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Voltage Divider Calculator

Computes the output voltage of a two-resistor divider (Vout = Vin·R2/(R1+R2)), the current through the chain and the power in each resistor, or solves for R2 when a target output voltage is given.

When to use

You need the voltage at the junction of two series resistors, the R2 that produces a target voltage, or the power each resistor must handle.

Do not use when: The output is loaded by a low-impedance load (the load parallels R2 and lowers Vout), you need a regulated supply, or you are limiting LED current (use led-resistor).

Formula

output_voltage_v = input_voltage_v × r2_ohm / (r1_ohm + r2_ohm); r2_ohm = r1_ohm × target_output_voltage_v / (input_voltage_v − target_output_voltage_v); current = input_voltage_v / (r1_ohm + r2_ohm); power = current² × R

Unloaded divider. A load RL across R2 replaces R2 by R2 ∥ RL; keep the output impedance at least 10 × smaller than the load for less than 10 % error.

Inputs

Parameter	Type	Unit	Required	Description
`input_voltage_v`	number	V	yes	Voltage Vin applied across R1 + R2. Range: > 0, ≤ 100000
`r1_ohm`	number	Ω	yes	Resistor between Vin and the output node, in ohms (1 kΩ = 1000). Range: > 0
`r2_ohm`	number	Ω	no	Resistor between the output node and ground. Omit and give target_output_voltage_v to solve for it. Range: > 0
`target_output_voltage_v`	number	V	no	Desired Vout; must be below input_voltage_v. Give either this or r2_ohm. Range: > 0

Outputs

Output	Type	Unit	Description
`solved_for`	string		output_voltage_v when R2 was given, r2_ohm when the target voltage was given.
`output_voltage_v`	number	V	Vout = Vin × R2 / (R1 + R2) with no load.
`r2_ohm`	number	Ω	Bottom resistor (computed as R1 × Vout / (Vin − Vout) when solving).
`division_ratio`	number		Vout / Vin = R2 / (R1 + R2).
`current_ma`	number	mA	I = Vin / (R1 + R2), the quiescent current drawn from the source.
`power_r1_w`	number	W	I² × R1.
`power_r2_w`	number	W	I² × R2.
`total_power_w`	number	W	Vin × I.
`output_impedance_ohm`	number	Ω	Thévenin source resistance seen by a load: R1 ∥ R2 = R1 × R2 / (R1 + R2).

Example

12 V, R1 = 10 kΩ, R2 = 4.7 kΩ: {"input_voltage_v":12,"r1_ohm":10000,"r2_ohm":4700} → {"solved_for":"output_voltage_v","output_voltage_v":3.8367,"division_ratio":0.319728,"current_ma":0.8163,"power_r1_w":0.006664,"power_r2_w":0.003132,"total_power_w":0.009796,"output_impedance_ohm":3197.28}

9 V to 5 V with R1 = 1 kΩ: {"input_voltage_v":9,"r1_ohm":1000,"target_output_voltage_v":5} → {"solved_for":"r2_ohm","r2_ohm":1250,"output_voltage_v":5,"current_ma":4,"power_r1_w":0.016,"power_r2_w":0.02}

GET https://tttkmbb.com/api/v1/calculate/voltage-divider?input_voltage_v=12&r1_ohm=10000&r2_ohm=4700

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/voltage-divider (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/voltage-divider · Markdown: https://tttkmbb.com/engineering/voltage-divider.md · JSON definition: https://tttkmbb.com/engineering/voltage-divider.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="voltage-divider"
OpenAPI operationId: calculate_voltage_divider
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

Wikipedia – Voltage divider (reference)
HyperPhysics – Voltage divider (reference)

FAQ

Why does the voltage drop when I connect a load?

The load is in parallel with R2, lowering the effective bottom resistance. Use resistors small enough that the output impedance is far below the load resistance, or add a buffer.

Can I use a divider to power a circuit?

Only for microamp loads or reference voltages; it wastes power continuously and the voltage sags with load. Use a regulator for supplies.

Related calculators

Ohm's Law Calculator — Voltage, current and resistance for one resistor.
Resistors in Series and Parallel Calculator — Equivalent resistance of the load in parallel with R2.
LED Resistor Calculator — Series resistor for an LED instead of a divider.