Library / Tools / Physics equation calculator / Potential divider

Potential divider calculator

GCSEA-level Electricity

Two resistors in series share the supply voltage in proportion to their resistances.

V_out = V_in R₂ / (R₁ + R₂)

This free calculator is for GCSE and A-level physics. Choose the quantity you want to find, enter the other values with their units, and it shows the working in the layout used in exams, with the answer to the number of significant figures you choose. The values shown are an example. Change them to your own.

Quantity to find

V_outOutput voltage to find 6.00 V

V_inInput voltage

R₁Resistor from the supply

R₂Resistor the output is taken across

Significant figures

Working

Write the equation
V_out = V_in R₂ / (R₁ + R₂)
Substitute the values
V_out = 9 × 2000 ÷ (1000 + 2000)
Calculate
V_out = 6 V
Give the answer to 3 significant figures
V_out = 6.00 V

Worked example

Find Vout, the output voltage. Given: input voltage Vin = 9.00 V; resistor from the supply R1 = 1000 Ω; resistor the output is taken across R2 = 2000 Ω.

Write the equation
V_out = V_in R₂ / (R₁ + R₂)
Substitute the values
V_out = 9 × 2000 ÷ (1000 + 2000)
Calculate
V_out = 6 V
Give the answer to 3 significant figures
V_out = 6.00 V

Symbols and units

Symbol	Quantity	SI unit
V_out	Output voltage	V
V_in	Input voltage	V
R₁	Resistor from the supply	Ω
R₂	Resistor the output is taken across	Ω

Potential divider: rearranged

To find output voltage: V_out = V_in R₂ / (R₁ + R₂)
To find input voltage: V_in = V_out (R₁ + R₂) / R₂
To find resistor from the supply: R₁ = R₂ (V_in − V_out) / V_out
To find resistor the output is taken across: R₂ = V_out R₁ / (V_in − V_out)

Exam tip

The output is across R₂. A load connected across R₂ changes the result, so the formula assumes no load.

Common questions

What is the formula for potential divider?

The equation is Vout = Vin R2 / (R1 + R2), where Vout is the output voltage (V); Vin is the input voltage (V); R1 is the resistor from the supply (Ω); R2 is the resistor the output is taken across (Ω).

What units should I use for potential divider?

In SI units: output voltage in V, input voltage in V, resistor from the supply in Ω, resistor the output is taken across in Ω. The calculator also accepts common alternatives such as kilometres, grams or minutes, and converts them for you.

How do I rearrange Vout = Vin R2 / (R1 + R2)?

Rearranging gives these forms: to find output voltage: Vout = Vin R2 / (R1 + R2). to find input voltage: Vin = Vout (R1 + R2) / R2. to find resistor from the supply: R1 = R2 (Vin − Vout) / Vout. to find resistor the output is taken across: R2 = Vout R1 / (Vin − Vout).

Practise potential divider questions

Include unit conversions (km, g, minutes…)

Press the button for a question with random numbers. Work it out on paper first, then type your answer.

Read the full explanation on the Potential divider formula page. More electricity equations: Charge, current and time, Ohm’s law, Electrical power, Electrical power, Energy transferred, Resistors in series, Resistors in parallel, Resistivity. See the full list on the physics equation calculator page.