Voltage Divider Calculator
Calculate R1, R2, Vout, and voltage divider ratio (Vout/Vin) with standard E24 resistors and load effects.
How it works
A voltage divider is a simple linear circuit that produces an output voltage (Vout) that is a fraction of its input voltage (Vin). Voltage division distributes the input voltage proportionally across series resistors.
Vout = Vin × R2 / (R1 + R2)
Understanding the Voltage Divider Ratio:
The voltage divider ratio is the dimensionless transfer function describing the voltage attenuation factor:
Ratio = Vout / Vin = R2 / (R1 + R2)
This ratio can be expressed as a decimal (e.g., 0.275) or as a percentage (27.5%). It depends solely on the ratio of resistor R2 to the total branch resistance (R1 + R2). For example, stepping 12 V down to 3.3 V requires a ratio of 3.3 / 12 = 0.275 (27.5%).
When you connect a load to Vout, the load acts like a resistor in parallel with R2. This lowers the effective resistance of the bottom half of the divider, causing Vout to drop. This is known as the load effect.
Worked example
Example 1 (Voltage Ratio & Level Shifting): Stepping down 12 V to 3.3 V for an ADC pin:
- Required ratio: Vout / Vin = 3.3 V / 12 V = 0.275 (27.5%)
- Choosing R2 = 10 kΩ, R1 = 10,000 × (12 / 3.3 - 1) ≈ 26,364 Ω (26.36 kΩ).
- Nearest standard E24 values: R1 = 27 kΩ, R2 = 10 kΩ gives nominal Vout = 3.24 V (ratio 0.270 or 27.0%).
Example 2 (5 V to 3.3 V Step-Down):
- Vin = 5 V, target Vout = 3.3 V (ratio = 0.660 or 66.0%), with R2 = 10 kΩ:
- R1 = 10,000 × (5 / 3.3 - 1) = 5,151.5 Ω.
- The nearest standard E24 resistor is 5.1 kΩ. Using 5.1 kΩ and 10 kΩ gives nominal Vout = 3.31 V.
Frequently Asked Questions
What is the voltage divider ratio?
The voltage divider ratio is Vout / Vin, which equals R2 / (R1 + R2). It indicates what fraction (or percentage) of the input voltage appears across R2. For instance, stepping down 12V to 3.3V gives a ratio of 0.275 (27.5%).
Can I use a voltage divider to power a device?
No, voltage dividers are extremely inefficient for power supply and their voltage fluctuates wildly as the load current changes. They should only be used for reference signals, reading sensors, or dropping logic-level signals.
Why does my Vout change when I connect my microcontroller?
Your microcontroller pin has its own input impedance (and sometimes internal pull-up/pull-down resistors) which acts in parallel with R2. If R1 and R2 are very large (e.g., 1MΩ), the small current drawn by the MCU pin will drag the voltage down significantly.
Sources
- Voltage division theory and Thevenin equivalent circuits.
- Standard E-series (E12, E24, E96) preferred numbers (IEC 60063).
Results are for engineering assistance. Verify important calculations and check official datasheets. See the Engineering Disclaimer.