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Voltage Divider Calculator

Model a simple divider locally; connected loads change the lower-leg resistance, and this page is not a wiring or mains-design tool.

Input voltage

R1 (Ω)

R2 (Ω)

Load (Ω, blank = none)

—Output voltage across R2 / load.

—Vout / Vin

—Divider current

—Effective lower R

—Unloaded source R

A connected load changes the lower-leg resistance. For an unloaded divider, leave Load blank.

Educational / low-voltage electronics calculator. Real components have tolerances and non-ideal behavior. Do not use this page as a substitute for qualified design or safety guidance for mains/high-energy electrical systems.

Electrical model check

Verify the circuit relation, units, and result

loaded or unloaded two-resistor divider

Vout = Vin·R2eff/(R1+R2eff)

Reading the current calculator inputs…

Load resistance changes the effective lower leg and therefore the output ratio.

Low-voltage educational scope. This review does not provide mains wiring, live-work, conductor-sizing, protection-device, or safety-certification instructions.

Loaded-divider & Thevenin audit

Expose source impedance, load-induced error and resistor power instead of treating the ideal divider equation as the whole circuit.

Loading evidence

Target Vout (optional)

Loading warning threshold (%)

Power display

Use the calculator above, then run this verification.

Verification uses the same visible inputs plus the assumptions shown here.

Input uncertainty & sensitivity envelope audit

Propagate editable input/component uncertainty through the current low-voltage calculation, then rank which input most changes the primary result. The review threshold is a user-selected triage signal, not a component, wiring, protection or safety certification.

Vin uncertainty (%)R1 / R2 tolerance (%)Load uncertainty (%)Review if result span exceeds (%)

Envelope layer	Min / base / max	Evidence

Change the calculator inputs above, then refresh this uncertainty audit.

Use component datasheets, measured values and qualified design review where safety or compliance matters.

Electrical uncertainty cluster

Calculate the nominal circuit, then expose what input spread can change

Nominal equations are only the center point. Use component tolerances and measurement uncertainty to inspect a min/base/max operating envelope, then verify real component ratings and datasheets separately.

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Match the calculator to the circuit model

Ohm’s law, dividers, RC/RL timing, reactance and battery estimates answer different questions. Keep units and assumptions visible instead of combining unrelated quantities.

Low-voltage educational scope

WebToolArc does not certify wiring, component safety, mains installations or high-energy systems. Use component datasheets and qualified guidance where safety matters.

Governing electrical relation

Vout = Vin·R2eff/(R1+R2eff) — loaded or unloaded two-resistor divider.

How to cross-check the result

For no load, verify Vout/Vin=R2/(R1+R2). With a load, first replace R2 by R2∥Rload and recalculate the ratio.

Real-component boundary

Load resistance changes the effective lower leg and therefore the output ratio. Component tolerance, temperature, parasitics, datasheet limits, installation rules, and hazardous-energy safety are not inferred by this idealized calculator.

Practical guide and verification

Use the product workflow above first. These notes add interpretation, verification and limits without displacing the inputs, results or visual review.

Start with the unloaded divider relation

For two series resistors, the ideal unloaded output is Vin × R2/(R1+R2) when the output is measured across R2. Swapping R1 and R2 changes the output ratio, so label which resistor is connected to the source and which is connected to ground.

A real load changes the effective lower resistance

When a load is connected from the output node to ground, it sits in parallel with R2. Recalculate with R2∥Rload rather than assuming the unloaded ratio still applies. Low load resistance can pull the output far below the nominal value.

Thevenin form makes loading easier to reason about

The divider can be reduced to an ideal Thevenin voltage and a Thevenin resistance. Comparing Rload with that source resistance quickly shows whether the load is large enough to leave the output nearly unchanged or small enough to cause material droop.

Check resistor power as well as voltage

The divider current and resistor voltage drops determine power dissipation. A numerically correct output voltage is not enough if a resistor’s power rating, tolerance, temperature coefficient or maximum working voltage is exceeded in the real circuit.

Measurement equipment can become the load

A meter, ADC input or sensor stage has finite input impedance. Include that impedance when it is not much larger than the divider resistance. For safety-critical or mains-connected circuits, use appropriate engineering design practices rather than a browser estimate alone.