Voltage Divider Calculator

Vout = Vin · R2 / (R1 + R2). Solve for any one of Vin/R1/R2/Vout given the other three.

Inputs

When > 0, R2 is paralleled with the load. Real-world voltage divider behavior — most divider failures come from forgetting this.

Result

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General calculation reads

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How to use this calculator

  • Pick the unknown — V_out is the most common; R1/R2 modes solve for a needed resistor value.
  • Enter V_in (supply voltage), R1 (top resistor), R2 (bottom resistor).
  • If your divider feeds a real load (microcontroller pin, op-amp input, etc.), enter the load impedance to see the sagged V_out.

About this calculator

A voltage divider is two resistors in series with the output taken at the midpoint. V_out = V_in · R2 / (R1 + R2) — the unloaded transfer function. The classic failure: forgetting that any LOAD draws current through R2 in parallel, reducing the effective R2 and dropping V_out below the no-load prediction. Rule of thumb: the divider current (V_in / (R1+R2)) should be 10-100× larger than the load current, or use a buffer (op-amp follower). For voltage references requiring precision, use a Zener diode or dedicated reference IC instead — voltage dividers have no inherent regulation and shift with V_in. The tool models both ideal (no-load) and loaded cases; toggle loadR > 0 to see the sag.

Frequently asked

Why does the loaded V_out differ from the formula?+
The load resistance shunts R2 in parallel, reducing the effective bottom-leg resistance. Loaded V_out = V_in · (R2 || R_load) / (R1 + R2 || R_load), always LESS than the no-load value.
What resistor values should I use?+
Trade-off: small values = better load tolerance but high quiescent current (battery drain); large values = low quiescent current but more loading sensitivity. For 5 V → 3.3 V level shift powering a high-impedance MCU pin: R1=R2=10 kΩ is fine.
Can I use a divider as a voltage REGULATOR?+
No — voltage dividers have no regulation. V_out scales with V_in and sags with load. For regulation use a linear regulator (LM7805 etc.) or switch-mode buck converter.
Source?+
Horowitz & Hill "The Art of Electronics" 3rd ed Ch. 1.2; National Semiconductor / TI app notes on voltage-divider design.

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