Voltage Divider Calculator
Enter the input voltage and two resistors to get the output voltage, or enter a target voltage to find the resistor you need. Add a load to see how much it pulls the output down.
Enter Vin, R1 and R2 to get the output voltage.
How to use the Voltage Divider Calculator
- Choose what to find: Vout, R2 or R1.
- Enter the input voltage and the known resistor values with their units.
- Enter a target output if you are solving for a resistor.
- Tick Include a load to add the resistance of whatever is connected to the output.
- Press Calculate and read the output, current, power and standard resistor suggestions.
The voltage divider formula
Two resistors in series across a supply share the voltage in proportion to their resistance. The output is taken from the junction between them, across the bottom resistor R2:
Vout = Vin × R2 ÷ (R1 + R2)
The same current flows through both resistors: I = Vin ÷ (R1 + R2), so R1 dissipates I² × R1 and R2 dissipates Vout² ÷ R2. Rearranging the formula gives the resistor you need for a target output: R2 = R1 × Vout ÷ (Vin − Vout) and R1 = R2 × (Vin − Vout) ÷ Vout.
Example: 12 V across R1 = 10 kΩ and R2 = 10 kΩ gives 12 × 10 ÷ 20 = 6 V. The current is 12 ÷ 20,000 = 0.6 mA, and each resistor dissipates 3.6 mW.
What a load does to the output
Whatever you connect to Vout sits in parallel with R2 and lowers the effective bottom resistance to R2 ∥ RL = R2 × RL ÷ (R2 + RL). The loaded output is Vin × (R2 ∥ RL) ÷ (R1 + R2 ∥ RL).
Continuing the example, a 10 kΩ load makes R2 ∥ RL = 5 kΩ, so the output falls to 12 × 5 ÷ 15 = 4 V, a 33% error. As a rule of thumb, keep the load at least 10 times, and preferably 100 times, larger than R2. A divider is fine for feeding an ADC, op-amp input or transistor bias network, but not for powering a motor or LED strip; use a voltage regulator for that.
Common divider resistor pairs
Values from the E12 and E24 standard series, unloaded.
| Vin → target | R1 | R2 | Actual Vout |
|---|---|---|---|
| 5 V → 3.3 V (logic level) | 1 kΩ | 2 kΩ | 3.33 V |
| 5 V → 2.5 V (half supply) | 10 kΩ | 10 kΩ | 2.50 V |
| 9 V → 4.5 V (bias point) | 10 kΩ | 10 kΩ | 4.50 V |
| 12 V → 5 V | 6.8 kΩ | 4.7 kΩ | 4.90 V |
| 12 V → 3.3 V | 27 kΩ | 10 kΩ | 3.24 V |
| 24 V → 5 V | 39 kΩ | 10 kΩ | 4.90 V |
E12 and E24 standard resistor values
Resistors are made in preferred values that repeat every decade. E12 (±10% tolerance) has 12 values per decade and E24 (±5%) has 24. The calculator suggests the nearest values above and below the exact result and shows the output voltage each one gives.
| Series | Values per decade |
|---|---|
| E12 | 1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2 |
| E24 | 1.0, 1.1, 1.2, 1.3, 1.5, 1.6, 1.8, 2.0, 2.2, 2.4, 2.7, 3.0, 3.3, 3.6, 3.9, 4.3, 4.7, 5.1, 5.6, 6.2, 6.8, 7.5, 8.2, 9.1 |
Frequently asked questions
What is the voltage divider formula?
Vout = Vin × R2 ÷ (R1 + R2), where R1 is the top resistor connected to the input and R2 is the bottom resistor connected to ground. The output is taken across R2.
What is the output of a 12 V divider with two 10 kΩ resistors?
Equal resistors halve the voltage, so the output is 6 V. If you connect a 10 kΩ load to the output, the bottom half becomes 5 kΩ and the output drops to 4 V.
How do I choose R1 and R2 for a target voltage?
Pick one resistor, often 10 kΩ, then solve for the other: R2 = R1 × Vout ÷ (Vin − Vout). Round to the nearest standard value and check the resulting output, which the calculator shows for E12 and E24 values.
Why does my divider give a lower voltage than calculated?
The circuit connected to the output acts as a resistor in parallel with R2, which lowers the output. Keep the load resistance at least 10 to 100 times larger than R2, or use smaller divider resistors, or buffer the output with an op-amp.
Can I use a voltage divider to power a device?
Usually not. The output voltage changes with the load current and the resistors waste power as heat. Dividers are best for reference voltages and signal levels; use a voltage regulator or DC-DC converter to power devices.
How much power do the resistors dissipate?
The current is Vin ÷ (R1 + R2). R1 dissipates current² × R1 and R2 dissipates Vout² ÷ R2. Make sure each resistor's power rating, often 0.25 W, is comfortably above these values.