A voltage divider uses two resistors to turn a higher voltage into a lower one. This free voltage divider calculator gives the output voltage from your resistor values, shows how a load changes it, and finds standard resistor pairs for the voltage you need.
Enter the input voltage and the two resistors, or switch to the “Find resistors” tab to start from the output voltage. Use the Full screen button at the top right for a larger view.
How to use the voltage divider calculator
- Enter the input voltage Vin and the two resistors. R1 is the top resistor and R2 is the bottom one. You can type values the engineering way, such as 4k7, 470R or 3V3.
- Read the output voltage, the current and the power in each resistor on the right.
- If something is connected to the output, enter its resistance as the load. The calculator shows how much the output drops.
- Choose the tolerance of your resistors to see the worst-case output range.
- Open the “Find resistors” tab when you know the output voltage you need and want standard resistor values.
Voltage divider formula
A voltage divider is two resistors in series between the input voltage and ground. The output is taken from the point between them:
$$V_{out} = V_{in} \times \frac{R_2}{R_1 + R_2}$$
The output is always lower than the input, because the ratio R2 / (R1 + R2) is always smaller than 1.
Example: with 12 V at the input, R1 = 27 kΩ and R2 = 10 kΩ:
$$V_{out} = 12\ \mathrm{V} \times \frac{10\ \mathrm{k\Omega}}{27\ \mathrm{k\Omega} + 10\ \mathrm{k\Omega}} = 3.24\ \mathrm{V}$$
How to choose R1 and R2
Only the ratio of the two resistors sets the output voltage. For a wanted output, the top resistor follows from the bottom one:
$$R_1 = R_2 \times \left(\frac{V_{in}}{V_{out}} – 1\right)$$
Many pairs give the same ratio, so the second choice is the total resistance. A current flows through the divider all the time:
$$I = \frac{V_{in}}{R_1 + R_2}$$
- Low total resistance (for example 1 kΩ): the output stays stable under a load, but the divider wastes current and the resistors get warm.
- High total resistance (for example 1 MΩ): almost no current is wasted, but even a small load pulls the output down, and noise is picked up more easily.
For sensor and ADC inputs, a total between 10 kΩ and 100 kΩ is a good start.
Check the power in each resistor as well:
$$P_{R1} = I^2 \times R_1 \qquad P_{R2} = I^2 \times R_2$$
With 24 V across 1 kΩ and 1 kΩ, the current is 12 mA and each resistor dissipates 144 mW. A 0.25 W resistor would run close to its limit, so a 0.5 W part is the safer choice. The calculator shows a warning when a resistor gets too hot.
The load effect
The formula above is only true when nothing draws current from the output. A load RL connected to the output sits in parallel with R2 and lowers the output voltage:
$$R_{2L} = R_2 \parallel R_L = \frac{R_2 \times R_L}{R_2 + R_L}$$
$$V_{out} = V_{in} \times \frac{R_{2L}}{R_1 + R_{2L}}$$
Seen from the load, the divider behaves like a voltage source with an output resistance equal to R1 and R2 in parallel:
$$R_{out} = R_1 \parallel R_2 = \frac{R_1 \times R_2}{R_1 + R_2}$$
A useful rule follows from this: the load must be at least 99 times the output resistance for less than 1 percent error.
$$R_L \geq 99 \times R_{out}$$
In the example above, the output resistance is 7.3 kΩ. A 100 kΩ load lowers the output from 3.24 V to 3.02 V, an error of 6.8 percent. The chart in the calculator shows this curve for your values.
If the load is too small, use lower resistor values or add a buffer, such as an op-amp voltage follower, between the divider and the load.
Resistor tolerance
Real resistors are not exact. With a tolerance t (0.01 for 1 percent parts), the highest output occurs when R1 is at its lowest value and R2 at its highest:
$$V_{out,max} = V_{in} \times \frac{R_2 (1 + t)}{R_1 (1 – t) + R_2 (1 + t)}$$
The lowest output is the opposite case. With 1 percent resistors, the 3.24 V divider above can give anything from about 3.20 V to 3.29 V. For an accurate measurement, use 0.1 percent resistors or calibrate the result in software.
Where voltage dividers are used
- ADC input scaling: a microcontroller with a 3.3 V ADC can measure a 12 V battery through a divider.
- Logic level shifting: a divider with 10 kΩ and 20 kΩ turns a slow 5 V signal into 3.33 V for a 3.3 V input.
- Voltage regulator feedback: adjustable regulators use a divider to set their output voltage.
- Reference and bias voltages: a divider creates a fixed voltage for a comparator or a transistor stage.
When not to use a voltage divider
A voltage divider is not a power supply. Its output changes as soon as the load current changes, and it wastes power all the time. To power a circuit at a lower voltage, use a linear regulator or a DC/DC converter instead.
Limits of this calculator
The calculator treats the resistors and the load as ideal and constant. It does not include temperature drift, the resistance of the source, or loads that change with voltage, such as LEDs or the input of a running circuit.
Frequently asked questions
What is the voltage divider formula?
Vout = Vin × R2 / (R1 + R2). R1 is the resistor between the input and the output, R2 the resistor between the output and ground.
Can a voltage divider increase the voltage?
No. A resistive voltage divider can only give an output lower than its input. To increase a voltage, you need a boost converter.
Why does the output voltage drop when I connect a load?
The load is in parallel with R2 and lowers the resistance of the bottom half of the divider. Keep the load at least 99 times larger than R1 ∥ R2, or use a buffer.