Enter the supply voltage, current, and resistance into the calculator to determine the voltage drop across a resistor.
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Resistor Voltage Drop Formula
The calculator uses Ohm's law and standard divider relationships, depending on the mode you choose.
Voltage drop mode — voltage across a resistor when current and resistance are known:
V = I * R
Series dropper mode — resistor needed to drop a supply down to a target voltage at a given load current:
R = (Vs - Vload) / I
Divider mode — output voltage across the bottom resistor of a two-resistor divider, with optional load:
Vout = Vs * (R2 || Rload) / (R1 + (R2 || Rload))
- V — voltage drop across the resistor, in volts (V)
- I — current through the resistor, in amps (A)
- R — resistance, in ohms (Ω)
- Vs — supply voltage (V)
- Vload — desired voltage at the load (V)
- R1 — top (series) resistor in a divider (Ω)
- R2 — bottom resistor in a divider (Ω)
- Rload — load resistance across R2 (Ω); blank means unloaded
Power dissipated in the resistor is P = I²R = V²/R. The calculator flags this and recommends a part with at least 2× the calculated power. The formulas assume DC, ideal resistors, and a stiff supply. R2 and Rload combine in parallel as R2*Rload / (R2+Rload).
Reference Tables
Common resistor power ratings and when they apply:
| Calculated dissipation | Recommended rating (2× margin) | Typical package |
|---|---|---|
| ≤ 60 mW | 1/8 W | 0805 SMD, small axial |
| ≤ 125 mW | 1/4 W | Through-hole axial |
| ≤ 250 mW | 1/2 W | Larger axial |
| ≤ 1 W | 2 W | Metal film, large axial |
| > 1 W | 5 W or wirewound | Ceramic / aluminum-clad |
Typical forward voltages for series-resistor LED calculations:
| LED color | Typical Vf | Typical drive current |
|---|---|---|
| Red | 1.8 – 2.1 V | 10 – 20 mA |
| Yellow / Orange | 2.0 – 2.2 V | 10 – 20 mA |
| Green | 2.1 – 3.3 V | 10 – 20 mA |
| Blue / White | 3.0 – 3.4 V | 15 – 20 mA |
Worked Example
Drive a red LED from 9 V at 15 mA. Use Vf = 2.0 V.
- Voltage to drop: 9 V − 2.0 V = 7.0 V
- Resistor: 7.0 V / 0.015 A = 467 Ω → use 470 Ω (E12)
- Power: 7.0 V × 0.015 A = 0.105 W → a 1/4 W part is fine
FAQ
Does the resistor's tolerance matter? For LEDs and most signal work, 5% is acceptable. For dividers feeding ADCs or precision references, use 1% or better.
Why does my divider output sag under load? Any load resistance smaller than about 10× R2 pulls the output down. Lower R1 and R2, or buffer the output with an op-amp.
Can I use this for AC? Only for purely resistive loads at low frequency. Reactive loads need impedance, not resistance.
Why double the power rating? Resistors derate at higher ambient temperatures and degrade over time near their limit. Running at 50% of rated power keeps them cool and reliable.