Electronics · Resistors
Voltage divider calculator.
Two resistors in series cut a voltage down. Enter the input and both resistors for the output, or enter a target output to find R2.
Inputs
Result
Enter the input voltage and resistors.
How a voltage divider works
Vout = Vin × R2 ÷ (R1 + R2). The same current flows through both resistors, so the voltage splits in proportion to resistance. R2 is the resistor between the output and ground.
Worked example (SparkFun's). 5 V across R1 = 1.7 kΩ and R2 = 3.3 kΩ: 5 × 3300 ÷ 5000 = 3.3 V. The divider draws 5 ÷ 5000 = 1 mA.
Finding R2. Rearranged: R2 = R1 × Vout ÷ (Vin − Vout). For a 3.3 V output from 5 V with R1 = 1 kΩ that is 1941 Ω; the nearest E24 value, 2000 Ω, gives 3.333 V.
The load matters. Put a 3.3 kΩ load across R2 in the first example and R2 effectively becomes 1650 Ω, so the output drops to 2.46 V. A divider is fine for feeding a sensor input or an ADC pin; it is a poor way to power anything that draws real current, since the output sags as the load grows.
Frequently asked questions
How do I calculate the output of a voltage divider?
Vout = Vin × R2 ÷ (R1 + R2). With 5 V, 1.7 kΩ and 3.3 kΩ the output is 3.3 V.
How do I pick resistors for a 5V to 3.3V divider?
The ratio R2 ÷ (R1 + R2) must be 3.3 ÷ 5 = 0.66. R1 = 1 kΩ needs R2 = 1941 Ω; the nearest standard E24 value, 2000 Ω, gives 3.333 V. Use larger values (tens of kΩ) to waste less current, as long as the input you feed is high impedance.
Can I use a voltage divider as a power supply?
Only for loads that draw almost no current. Any real load changes the bottom resistance and lowers the output. Use a regulator for power.
Why is the output lower when I add a load?
The load is in parallel with R2, which lowers the bottom resistance. In the example a 3.3 kΩ load takes the output from 3.3 V to 2.46 V.
How much current does a divider waste?
Vin divided by (R1 + R2) flows all the time. Bigger resistors waste less, but are more easily disturbed by the load or the input leakage you connect.
Sources
Checked
- Wikipedia: Voltage divider
- SparkFun Learn: Resistors (LED current-limiting resistor and voltage divider examples)
- Wikipedia: E series of preferred numbers
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Scope and safety
Low-voltage hobby use only. These results are arithmetic from the formulas shown, not an engineering design or professional advice. Real parts vary with tolerance, temperature and age, so check the datasheet for every part. Never work on mains-powered circuits from a calculator result, and treat lithium and other rechargeable batteries with care.