How to use the Voltage Divider Calculator
- 1. Choose what to solveOutput voltage, or the bottom resistor R2 for a target Vout.
- 2. Enter Vin and resistorsUse k, M and other SI prefixes. Add RL if a load is connected.
- 3. Get the resultSee Vout, current, power and the nearest standard resistor.
Voltage divider formula
Two resistors in series share the input voltage in proportion to their values: Vout = Vin × R2 / (R1 + R2), where Vout is taken across R2. The same current I = Vin / (R1 + R2) flows through both resistors.
Worked example: Vin = 12 V, R1 = 10 kΩ, R2 = 4.7 kΩ. Vout = 12 × 4.7 / 14.7 = 3.837 V and I = 12 / 14.7 kΩ = 0.816 mA. Now connect a 10 kΩ load across R2. R2 ∥ RL = 4.7 × 10 / 14.7 = 3.197 kΩ, so Vout drops to 12 × 3.197 / 13.197 = 2.907 V. This is why a divider is good for reference and sensing voltages but poor as a power supply.
Design rules and exam questions
- Loading: keep the load at least 10 times R2 so Vout changes by less than about 10%.
- Finding R2: R2 = R1 × Vout / (Vin − Vout).
- Common use: scaling a 12 V signal down to a microcontroller's 3.3 V or 5 V ADC input.
Exam questions often ask: find Vout with and without load, find R2 for a given output, or find the power rating needed for each resistor (P = I²R). The tool also rounds R2 to the nearest E24 value and shows the output you would really get.
Frequently asked questions
Why does the output drop when I connect a load?
The load is in parallel with R2, so the bottom part of the divider becomes smaller than R2. A smaller bottom resistance gives a lower share of Vin.
Can I use a voltage divider as a power supply?
Only for tiny currents. The output changes with load current, and a lot of power is wasted in R1. Use a voltage regulator for real loads.
What resistor values should I choose?
For sensing and ADC inputs, values from 1 kΩ to 100 kΩ are common. Lower values waste more current; very high values pick up noise and are affected by input leakage.
Is this tool free?
Yes. You enter your name and WhatsApp number once to see your result, and all other Uncodemy tools then open without the form.