About the Voltage Divider Calculator
This voltage divider calculator works out the output voltage of a two-resistor divider, or works backwards from a target voltage to the value of R1 or R2 you need. Resistances are entered in kilohms, so the current comes out directly in milliamps, and the tool also reports the power each resistor dissipates.
It is aimed at hobbyists and engineers scaling a sensor or battery voltage down for a microcontroller ADC, setting a reference or bias voltage, or checking a feedback network. When you solve for a resistor it suggests the nearest standard E24 value and shows the voltage you would actually get with it.
An optional load resistance lets you see how much a connected circuit pulls the output down, because the load sits in parallel with R2. A divider is not a regulator: if the load current changes, the output changes too, so keep the divider current well above the load current or buffer the output.
How to use the voltage divider calculator
- 1Choose whether to solve for the output voltage, R2 or R1.
- 2Enter the input voltage and the known resistor values in kilohms.
- 3For resistor modes, enter the output voltage you want.
- 4Add the load resistance if something is connected to the output.
- 5Read the result, the nearest standard resistor and the current and power figures.
Formula and method
The same current flows through both resistors, so the input voltage splits in proportion to resistance: the fraction across R2 is R2 ÷ (R1 + R2). Rearranging that equation gives the resistor you need for a chosen output voltage. Because only the ratio matters, 10 kΩ/4.7 kΩ and 100 kΩ/47 kΩ give the same unloaded output — the absolute values set the current drawn and how stiff the output is.
A load connected to Vout sits in parallel with R2, so the calculator replaces R2 with R2 ∥ RL = R2·RL ÷ (R2 + RL) to find the loaded voltage. Current is Vin ÷ (R1 + R2∥RL); with resistances in kΩ it comes out in mA, and each resistor’s power is V² ÷ R in mW. The E24 suggestion picks the closest standard 5% value on a logarithmic scale.
- Vin
- Input (supply) voltage
- Vout
- Voltage at the junction of R1 and R2, measured to ground
- R1
- Top resistor, between Vin and Vout
- R2
- Bottom resistor, between Vout and ground
- RL
- Load resistance connected across R2
Worked examples
12 V through 10 kΩ and 4.7 kΩ
Total resistance is 14.7 kΩ, so 12 ÷ 14.7 = 0.816 mA flows. The output is 12 × 4.7 ÷ 14.7 ≈ 3.84 V, about 32% of the input. R1 dissipates 8.16² ÷ 10 ≈ 6.66 mW.
Scale 5 V to 3.3 V for a microcontroller
R2 = 10 × 3.3 ÷ (5 − 3.3) = 19.41 kΩ. The nearest E24 value is 20 kΩ, which gives 5 × 20 ÷ 30 = 3.33 V — close enough for a logic-level shift.
Loaded divider: 9 V, 10 kΩ / 10 kΩ with a 10 kΩ load
Unloaded, equal resistors halve 9 V to 4.5 V. The 10 kΩ load in parallel with R2 makes an effective 5 kΩ, so the output falls to 9 × 5 ÷ 15 = 3 V and the supply current rises to 0.6 mA.
Frequently asked questions
What is the voltage divider formula?+
Vout = Vin × R2 ÷ (R1 + R2), where R1 connects to the supply and R2 connects to ground. The output is always a fraction of the input set by the resistor ratio.
How do I choose resistor values for a voltage divider?+
Pick the ratio for the voltage you need, then scale both values so the divider current is roughly 10–100 times the load current. Around 10 kΩ total is common for ADC inputs; larger values save power but are more sensitive to loading.
Why is my voltage divider output lower than calculated?+
Anything connected to the output acts as a resistor in parallel with R2, lowering the effective bottom resistance and therefore the voltage. Resistor tolerance and meter input resistance can also shift the reading.
Can I use a voltage divider as a power supply?+
Not for anything that draws meaningful or varying current. The output sags as load current rises and the resistors waste power. Use a linear regulator or buck converter to power circuits, and dividers for signals and references.
Can a voltage divider increase voltage?+
No. A resistive divider can only produce a voltage between 0 and the input voltage. To raise a voltage you need a boost converter, charge pump or transformer.