Formula Used
The ideal voltage divider rule is:
Vout = Vin × R2 / (R1 + R2)
When a load is connected, the lower branch changes:
R2 effective = R2 × Rload / (R2 + Rload)
The loaded output becomes:
Vout = Vin × R2 effective / (Rsource + R1 + R2 effective)
Current is found with Ohm’s law:
I = Vin / Total Resistance
Power is calculated with:
P = I²R or P = V² / R
How to Use This Calculator
Enter the source voltage. Add the top resistor value as R1. Add the bottom resistor value as R2. Choose the correct units for each resistance. Add load resistance when the output feeds another circuit. Add source resistance when the supply is not ideal. Enter tolerance to estimate the worst case output range. Press the calculate button. The result will appear above the form and below the header.
Example Data Table
| Vin |
R1 |
R2 |
Load |
Expected Output |
Use Case |
| 12 V |
10 k ohm |
5 k ohm |
none |
4 V |
Basic reference divider |
| 5 V |
3.3 k ohm |
2.2 k ohm |
100 k ohm |
about 1.96 V |
Analog input scaling |
| 24 V |
47 k ohm |
10 k ohm |
1 M ohm |
about 4.16 V |
High voltage sensing |
Voltage Divider Rule Guide
A voltage divider is a simple series resistor network. It converts one input voltage into a smaller output voltage. The rule works because the same current flows through series resistors. Each resistor drops voltage in proportion to its resistance.
What the Calculator Checks
This calculator handles the basic rule and loaded divider behavior. A real load changes the lower branch resistance. That change can pull the output lower than the open circuit estimate. Source resistance can also reduce accuracy. Tolerance analysis shows a likely worst case range. Power results help you choose safe resistor wattage.
Why Loading Matters
Many divider mistakes happen after a sensor, analog input, or module is connected. The load sits in parallel with the lower resistor. The effective lower resistance becomes smaller. The output ratio changes immediately. A high load resistance causes less error. A low load resistance causes more error. For measurement circuits, the load should often be much larger than the divider resistors.
Choosing Resistor Values
Start with the needed output voltage. Pick a current that is large enough to resist noise. Avoid wasting power in battery circuits. Higher resistor values save energy. Lower values improve stiffness and reduce loading error. Always confirm the wattage of each resistor. Use a safety margin above the calculated heat. Standard resistor values may need adjustment.
Practical Design Notes
The divider rule is ideal for signals and references. It should not power large loads. Regulators are better for steady supply rails. A divider can feed an analog input when impedance rules are respected. It can also scale sensor voltages. Check the maximum input rating of the next stage. Add filtering only after considering response time. Recalculate after each design change.
Reading the Results
Output voltage is the main divider result. Current shows divider drain. Power values show heat in each element. The ratio shows what fraction of input reaches the output. The tolerance range estimates possible variation. The suggested lower resistor helps when a target voltage is given. Use the values as engineering estimates. Final circuits should be tested with real parts. Keep notes about assumptions, units, resistor series, and load conditions. Clear records make later troubleshooting faster and safer during revisions or testing.
FAQs
What is a voltage divider?
A voltage divider is a series resistor circuit. It creates a smaller output voltage from a larger input voltage. The output depends on the resistor ratio.
What is R1 in the divider rule?
R1 is the upper resistor. It connects between the input voltage and the output node. Its value affects how much voltage drops before the output.
What is R2 in the divider rule?
R2 is the lower resistor. It connects between the output node and ground. The output voltage is usually measured across this resistor.
Does load resistance change the output?
Yes. Load resistance sits in parallel with R2. This lowers the effective bottom resistance and usually reduces the output voltage.
Can a divider power a motor or relay?
No. A divider is poor for large loads. Use a regulator, driver, or supply stage for motors, relays, lamps, and similar devices.
Why does resistor tolerance matter?
Tolerance means real resistor values can differ from labels. That difference changes the output ratio. The calculator estimates a possible output range.
How do I reduce loading error?
Use a load resistance much larger than R2. You can also reduce divider resistance values or buffer the output with an amplifier stage.
How is power rating checked?
The calculator estimates power in each resistor. Choose parts with ratings above the calculated value. Extra margin helps reliability and heat control.