Voltage Divider Calculator

Calculates the output voltage of a two-resistor divider (R1 and R2), the current and the power in each resistor, and finds standard resistor pairs for a target voltage.

How to use

1. Fill in three of "Input voltage (Vin)", "Top resistor (R1)", "Bottom resistor (R2)" and "Output voltage (Vout)"; the empty one is calculated. Values such as 10k or 4k7 are accepted.
2. If something is connected to Vout, enter "Load resistance (RL)" to see the real output voltage.
3. Under "Find standard resistors for a target voltage", set the "Target Vout", the "Standard series" and the total resistance range. Pairs are listed by error.
4. Press "Use" next to a pair to copy it into the calculation above.

How it works

• Vout = Vin × R2 ÷ (R1 + R2), and the current is Vin ÷ (R1 + R2). With a load, R2 is replaced by R2 ‖ RL.
• A warning appears when the load lowers the output by more than 5%. A load at least 20 times the output resistance (R1 ‖ R2) keeps the drop below 5%.
• A divider suits signals and reference voltages that draw almost no current. It is not a power supply; use a voltage regulator for that.
• For learning and low-voltage electronics, not for mains wiring.

Questions and answers

What is the voltage divider formula?

Vout = Vin × R2 ÷ (R1 + R2), taken across R2 on the ground side. With 5 V, R1 = 10 kΩ and R2 = 20 kΩ, Vout is about 3.33 V.

Which resistors divide 5V down to 3.3V?

Any pair where R2 ÷ (R1 + R2) is 0.66. Enter 3.3 as the "Target Vout" to see standard pairs close to this ratio, each with its error.

Why does the output voltage drop when a load is connected?

The load is in parallel with R2, so the bottom part of the divider becomes smaller. Enter "Load resistance (RL)" to see the new voltage and the change.

Can I calculate R1 or R2 from the voltages?

Yes. Enter the voltages and one resistor, and leave the other empty. It is marked "calculated" in the results.