs − Vf) / If
Nearest standard (round up)
Resistor power
60 mW
Use ⅛ W (2× safety margin)
Practical Examples
- Red LED on 5 V USB — 5 V supply, Vf 2.0 V, 20 mA → 150 Ω. E24 lands exactly on 150 Ω, dissipates only 60 mW, so a ⅛ W resistor is plenty (¼ W is the common stock part).
- Blue indicator on 12 V — 12 V, Vf 3.2 V, 20 mA → 440 Ω. Round up to E24 470 Ω — slightly dimmer but safer. Resistor dissipates 0.18 W, so a ¼ W part is right.
- Three white LEDs in series on 12 V — 12 V − 3 × 3.2 V = 2.4 V across R, at 20 mA → 120 Ω. One resistor drives the whole string — cheaper and more efficient than one per LED, since the total resistor power drops with N.
How it works
An LED is a current-driven device with a near-fixed forward voltage drop Vf. The resistor's job is to drop the leftover voltage and set the operating current: R = (Vs − N · Vf) / If, where N is the number of LEDs in series.
The power dissipated by the resistor is PR = (Vs − N · Vf) · If. Always pick a part rated for at least 2× that — a 60 mW load should run on a ¼ W resistor, not a ⅛ W. Headroom keeps the resistor cool and avoids drift.
LEDs in series share the same current, so one resistor sets the current for the whole string. LEDs in parallel do not share current evenly because Vf varies part-to-part — each parallel branch needs its own resistor.
Design rules of thumb
- Round up to the next E-series value — Slightly less current, slightly less brightness — but the LED stays well inside its rating as Vf drifts with temperature.
- Derate the resistor to 50% of its rating — A ¼ W part running at 200 mW is borderline; 100 mW is comfortable. The 2× rule is the standard reliability target.
- One resistor per parallel branch — Sharing one resistor across parallel LEDs lets the lowest-Vf LED hog the current and run hot. Give each branch its own current limit.
- Leave 1.5 V of headroom across the resistor — With Vs − N·Vf below ~1.5 V, the LED current becomes sensitive to supply and Vf tolerance. The resistor stops doing its job.
- Use a current source above ~100 mA — High-power LEDs waste too much heat in a resistor. A simple constant-current driver IC is both cooler and more accurate.
- Check the LED's absolute max current — Datasheets list If(max). Design for 60–70% of that — modern LEDs are bright at well under the maximum and last much longer.
FAQ
Why does my LED need a resistor at all?
LEDs have a soft V/I curve — a tiny change in voltage causes a huge change in current. Without a resistor (or a constant-current driver), the LED runs away thermally and burns out. The resistor sets the operating current and absorbs the difference between the supply voltage and the LED's forward voltage.
What's the right LED current to design for?
For standard 5 mm indicator LEDs, 10–20 mA is normal. Modern high-efficiency LEDs are visibly bright at 2–5 mA. The datasheet's If(typ) is the usual target; If(max) is the don't-exceed. Designing at 60–70% of the maximum gives reliable long-term operation.
Should I round to the nearest E24 or E96 value?
Both work, but always round up — a slightly higher resistance gives slightly less current, which protects the LED as Vf drifts with temperature. E24 (±5%, 24 values/decade) is fine for indicators. E96 (±1%, 96 values/decade) is worth it when matching brightness across multiple LEDs or driving close to the LED's current limit.
Can I put LEDs in parallel with one resistor?
Only if the LEDs are tightly matched (same bin, same temperature). Otherwise the LED with the lowest forward voltage takes most of the current and gets brighter — and hotter, which makes its Vf drop further, which makes it hog even more current. Safer: one resistor per LED branch.
How many LEDs can I drive in series?
Up to floor((Vs − headroom) / Vf). Keep at least 1.5 V of headroom across the resistor so that the LED current is set by R, not by supply variation or Vf tolerance. With a 12 V supply and 3.2 V white LEDs, three is comfortable; four leaves no margin.
What about PWM dimming?
PWM swings the current between zero and full — the average current sets perceived brightness. Design the resistor for the full ("on") current as if PWM weren't there; the average power dissipated is duty cycle × Pres. The LED sees the same instantaneous current as a non-PWM design, so the resistor sizing doesn't change.
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