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Voltage Divider Calculator

Two resistors in series split the supply voltage in proportion to their values. It is the simplest way to scale a voltage down, and the easiest circuit to misuse: a divider is a reference, not a power supply.

Voltage Divider — Vₒᵤₜ = Vᵢₙ × R₂ / (R₁ + R₂)

Compute output voltage, divider ratio, current draw and per-resistor power.

Output voltage
Ratio (Vout/Vin)
Current draw
R1 power
R2 power
  1. 1.Given: Vᵢₙ = 5 V, R₁ = 10 kΩ, R₂ = 10 kΩ
  2. 2.FormulaR_total = R₁ + R₂SubstituteR_total = 10000 + 10000ResultR_total = 20 kΩ
  3. 3.FormulaVₒᵤₜ = Vᵢₙ × R₂ ÷ (R₁ + R₂)SubstituteVₒᵤₜ = 5 × 10000 ÷ 20000ResultVₒᵤₜ = 2.5 V
  4. 4.FormulaI = Vᵢₙ ÷ (R₁ + R₂)SubstituteI = 5 ÷ 20000ResultI = 250 µA
  5. 5.FormulaP = I² × R (each resistor)SubstituteP(R₁) = 250e-6² × 10000, P(R₂) = 250e-6² × 10000ResultP(R₁) = 625 µW, P(R₂) = 625 µW
Common trap: A divider only holds its ratio while the load draws far less current than the divider itself. Rule of thumb: divider current ≥ 10× the load current — and never power a real load from a divider; use a regulator.

The formula

V_out = V_in × R₂ / (R₁ + R₂)

V_in
input voltage across the pair (volts)
R₁
top resistor, input to output (ohms)
R₂
bottom resistor, output to ground (ohms)
V_out
voltage at the junction (volts)

Worked example

5 V in, R₁ = 10 kΩ, R₂ = 4.7 kΩ.

  1. V_out = 5 × 4700 / (10000 + 4700)
  2. V_out = 5 × 4700 / 14700 = 5 × 0.3197
  3. Current through the chain: I = 5 / 14700 = 340 µA

V_out = 1.60 V, drawing 340 µA from the supply.

Where you'll use it

Scaling a signal into an ADC's range, setting a reference, or reading a sensor. Remember the output sags as soon as you draw current from it — anything more than a tenth of the chain current needs a buffer.

The laws behind it

Parts this applies to

Potentiometer / Rheostat / TrimmerPhotoresistor (LDR)555 Timer ICRectifier DiodeZener DiodeBridge RectifierSCR (Thyristor)TRIAC

Related calculators

Reverse Voltage Divider CalculatorR₂ / (R₁ + R₂) = V_out / V_inOhm's Law CalculatorV = I × RSeries and Parallel Resistance CalculatorSeries: R = R₁ + R₂ + …Op-Amp Gain CalculatorInverting: A_v = −R_f / R_in