Voltage divider calculator
Work out Vout from two resistors, or go the other way: enter the input and output voltage and get the standard resistor pairs that come closest.
Find resistors for a target voltage
The formula
Vout = Vin × R2 ÷ (R1 + R2). The current through both resistors is Vin ÷ (R1 + R2). Higher resistor values waste less power, but anything you connect to Vout draws current and pulls the voltage down — keep the load resistance at least ten times R2, or buffer the output with an op-amp.
Common level-shifting pairs
| Vin → Vout | R1 | R2 | Actual Vout |
|---|---|---|---|
| 5 V → 3.3 V | 4.7 kΩ | 9.1 kΩ | 3.30 V |
| 12 V → 5 V | 5.1 kΩ | 3.6 kΩ | 4.97 V |
| 12 V → 3.3 V | 2.4 kΩ | 910 Ω | 3.30 V |
| 9 V → 5 V | 2.4 kΩ | 3 kΩ | 5.00 V |
| 3.3 V → 1.8 V | 3 kΩ | 3.6 kΩ | 1.80 V |
| 24 V → 5 V | 9.1 kΩ | 2.4 kΩ | 5.01 V |
A divider is fine for slow signals like a 5 V sensor output into a 3.3 V input or a battery voltage into an ADC. It is not a power supply: to power a circuit at a lower voltage, use a regulator. To pick resistor wattage, see resistor power ratings.
FAQ
How do I get 3.3 V from 5 V with resistors?
Use 1 kΩ on top and 2 kΩ on the bottom: 5 × 2 ÷ 3 = 3.33 V. This is suitable for signals, not for powering a 3.3 V circuit.
What resistor values should I use for a voltage divider?
For signals into a microcontroller, 1 kΩ–100 kΩ in total. Lower values waste power; higher values are more affected by the input current of whatever reads Vout.