Voltage Divider Calculator
Two resistors in series, output tapped between them. Change any value and watch Vout move on the diagram.
How the divider divides
The same current flows through both resistors, so the input voltage splits across them in proportion to their values. Vout is the share that lands on R2: Vout = Vin × R2 ÷ (R1 + R2). Equal resistors halve the voltage. A 10k over a 4.7k turns 12 V into 3.83 V.
The catch nobody mentions
A divider only holds its ratio while almost nothing is drawn from the tap. Connect a load comparable to R2 and it sits in parallel with R2, dragging Vout down. The rule of thumb: whatever reads the divider should have at least ten times the impedance of R2. That's fine for a microcontroller ADC pin, and hopeless for powering anything. A divider is a reference, not a power supply.
Resistor choice is a trade: small values hold the ratio stiffly but burn current all day; big values save power but get noisy and slow. For sensor work, tens of kilohms is the usual sweet spot.