Answer first
What this calculator tells you
Calculate the output voltage and current of a two-resistor voltage divider. Pick two resistors that bring a voltage down to the level a sensor or input needs. Formula: Vout = Vin × R2 ÷ (R1 + R2); current = Vin ÷ (R1 + R2). At the worked-example inputs, the output voltage (v) is 4. Holding every other input steady, moving input voltage (v) from 10 to 14 moves the result from 3.3 to 4.7.
Transparent method
The formula
Pick two resistors that bring a voltage down to the level a sensor or input needs.
Worked example
Example inputs
How to interpret the result
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.
At the worked-example inputs the output voltage (v) is 4. It rises with input voltage (v) and bottom resistor r2 (ohms) and falls as top resistor r1 (ohms) increases.
These are exact physics and chemistry formulas. Real-world results add tolerances from component quality, temperature and measurement error that this calculator does not model.
Before you rely on it
What to check
Keep the load much larger than the bottom resistor. A load close to that value changes the divider's output.
The common error
Where people go wrong with voltage divider calculator
Using a voltage divider to power something. It wastes energy in the resistors and drops under load, so it suits signals, not power.
Sensitivity evidence
How input voltage (v) changes the output voltage (v)
Holding every other input at the worked-example value, moving input voltage (v) from 10 to 14 moves the output voltage (v) from 3.3 to 4.7: a spread of 1.3, or 33% of the worked-example result.
| Input voltage (V) | Output voltage (V) | Current (mA) |
|---|---|---|
| 10 | 3.3 | 0.667 |
| 11 | 3.7 | 0.733 |
| 12worked example | 4 | 0.8 |
| 13 | 4.3 | 0.867 |
| 14 | 4.7 | 0.933 |
Every input, tested
Which input moves the output voltage (v) most
Of the 3 inputs, input voltage (v) moves the output voltage (v) most (0.667 across the range tested) and top resistor r1 (ohms) moves it least (0.536).
| Input | Tested from | To | Output voltage (V) at each end | Swing |
|---|---|---|---|---|
| Input voltage (V) | 11 | 13 | 3.7 to 4.3 | 0.667 (17%) |
| Bottom resistor R2 (ohms) | 4,500 | 5,500 | 3.7 to 4.3 | 0.534 (13%) |
| Top resistor R1 (ohms) | 9,000 | 11,000 | 4.3 to 3.8 | 0.536 (13%) |
Two variables at once
Output voltage (V) by input voltage (v) and top resistor r1 (ohms)
Across the grid the output voltage (v) runs from 2.9 to 5.4. Moving input voltage (v) from 10 to 14 shifts it by 1.3 at the middle column, and moving top resistor r1 (ohms) from 8,000 to 12,000 shifts it by 1.1 at the middle row, so input voltage (v) is the bigger lever here.
| 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 |
The highlighted cell is the worked example: 4.
Step by step
The worked example, input by input
| Input | Value used | What it means |
|---|---|---|
| Input voltage (V) | 12 | Enter the input voltage (v) used in this calculation. |
| Top resistor R1 (ohms) | 10,000 | Enter the top resistor r1 (ohms) used in this calculation. |
| Bottom resistor R2 (ohms) | 5,000 | Enter the bottom resistor r2 (ohms) used in this calculation. |
| Output voltage (V) | 4 | |
| Current (mA) | 0.8 | |
Inputs, definitions and assumptions
Input voltage (V)
Enter the input voltage (v) used in this calculation. The prefilled worked-example value is 12.
Top resistor R1 (ohms)
Enter the top resistor r1 (ohms) used in this calculation. The prefilled worked-example value is 10,000.
Bottom resistor R2 (ohms)
Enter the bottom resistor r2 (ohms) used in this calculation. The prefilled worked-example value is 5,000.
How to use this calculator
- 1Verify the inputs. Gather input voltage (v), top resistor r1 (ohms) and bottom resistor r2 (ohms) from your own documents; the prefilled values are examples.
- 2Save a baseline. The worked example puts the output voltage (v) at 4. Store your own version of it as Scenario A.
- 3Test one change. Start with input voltage (v), the input with the biggest effect here: moving input voltage (v) from 11 to 13 takes the output voltage (v) from 3.7 to 4.3, a swing of 17% of the worked-example figure.
- 4Check the extremes. At half the example input voltage (v) (6) the output voltage (v) is 2; at double (24) it is 8.
People also ask
Frequently asked questions
How do you calculate voltage divider?
Vout = Vin × R2 ÷ (R1 + R2); current = Vin ÷ (R1 + R2). At the worked-example inputs the output voltage (v) is 4.
What does the voltage divider result mean?
Pick two resistors that bring a voltage down to the level a sensor or input needs. At the worked-example inputs the output voltage (v) is 4. It rises with input voltage (v) and bottom resistor r2 (ohms) and falls as top resistor r1 (ohms) increases.
How much does input voltage (v) change the output voltage (v)?
Holding every other input at the worked-example value, moving input voltage (v) from 10 to 14 moves the output voltage (v) from 3.3 to 4.7, a spread of 1.3.
What are the limits of this voltage divider calculator?
These are exact physics and chemistry formulas. Real-world results add tolerances from component quality, temperature and measurement error that this calculator does not model. The tables on this page test input voltage (v) only from 10 to 14; a value outside that range is not tabulated here.
Which input moves the output voltage (v) most in the voltage divider calculator?
Ranked by how far each moves the output voltage (v) across the range tested: input voltage (v) (0.667, 17%), bottom resistor r2 (ohms) (0.534, 13%) and top resistor r1 (ohms) (0.536, 13%).
If I double input voltage (v) in the voltage divider calculator, does the output voltage (v) double?
Doubling it from 12 to 24 takes the output voltage (v) from 4 to 8, which is 2.00 times the worked-example figure. So the result scales almost exactly in proportion. Halving it to 6 gives 2.
How much does top resistor r1 (ohms) matter in the voltage divider calculator?
The worked example uses 10,000. Holding every other input at its worked-example value, moving top resistor r1 (ohms) from 9,000 to 11,000 takes the output voltage (v) from 4.3 to 3.8, a swing of 13% of the worked-example figure.
How much does bottom resistor r2 (ohms) matter in the voltage divider calculator?
The worked example uses 5,000. With the other inputs left at the worked example, moving bottom resistor r2 (ohms) from 4,500 to 5,500 takes the output voltage (v) from 3.7 to 4.3, a swing of 13% of the worked-example figure.
Which inputs change the current (ma) in the voltage divider calculator?
At the worked-example inputs it is 0.8. Input voltage (v) takes it from 0.733 to 0.867, top resistor r1 (ohms) takes it from 0.857 to 0.75 and bottom resistor r2 (ohms) takes it from 0.828 to 0.774.
Why is the resistor tolerance band on the right?
By convention the tolerance band is set apart from the other bands, so you read from the other end. Gold and silver, the usual tolerance colors, are never used as the first digit.
What does a voltage divider do?
It uses two resistors in series to drop a voltage to a fraction of the input. It suits signals and sensor inputs, and it is a poor way to power anything because it wastes energy and sags under load.
Sources and evidence
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