What is a Voltage Divider?
A voltage divider is one of the most fundamental circuits in electronics: two resistors in series across a supply voltage. The output is taken from the junction between them, producing a fraction of the input voltage. It is used to scale signals, set bias points, create reference voltages, and read sensors such as potentiometers and thermistors.
How to Use This Calculator
Enter the input voltage Vin (the supply applied across both resistors), the value of the top resistor R1, and the bottom resistor R2 (the one the output is measured across). The calculator returns the output voltage, the divider ratio, the series current, and the total power dissipated.
The Formula Explained
The output voltage is given by $$V_{out} = \text{V}_{in} \cdot \frac{\text{R}_2}{\text{R}_1 + \text{R}_2}$$ Because the same current flows through both resistors, the voltage splits in proportion to resistance. The fraction \(\frac{\text{R}_2}{\text{R}_1 + \text{R}_2}\) is the divider ratio — multiply it by Vin to get Vout. The series current is \(I = \frac{\text{V}_{in}}{\text{R}_1 + \text{R}_2}\), and the power dissipated is \(P = I^2(\text{R}_1 + \text{R}_2)\). This formula assumes a negligible load on the output; a heavy load in parallel with R2 will lower Vout.
Worked Example
Suppose Vin = 9 V, R1 = 1000 Ω and R2 = 2000 Ω. Then $$V_{out} = 9 \times \frac{2000}{1000 + 2000} = 9 \times \frac{2000}{3000} = 9 \times 0.6667 = 6 \text{ V}$$ The series current is \(\frac{9}{3000} = 0.003 \text{ A}\) (3 mA), and total power is \(0.003^2 \times 3000 = 0.027 \text{ W}\).
FAQ
Why is my real output lower than calculated? Connecting a load across R2 forms a parallel combination, reducing the effective resistance and the output voltage. Keep the divider impedance low relative to the load, or buffer with an op-amp.
Which resistor is R1 and which is R2? R2 is the resistor that the output is measured across (between the output node and ground). R1 is between the input and the output node.
Can I swap R1 and R2? Yes — swapping them gives the complementary fraction, so Vout becomes \(\text{V}_{in} \times \frac{\text{R}_1}{\text{R}_1 + \text{R}_2}\).
