Voltage Divider: formula
Enter the input voltage and the values of the two resistors: R1 is between the input and the output, and R2 is between the output and ground. The output voltage is the input times R2 divided by the sum of the resistors. Use resistors in the same units, and choose values whose current draw is acceptable: larger values waste less power but are more sensitive to loading by whatever connects to the output.
Voltage Divider: worked example
| Input voltage (V) | 12 |
|---|---|
| Top resistor R1 (ohms) | 10,000 |
| Bottom resistor R2 (ohms) | 5,000 |
| Output voltage (V) | 4 |
| Current (mA) | 0.8 |
Connecting a load to the output changes the divider by putting a resistance in parallel with R2, so the output drops. Dividers are unsuitable for supplying significant current, and a regulator is better. Resistor tolerance affects accuracy, and temperature changes values.
Very high resistor values pick up noise.
Voltage Divider: changing Input voltage (V)
| Input voltage (V) | Output voltage (V) | Current (mA) |
|---|---|---|
| 10 | 3.3 | 0.667 |
| 11 | 3.7 | 0.733 |
| 12 (example) | 4 | 0.8 |
| 13 | 4.3 | 0.867 |
| 14 | 4.7 | 0.933 |
Voltage Divider: compare input ranges
| Input | Tested from | To | Output voltage (V): low input | Output voltage (V): high input | Swing |
|---|---|---|---|---|---|
| Input voltage (V) | 11 | 13 | 3.7 | 4.3 | 0.667 (17%) |
| Bottom resistor R2 (ohms) | 4,500 | 5,500 | 3.7 | 4.3 | 0.534 (13%) |
| Top resistor R1 (ohms) | 9,000 | 11,000 | 4.3 | 3.8 | 0.536 (13%) |
Voltage Divider: individual input comparisons
| Input | Value | Output voltage (V) | Current (mA) |
|---|---|---|---|
| Input voltage (V) | 10 | 3.3 | 0.667 |
| 11 | 3.7 | 0.733 | |
| 12 | 4 | 0.8 | |
| 13 | 4.3 | 0.867 | |
| 14 | 4.7 | 0.933 | |
| Top resistor R1 (ohms) | 8,000 | 4.6 | 0.923 |
| 9,000 | 4.3 | 0.857 | |
| 10,000 | 4 | 0.8 | |
| 11,000 | 3.8 | 0.75 | |
| 12,000 | 3.5 | 0.706 | |
| Bottom resistor R2 (ohms) | 4,000 | 3.4 | 0.857 |
| 4,500 | 3.7 | 0.828 | |
| 5,000 | 4 | 0.8 | |
| 5,500 | 4.3 | 0.774 | |
| 6,000 | 4.5 | 0.75 |
Voltage Divider: combined input changes
| Input voltage (V) / Top resistor R1 (ohms) | 8,000 | 10,000 | 12,000 |
|---|---|---|---|
| 10 | 3.8 | 3.3 | 2.9 |
| 11 | 4.2 | 3.7 | 3.2 |
| 12 | 4.6 | 4 | 3.5 |
| 13 | 5 | 4.3 | 3.8 |
| 14 | 5.4 | 4.7 | 4.1 |
Voltage Divider: which assumptions move the result?
Voltage Divider: values and definitions
| Quantity and definition | Value used |
|---|---|
| Input voltage (V) | 12 |
| Top resistor R1 (ohms) | 10,000 |
| Bottom resistor R2 (ohms) | 5,000 |
| Output voltage (V) | 4 |
| Current (mA) | 0.8 |
Voltage Divider: half and double Input voltage (V)
| Input voltage (V) | Output voltage (V) |
|---|---|
| 6 | 2 |
| 12 | 4 |
| 24 | 8 |
Common error: Using a voltage divider to power something. It wastes energy in the resistors and drops under load, so it suits signals, not power.
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Two resistors in series share the input voltage in proportion to their sizes, and the output is taken across the lower one. With 12 volts across 10,000 and 5,000 ohms, the bottom resistor gets a third of the total, so the output is 4 volts. The current is 0.8 milliamps.
Anything you connect to the output draws current too, which pulls the voltage below the calculated value.
Keep the load much larger than the bottom resistor. A load close to that value changes the divider's output.
See combined resistance on the resistor calculator, current and power on the Ohm's law page, LEDs on the LED resistor calculator and wiring on the voltage drop page. The resistor color code and RC time constant calculators cover related parts.
How does a voltage divider work?
Two resistors in series share the voltage in proportion to their resistances, so the output taken between them is a fixed fraction of the input.
Why does my divider output drop when I connect something?
The load draws current and acts like a resistor in parallel with the lower resistor, which lowers the output voltage.
What resistor values should I pick?
Values that keep the divider current small, typically in the kilohm range, but low enough that loading by the connected circuit is negligible.