Voltage divider calculator
Vout = Vin × R2 / (R1 + R2). Enter any Vin, R1 and R2 — results update instantly. Switch to reverse mode to find the resistor ratio for a target output voltage.
Frequently asked questions
How do you calculate the output voltage of a resistor divider?
The calculator uses Vout = Vin × R2 ÷ (R1 + R2). With its defaults of Vin = 5 V and R1 = R2 = 1 kΩ the output is 2.5 V, and it also reports the divider current I = Vin ÷ (R1 + R2) = 2.5 mA and the percentage of Vin dropped across R2.
How do I find the resistor ratio for a target output voltage?
Switch to reverse mode and enter the target Vout with one resistor fixed. The tool solves the ratio R1 ÷ R2 = (Vin − Vout) ÷ Vout and then the absolute R1 = (Vin − Vout) × R2 ÷ Vout. With Vin = 5 V and a target of 2.5 V the ratio is 1:1, so equal resistors give 2.5 V.
How much current does a voltage divider draw from the source?
The divider loads the source with R1 + R2, so I = Vin ÷ (R1 + R2). At the defaults of 5 V and 1 kΩ + 1 kΩ that is 2.5 mA. Keep the divider current well above the load you are feeding, otherwise the load pulls the output below the unloaded value.
Does the calculator account for the load on the divider?
No — it assumes an ideal resistive divider with negligible load. If you need to see how a real load sags the output, use the loaded voltage divider in the resistor network calculator, which models the load in parallel with R2 and reports the sag percentage.
Results assume an ideal resistive divider with negligible load. ICBOMS provides this tool for reference only; verify designs against datasheets before production.