Resistive Voltage Divider Calculator

Unloaded resistive voltage divider: Vout = Vin × R2 / (R1 + R2). Also shows divider ratio and quiescent current through the resistor chain — critical for low-power designs.

Resistive Voltage Divider

ParameterValue
Vout--
Divider Ratio--
Quiescent Current--

Formula & Theory

  • Output voltage: Vout = Vin × R2 / (R1 + R2)
  • Divider ratio: k = R2 / (R1 + R2) = Vout / Vin
  • Quiescent current: Iq = Vin / (R1 + R2)

When a load RL is connected in parallel with R2: R2_eff = R2 ∥ RL = R2 × RL / (R2 + RL). Substitute into Vout formula for loaded result.

Worked Example

Scale 12 V to 3.3 V for an ADC input:

  • Required ratio: 3.3/12 = 0.275
  • Choose R2 = 10 kΩ, solve R1: R1 = R2 × (1/0.275 − 1) = 10 × 2.636 = 26.36 kΩ → use 27 kΩ (nearest E24)
  • Vout = 12 × 10 / (27 + 10) = 3.24 V (−1.8% error — acceptable for most ADC ranges)
  • Iq = 12 / 37,000 = 324 µA → 3.9 mW quiescent

Assumptions & Limitations

  • Unloaded — no current drawn by the circuit connected to Vout
  • Ideal resistors — no temperature coefficient or tolerance considered
  • DC analysis only — for AC signals, capacitive loading at Vout changes frequency response

Common Mistakes

  • Ignoring load current: If your load draws current comparable to Iq, Vout will be significantly lower than calculated. Set Iq at least 10× your load current.
  • Using 5% resistors for precision references: 1% or better resistors are needed when output accuracy matters. Two 5% resistors can combine worst-case to >7% error.
  • No bypass capacitor on Vout: For fast-switching loads (ADC input, MOSFET gate via divider), a small capacitor (1–10 nF) on Vout stabilizes the output during transients.

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Frequently Asked Questions

Does this calculator account for load current?

No — this calculator assumes an unloaded divider. When a load is connected, it appears in parallel with R2 and reduces Vout. For load current IL much smaller than the quiescent current through R1+R2, the error is small. If IL is comparable to Iq, recalculate with R2 replaced by R2 ∥ Rload.

How do I choose R1 and R2 for a feedback network?

For a switching regulator feedback pin, the IC datasheet typically specifies a reference voltage (e.g. 0.8 V) and a recommended R2 value (e.g. 100 kΩ). Then R1 = R2 × (Vout/Vref − 1). Use 1% tolerance resistors to maintain output accuracy.

What resistor values should I use to minimize power waste?

Higher R values reduce quiescent current but increase susceptibility to noise and layout parasitics. A common rule: set quiescent current 10–100× above the feedback bias current of the IC. For a 1 µA bias current, 10–100 µA through the divider (R1+R2 = 50–500 kΩ) is reasonable.