Voltage Divider Calculator

Calculate output voltage and resistor values for voltage divider circuits.

Tested tool guide Tested browser tools Checked August 16, 2026

What Voltage Divider Calculator does, with a checked example

A voltage divider is the two-resistor circuit that scales a supply voltage down to a chosen fraction: the output node sits between the resistors, and the ratio of the two resistances decides the voltage there. This tool computes that output from any resistor pair, and works the other way too, finding resistor values that produce a target voltage. The one thing most people get wrong: the formula holds only while nothing is connected to the output. Hook up a load and the voltage sags, because the load sits in parallel with the bottom resistor and quietly changes the ratio.

Worked example

A concrete input and expected output from the current implementation.

Input

Supply: 9 V; R1 = 1 kohm (supply to output); R2 = 2 kohm (output to ground)

Expected output

Vout = 6.00 V. Divider current = 3.00 mA.

Vout = 9 V x 2000/(1000 + 2000) = 9 x 2/3 = 6 V. The 3 mA figure is the current the divider draws continuously from the supply: 9 V divided by the 3 kohm total resistance.

How the result is produced

The divider rule

The output is set by the ratio of the two resistors: Vout = Vin x R2/(R1 + R2), where R1 runs from the supply to the output node and R2 runs from that node to ground. Only the ratio matters, so 10 kohm + 10 kohm halves the input exactly like 1 kohm + 1 kohm. The tool plugs in the entered values and applies this ratio. The divider current, Vin/(R1 + R2), flows continuously through both resistors.

Sizing for a target voltage

Working backwards, the tool starts from the desired output, solves the ratio R2/(R1 + R2) that yields it, and lands on real resistor values, which come in fixed steps called E-series values. The voltage actually achieved is the nearest realizable value, not the ideal target. The absolute size of the pair is a trade-off: small values hold the output under load but waste more current as heat; large values do the reverse.

Good uses

Scaling a sensor's 0-5 V output into a 3.3 V microcontroller ADC range so the full measurement fits the converter without clipping.
Setting a fixed reference point in an analog circuit, such as the midpoint bias for an op-amp stage or a comparator threshold that must stay at a specific fraction of the supply.
Checking a divider before building it, using the exact resistor values from the parts drawer, including non-ideal standard values, to see what voltage it will actually produce.

Limits and checks

The formula assumes an unloaded output. Any load across R2 behaves as a resistor in parallel with it, lowering the effective ratio, so the output sags; a multimeter's 10 Mohm input is usually harmless, but a real load current needs comparing with the divider's own current.
The divider itself is a weak source: its output impedance is R1 in parallel with R2, typically kilo-ohms, so it cannot supply meaningful current, and the divider current runs continuously, wasting power as heat in both resistors.
Resistors come in standard E-series values with tolerances, so a divider sized for a target voltage lands on the nearest available pair; the real output can differ from the ideal by the tolerance, typically 1% or 5%.

Common questions

Why is the voltage I measure lower than what the calculator says?

Almost always because something is loading the output. Any current drawn through the divider's output impedance, R1 in parallel with R2, drops voltage across it. Compare the load current with the divider's own current: if the load draws more than a few percent of it, buffer the output with an op-amp, or use smaller resistors and accept the extra wasted current.

Can I use a divider to power a circuit, like dropping 9 V to 5 V?

Not for anything that draws real current. A divider only divides, it cannot regulate: the output falls as load current rises, and the resistors burn the excess voltage as heat. It is right for signals and references feeding high-impedance inputs. To power a load, use a linear or switching regulator instead.

References and verification

The example and behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.

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