Voltage divider.
Voltage divider: Calculate the output voltage of a resistor voltage divider given Vin, R1, and R2.
Two-resistor voltage divider — output voltage, current draw, and power dissipation in each resistor. Plus the closest E12 and E24 standard resistor values.
How the math works
Vout = Vin × R2 / (R1 + R2) Current I = Vin / (R1 + R2) Power R1 P1 = I² × R1 Power R2 P2 = I² × R2
The formula assumes no load on the output — i.e., the downstream circuit draws negligible current compared to what flows through the divider. If the load draws current comparable to I, the actual Vout will sag (it forms a Thévenin source with output impedance R1∥R2).
Rule of thumb: load resistance ≫ R1∥R2
For the voltage divider to work as designed, the load resistance should be at least 10× higher than the parallel combination of R1 and R2. If your load is 10 kΩ, your divider's R1∥R2 should be ≤ 1 kΩ.
This trades off against power dissipation: lower R values mean more current flows continuously, which means more wasted power and more heat. The classic compromise is to make R1+R2 around 10-50× the load impedance.
Common pitfalls
- Don't power a microcontroller pin via a voltage divider for ANY current draw. Voltage dividers only work for sensing (ADC inputs, comparator references). For powering anything, use a regulator (linear or switching).
- Resistor tolerance affects the output. Standard ±5% resistors can give ±5% error on the divider output. Use 1% precision resistors when accuracy matters.
- Power rating matters. A 1/4 W resistor dissipating 0.3 W will burn out. Always check P calculation against resistor power rating.
- Thermistor and photoresistor dividers are non-linear. A voltage divider with a thermistor on top gives a temperature-dependent voltage, but the relationship isn't linear with temperature — calibrate or use a lookup table.
- Source impedance affects ADC accuracy. Most microcontrollers want source impedance ≤ 10 kΩ on ADC inputs for accurate sampling. If R1∥R2 is much higher, add a buffer (op-amp follower) before the ADC.
Common questions
What is a voltage divider?
Two resistors in series across a voltage source. The output, tapped between them, is a fixed fraction of the input: Vout = Vin × R2 ÷ (R1 + R2).
Can I power a circuit from a voltage divider?
Generally no. A divider only holds its calculated voltage when almost no current is drawn from the tap. As soon as a load pulls current, the output sags. Use dividers for reference or sense voltages — for supplying power, use a voltage regulator or buck converter.
How do I choose R1 and R2 values?
Set the ratio for your target voltage, then scale both resistors for your current budget. Smaller values waste more power but tolerate heavier loads; larger values save power but are more sensitive to loading and noise. A combined 10k–100k Ω is a common compromise.
How much power do the resistors dissipate?
A standing current of I = Vin ÷ (R1 + R2) flows through the pair continuously, dissipating P = Vin² ÷ (R1 + R2). With low-value resistors this can be significant — check each resistor's wattage rating against its I²R.
Why is my measured output different from the calculation?
Loading (your meter or the downstream circuit draws current), resistor tolerance (±5% or ±1%), and temperature all shift it. A 10 MΩ multimeter barely loads a 10k divider but noticeably loads a 1 MΩ one.
What are the E12 and E24 resistor series?
They are the standard manufactured resistor values: E12 has 12 values per decade (±10% parts), E24 has 24 (±5% parts). The calculator suggests the nearest standard pair because arbitrary resistance values aren't stocked.
Sources
- Standard resistor values: IEC 60063 — Preferred number series for resistors and capacitors (E6, E12, E24, E48, E96, E192 series).
- Voltage divider analysis: Any basic electronics textbook (Horowitz & Hill, The Art of Electronics, Chapter 1).
Disclaimer. Voltage dividers are appropriate for level-shifting and sensing applications. For power delivery, use a voltage regulator (LDO, switching regulator) instead.