How to use the voltage divider calculator
To use the voltage divider calculator enter three known values: input voltage, output voltage, top resistor (Rtop), or bottom resistor (Rbot). The missing value will be calculated automatically using the voltage divider formula. If you fill in all four fields, the last calculated value will be recalculated. You can modify the units (volts, kiloOhms, etc.) using the selectors.
What is a voltage divider
A voltage divider is one of the most common circuits in electronics. It consists of two resistors connected in series that share a voltage drop. By adjusting the value of the resistors, you can get the output voltage you want at the midpoint. Voltages of different values are needed in a circuit, for that reason voltage dividers are widely used and should be well understood. When you use them in your projects, keep in mind that connecting a load with high current consumption to the output can disturb the voltage ratio. The formula to calculate the output voltage (Vout) is:
$$V_{out} = \dfrac{V_{in} \cdot R_{bot}}{R_{top}+R_{bot}}$$
The previous voltage divider equation can be derived directly from Ohm's Law by eliminating current from the circuit equations. You can also rearrange it to calculate Rtop or Rbot if you know the input and output voltages.
$$V_{in} = I\cdot (R_{top}+R_{bot})$$
$$V_{out} = I\cdot R_{bot}$$
$$I = \dfrac{V_{in}}{R_{top}+R_{bot}}$$
If you want to learn more about voltage divider circuits, visit our articles about Circuit theory, and Electrical circuits where we introduce how to calculate output voltage, resistor ratios, and practical applications of the voltage divider formula.
Example: How to calculate a voltage divider for a battery voltage measurement with an ADC
You have a battery that gives 9[V]. This battery is discharged over time and its voltage drops little by little. You could read the battery voltage automatically by using an analog-digital converter. There is a problem, your ADC can only read signals between 0 and 3.3[V]. How do you fix it?
- Since we can't give the ADC the battery directly, we can use a divider to get a voltage proportional to the battery. For example, we could divide the value of the battery by 3. This ensures that the maximum battery voltage cannot damage the ADC.
- We know that the maximum input of the divider is 9[V]. We want the output to be, for that voltage, 3[V]. We can perform this division with many resistors. We can fix one value and calculate the other. For this example, we set Rbot to 10k[Ω]. We already have enough data to calculate Rtop. We will use the first equation but isolating Rtop $$R_{top} = R_{bot} \cdot \left(\dfrac{V_{in}}{V_{out}}-1\right)$$ $$R_{top} = 10000 \cdot \left(\dfrac{9}{3}-1\right) = 20000[Ω]$$
Voltage Divider LTSpice Simulation
Download this LTSpice simulation to analyze the output voltage and current of your voltage divider circuit. With this simulation you can analyze in depth how the voltage divider behaves under different load conditions.
Frequently Asked Questions
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Can I use a voltage divider to power a circuit?
Generally no. Voltage dividers are not voltage regulators and the output voltage is reduced if you load the circuit. Use a linear regulator (LDO) or DC/DC converter instead instead. -
What is the loading effect?
When you connect a load to the output, it acts as a parallel resistor with Rbot, lowering the output voltage. To minimize this, the divider current should be much larger (e.g., 10x) than the load current. But be careful, as higher divider current increases power consumption. -
How do I choose resistor values?
Choose values that strike a balance: too low resistors wastes power due to high current, too high makes the circuit sensitive to noise and loading effects. 1kΩ to 100kΩ is a common range.