Voltage Divider Formula
Vout = Vin × R2 ÷ (R1 + R2). Two resistors in series split the supply voltage proportionally to their values. If R1 = 1kΩ and R2 = 1kΩ, the output is exactly half the input. If R1 = 9kΩ and R2 = 1kΩ, the output is one-tenth of the input.
Load Effect
The formula assumes infinite load impedance. In practice, any device connected to Vout draws current, reducing the output below the calculated value. As a rule of thumb, load impedance should be at least 10× (R1 + R2) for the output to stay within 10% of the ideal value. If the load impedance is too low, the divider output collapses toward zero.
Practical Applications
Voltage dividers are used in sensor interfacing (stepping a 5V signal down to 3.3V for a microcontroller input), ADC reference generation, and logic level shifting between different families. Do not use a simple divider as a power supply—use a voltage regulator. Dividers waste power as heat continuously, making them unsuitable for high-current loads.
Frequently Asked Questions
Can I use a voltage divider to power a circuit?
Not reliably. A voltage divider’s output voltage drops when load current is drawn (poor load regulation). For light-load sensing — like a microcontroller ADC input — it can work. For powering circuits, use a linear voltage regulator (e.g., LM7805) or a switching supply. Output voltage is only stable when load impedance is much greater than R2.
How do I choose R1 and R2 values?
Start from the ratio: Vout/Vin = R2/(R1+R2). For low quiescent current, use values in the 10kΩ–100kΩ range. Smaller resistors waste more current as heat; larger resistors make the output more sensitive to load variation. Keep load impedance at least 10× higher than R2 to maintain output accuracy.
Can I convert between unit systems?
Yes — as long as both R1 and R2 use the same unit, the ratio stays correct. 10kΩ/20kΩ gives the same result as 10,000Ω/20,000Ω. When mixing prefixes (Ω, kΩ, MΩ), convert to the same unit before entering values.
Do the formulas apply to all conditions?
The resistive voltage divider formula applies to DC circuits and to AC circuits where both components are pure resistors. For AC with reactive components (capacitors or inductors), use the impedance form: Vout = Vin × Z2/(Z1+Z2), where Z values are complex impedances that vary with frequency.
Is this precise enough for academic work?
For introductory circuit analysis and lab preparation, yes. The calculator uses exact floating-point arithmetic. For precision analog design, account separately for resistor tolerance (1% vs 5%), temperature coefficient drift, and PCB trace resistance — factors this calculator does not model.