Ohmsums

Voltage divider calculator

Output voltage of two resistors in series, what a load does to it, and the R2 to buy for a wanted output. Results update as you type.

Indicative calculation; verify before use.

Divider

V
Ω
Ω
Ω

Resistors accept 4k7, 2R2, 1M or plain ohms.

Output

Vout without load
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With load
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Ratio R2 / (R1 + R2)
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Divider current
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Power in R1
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Power in R2
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Output resistance R1 ∥ R2
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Find R2 for a wanted output

V

Exact R2
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NearestValueGives VoutError
E12–
E24–

How it works

V_out = V_in × R2 / (R1 + R2) I = V_in / (R1 + R2), P = I² × R R2 = R1 × V_out / (V_in − V_out) Load RL across R2: R2' = R2 × RL / (R2 + RL)

The same current flows through both resistors, so the voltage splits in proportion to the resistances. A load across R2 is in parallel with it, which lowers the effective bottom resistance and pulls the output down. The output behaves like a source with internal resistance R1 ∥ R2: keep the load much larger than that (100× gives about 1 % drop), or buffer the output.

A divider is for setting a reference or reading a voltage with an ADC, not for powering a circuit: the output changes with the load current. Use a regulator for that. Resistor tolerance (±1 % or ±5 %) adds to the error shown.

Worked example: 12 V in, R1 = 10 kΩ, R2 = 4.7 kΩ. V_out = 12 × 4.7 / (10 + 4.7) = 3.837 V, and I = 12 V / 14.7 kΩ = 0.816 mA. A 100 kΩ load across R2 gives R2' = 4.7 × 100 / (4.7 + 100) = 4.489 kΩ, so V_out drops to 12 × 4.489 / (10 + 4.489) = 3.718 V, 3.1 % lower.

Sources (checked 2026-10-31):