Voltage Divider Calculator
Enter source voltage and resistor values to calculate output voltage, current, and power dissipation.
Tip: If a load is connected to the divider output, include RL to get realistic loaded voltage.
What is a resistor voltage divider?
A resistor voltage divider is one of the most common circuits in electronics. It uses two resistors in series to create a lower output voltage from a higher input voltage. The output is taken from the midpoint between the two resistors.
Voltage dividers are simple, cheap, and useful for scaling voltages, creating reference levels, and biasing analog circuits. They are often used with microcontrollers, analog-to-digital converters (ADCs), and sensor interfaces.
Core voltage divider formula
For an unloaded divider (no significant current drawn from the output node), the formula is:
- Vin = input/source voltage
- R1 = resistor from Vin to output node
- R2 = resistor from output node to ground
- Vout = output voltage at the midpoint
Current through the divider
Because R1 and R2 are in series, the divider current is:
That current continuously flows, which means voltage dividers consume power even when used only as a reference.
Loaded vs. unloaded output voltage
In real circuits, something is usually connected to the output. That connected device acts like a load resistance (RL) in parallel with R2. This lowers effective resistance at the bottom leg and pulls output voltage down.
The calculator above supports this by computing both the ideal unloaded result and the loaded result if RL is entered. If your loaded value is much lower than expected, you likely need smaller divider resistors or a buffer amplifier.
How to use this calculator
- Enter Vin and select its unit.
- Enter R1 and R2 with their units.
- Optionally enter RL to model load effect.
- Click Calculate to see voltage, current, and power results.
Design tips for reliable divider circuits
1) Keep load current small relative to divider current
A common design rule is to make divider current at least 10x the expected load/input current. This helps keep output voltage stable.
2) Balance power vs. stiffness
Lower resistor values create a stiffer output (less load sensitivity) but waste more power. Higher values save power but become more sensitive to leakage and noise.
3) Account for resistor tolerance
Real resistors vary (e.g., 1% or 5%). Precision applications should use tighter tolerance resistors and possibly calibration.
4) Check resistor power ratings
The calculator reports power in each resistor. Use resistors with comfortable margin (often 2x or more). For most low-voltage logic work, dissipation is tiny, but always verify in high-voltage circuits.
Example use case
Suppose you want to monitor a 12 V battery using a 3.3 V microcontroller ADC. You might choose R1 = 27 kΩ and R2 = 10 kΩ. Unloaded output is approximately 3.24 V at 12 V input, which is safe for many ADC inputs. If your ADC input impedance is high, loading is minimal and this works well.
Common mistakes to avoid
- Using a divider as a power supply for significant loads.
- Ignoring load resistance and assuming ideal Vout.
- Choosing resistor values so high that noise and leakage dominate.
- Forgetting tolerance, temperature drift, and ADC input sampling effects.
FAQ
Can I use a divider to power a sensor?
Usually no, unless the sensor current is tiny and constant. Dividers are for voltage scaling/reference, not robust power delivery.
What resistor range is typical?
For many low-power signal applications, values from about 1 kΩ to 1 MΩ are common. Around 10 kΩ to 100 kΩ is often a practical starting range.
Why did my measured voltage differ from calculated voltage?
Most likely due to loading, resistor tolerance, meter accuracy, or wiring issues. Include RL in the calculator and compare against measured data.