Voltage Divider Calculator, With and Without a Load

Enter the input voltage and both resistors to get the output, add a load to see how far it pulls the output down, or solve for R2.

Calculator

Output Vout
3.837 V
Current from the source
0.8163 mA
Power in R1
6.664 mW

Type numbers like 4.7, 0.002 or 2e-3. Results are rounded to 4 significant figures.

The formulas

Vout=Vin R2R1+R2V_{out} = V_{in}\,\dfrac{R_2}{R_1 + R_2}

Unloaded: the same current flows through both resistors, so the voltage splits in proportion to resistance.

Vout=Vin R2∥RLR1+R2∥RLV_{out} = V_{in}\,\dfrac{R_2 \parallel R_L}{R_1 + R_2 \parallel R_L}

Loaded: the load RL sits in parallel with R2, which lowers the output. R2 ∥ RL = R2 RL / (R2 + RL).

R2=R1 VoutVin−VoutR_2 = R_1\,\dfrac{V_{out}}{V_{in} - V_{out}}

Solving the unloaded form for R2.

  • A divider only holds its voltage when the load resistance is much larger than R2, about ten times or more.
  • It is not a power supply: use a regulator to power anything that draws real current.
  • The Thevenin resistance seen from the output is R1 ∥ R2.

Worked example: 12 V, R1 = 10 kΩ, R2 = 4.7 kΩ

  1. Unloaded: Vout = 12 V × 4.7 / (10 + 4.7) = 12 × 0.3197 = 3.837 V.
  2. Current through both resistors: 12 V / 14.7 kΩ = 0.8163 mA.
  3. Add a 10 kΩ load: R2 ∥ RL = 4.7 × 10 / 14.7 = 3.197 kΩ.
  4. Loaded: Vout = 12 V × 3.197 / (10 + 3.197) = 2.907 V, almost a volt lower, because the load is only about twice R2.
  5. For 3.3 V from 5 V with R1 = 10 kΩ: R2 = 10 kΩ × 3.3 / 1.7 = 19.41 kΩ.

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