Voltage Divider Calculator

How it works

Two resistors in series across a supply share its voltage in proportion to their resistance. With R1 from the input to the output node and R2 from the output node to ground, the same current I = Vin ÷ (R1 + R2) flows through both, so the voltage across R2 is Vout = I × R2 = Vin × R2 ÷ (R1 + R2) and the rest, Vin × R1 ÷ (R1 + R2), appears across R1.

To hit a target output, the formula is turned around: R2 = R1 × Vout ÷ (Vin − Vout). The target must lie strictly between zero and the input voltage, because an unloaded divider can only scale a voltage down.

The power each resistor turns into heat is P = I² × R, which is what decides the resistor's wattage rating. Higher resistances waste less power but make the output more sensitive to whatever is connected to it.

Formula

I    = Vin ÷ (R1 + R2)
Vout = Vin × R2 ÷ (R1 + R2)
VR1  = Vin × R1 ÷ (R1 + R2)
R2   = R1 × Vout ÷ (Vin − Vout)     (for a target Vout)
P1 = I² × R1;   P2 = I² × R2

Example

A 12 V supply across R1 = 20 Ω and R2 = 40 Ω drives I = 12 ÷ 60 = 0.2 A, shown as 200 mA. The output is 12 × 40 ÷ 60 = 8 V, with 4 V across R1. R1 dissipates 0.2² × 20 = 0.8 W (800 mW) and R2 0.2² × 40 = 1.6 W.

Turned around, a target of 8 V from 12 V with R1 = 20 Ω needs R2 = 20 × 8 ÷ (12 − 8) = 40 Ω.

Assumptions and limitations

• The divider is unloaded: nothing draws current from the output. A load connected across R2 acts in parallel with it and pulls the output below the value shown, by an amount that grows as the load resistance approaches R2.
• Resistors are exact. Real resistors differ from their marked value by their tolerance, and the R2 found for a target is generally not a standard value; the nearest available value gives a slightly different output.
• The supply is ideal, with no internal resistance.
• A voltage divider is not a regulated supply: its output follows any change in the input voltage or the load. Results are estimates for learning and circuit sketching, not a substitute for the manufacturer's data sheet or a qualified engineer's design review.

Frequently asked questions