Math
Voltage Divider Calculator
Compute V_out, R1, R2, or V_in for a two-resistor divider. Get current, power per resistor, division ratio, and a loading warning when V_out is loaded.
Voltage divider calculator
V
Leave blank for an unloaded divider.
V_out
5 V
V_in
10 V
V_out
5 V
Division ratio
0.5
R1
10 kΩ
R2
10 kΩ
Current
0.5 mA
P (R1)
2.5 mW
P (R2)
2.5 mW
P (total)
5 mW
Frequently Asked Questions about the Voltage Divider Calculator
What is a voltage divider?
A voltage divider is two resistors in series that scale an input voltage down by the ratio R2 / (R1 + R2). V_out = V_in x R2 / (R1 + R2). It is one of the most common building blocks in analog electronics.
Why does the calculator warn about loading?
A voltage divider's output only equals V_in x R2 / (R1 + R2) when the load draws no current. As soon as you connect a real load (an ADC, an opamp input, a microcontroller pin), the output sags. The warning fires when the load resistance is within 10x of R2, where the error becomes significant.
How do I size R1 and R2?
Pick R1 + R2 so the divider draws acceptable current: V_in / (R1 + R2) is the standing current. For a 5 V divider with R1 + R2 = 10 kOhm, that is 0.5 mA, fine for most signal-conditioning. For battery-powered work, prefer 100 kOhm to 1 MOhm to save power, but watch loading.
Can I solve for R1 or R2 instead of V_out?
Yes. The calculator has four modes. Given V_in, V_out, and one resistor, it solves for the other. Given V_in and V_out and a target current, it can size the pair. That is useful when you have one resistor on hand and need the matching partner.
Why does power dissipation matter?
Each resistor dissipates I^2 x R as heat. A 5 V divider with two 100 Ohm resistors burns 0.125 W per resistor, near the limit of a standard 1/8 W part. The calculator shows per-resistor power so you can pick a wattage rating that does not smoke.