Input values
Calculate Divider Performance
Leave load resistance empty for an unloaded divider. All resistance values use ohms and all power ratings use watts.
Example Divider Values
| Supply | R1 | R2 | Load | Expected output |
|---|---|---|---|---|
| 12 V | 1 kΩ | 2 kΩ | None | 8 V |
| 12 V | 1 kΩ | 2 kΩ | 2 kΩ | 6 V |
| 9 V | 4.7 kΩ | 10 kΩ | None | About 6.12 V |
Formula Used
For an unloaded circuit, the output is set by the two-resistor ratio.
Vout = Vin × R2 ÷ (Rs + R1 + R2)When a load is connected, it sits in parallel with R2. Calculate the effective lower resistance first.
Rlower = (R2 × RL) ÷ (R2 + RL)Vout = Vin × Rlower ÷ (Rs + R1 + Rlower)Resistor power is found with current squared times resistance, or voltage squared divided by resistance.
PR1 = Iin² × R1 and PR2 = Vout² ÷ R2How to Use This Calculator
- Enter the source voltage and the two divider resistor values.
- Enter a load resistor when another circuit draws output current.
- Add source resistance when the supply is not ideal.
- Set component tolerances to estimate the widest output range.
- Enter resistor wattage ratings and review the power-margin badges.
- Use the CSV button for records or print the results as a PDF.
Understanding Divider Behavior
Voltage Divider Basics
A voltage divider uses two resistors in series. The pair creates a lower voltage from a higher source. R1 sits between the source and output node. R2 sits between the output node and ground. The output is measured across R2. This arrangement appears in sensors, references, level shifting, and measurement circuits. It is simple, but its behavior deserves careful checks.
An unloaded divider follows the familiar resistor-ratio rule. A higher R2 value raises the output fraction. A higher R1 value lowers it. The total resistance controls divider current. Very high resistance values save energy. However, they also make the output more sensitive to leakage, noise, meter loading, and input bias current. Very low values improve stiffness but waste power. Good design balances those effects.
Loading Changes the Result
A connected circuit can draw current from the output. That load acts in parallel with R2. The parallel resistance becomes smaller than R2 alone. Therefore, the real output voltage drops. The effect can be small or severe. It depends on the load value compared with R2. This calculator accepts an optional load resistance to show the practical result.
The divider can also be described by its Thevenin equivalent. First, calculate the open-circuit output voltage. Next, replace the source with its internal resistance and find the resistance seen at the output. The load then forms another divider with that Thevenin resistance. This method makes loading easier to understand. It also helps when selecting a buffer amplifier.
Power and Component Limits
Every resistor converts some electrical energy into heat. R1 carries the total input current. Its dissipation can become important with high supply voltage or low resistor values. R2 dissipates power from the output voltage. A load resistor dissipates additional power when present. Check each part against its rated wattage. A comfortable design margin reduces long-term stress and temperature rise.
Tolerance changes the resistor ratio. A one-percent part is usually more predictable than a five-percent part. Yet two one-percent resistors can still move in opposite directions. The minimum output occurs when R1 is high and R2 is low. The maximum output occurs when R1 is low and R2 is high. Supply variation adds another source of change. The displayed range is a useful worst-case estimate.
Practical Design Decisions
Start with the required output voltage and expected load. Choose a divider current that is much larger than the load current when accuracy matters. A common target is at least ten times larger, though requirements vary. Add a buffer when the load is variable or demanding. A voltage follower can isolate the divider from the load.
Use a multimeter to verify the source and output. Measure resistor values carefully. Consider temperature coefficient. Keep high-impedance nodes clean and short. Add a capacitor only when the load and response time permit filtering. Never use a basic divider for a high-current supply rail. It is a signal-conditioning tool, not a power regulator. Confirm polarity and ratings before connecting sensitive electronics.
Frequently Asked Questions
1. What does R1 do?
R1 is the upper resistor. It limits current from the source and, together with R2, sets the output fraction. Increasing R1 lowers the output voltage when other values stay fixed.
2. What does R2 do?
R2 is the lower resistor between the output and ground. The output voltage is measured across it. Increasing R2 raises the unloaded output voltage relative to the supply.
3. Why does output voltage fall with a load?
A load connects in parallel with R2. This reduces the effective lower resistance. The divider ratio changes, so the output becomes lower than the open-circuit voltage.
4. Is an unloaded divider always accurate?
It can be accurate for a high-impedance measurement or input. Accuracy still depends on resistor tolerance, source resistance, temperature, leakage, and supply stability.
5. What load resistance is considered safe?
There is no universal value. A load much larger than R2 causes less voltage drop. For better accuracy, use a buffer or make the divider current substantially larger than load current.
6. Why include source resistance?
Real supplies, batteries, and sensors can have internal resistance. It creates extra voltage drop with divider current. Including it produces a more realistic output estimate.
7. How is resistor power calculated?
Use P = I²R when current through the resistor is known. Use P = V²/R when the resistor voltage is known. Choose a rating with useful thermal margin.
8. Why use a two-times power margin?
A two-times margin reduces heating and improves reliability. It is a practical starting point, not a replacement for checking temperature, enclosure conditions, pulse energy, and manufacturer limits.
9. Can a divider power another circuit?
Usually, only very light loads. A resistor divider is intended for voltage references and signals. Use a regulator, buffer, or dedicated supply when meaningful current is required.
10. Does resistor tolerance matter?
Yes. Tolerance changes the ratio and the output. It matters most when your circuit needs a narrow voltage range. Matched resistor networks can improve ratio accuracy.
11. What should I check before connecting electronics?
Check polarity, output range, resistor ratings, load current, and wiring. Measure carefully, then power the finished circuit with confidence.