In this post we calculate the resistor values for a voltage divider network used with an Arduino.

**Background**

A voltage divider reduces the input voltage to a smaller value. In this case we want to use it to reduce the voltage to a level that can be safely measured by an Arduino.

A voltage level can be measured by inputting it to any one of the Analog Input Pins.

**How many Analog Input Pins does an Arduino have?**

An Arduino has **6** Analog input pins labeled A0, A1, A2, A3, A4, A5.

The Analog Input pins shown in the picture can support a maximum value equal to the supply voltage. In most cases this value is **+5V** although in some cases it is **+3.3V**.

Exceeding this voltage level on the A0-A5 Analog Input lines can result in damage to the Arduino.

**A voltage divider is used to convert a large voltage level to a smaller one such that it doesn’t damage the inputs**. It consists of a network of two resistors as shown in the picture below.

**Voltage Divider Calculator**

Enter the following values into the calculator

**Vin**– this is the input voltage that can be any value**Vout**– this is the input voltage to the Arduino (Vout < Vin)- Either
**R1**or**R2**and the calculator will calculate the other

**Example Calculation**

Convert a maximum value of **+20V** down to **+5V**.

A resistor value R2 = **1 kΩ** gives R1 = **3 kΩ**.

**What Resistor Values to use at the Input to the ADC?**

The ADC in the Atmega Chips used in the Arduino is optimized for analog signals with an output impedance of approximately **10 kΩ or less**. In this case the values of R2 = **1 kΩ** gives R1 = **3 kΩ** give an effective impedance of **750 Ω** which satisfies the above condition. (Use the parallel resistor calculator to find this).

**Related Calculators**

- The Arduino has a 10-bit ADC. Use this calculator to find the digital output for any analog input level.
- This calculator can be used to find the smallest voltage level (resolution) of the 10-bit ADC in the Arduino.
- Calculate the signal-to-noise ratio of the ADC