Reverse Voltage Divider Calculator

The divider formula run backwards. Rather than telling you what two resistors do, this finds pairs that hit your target — restricted to values you can actually buy, because an ideal ratio you cannot source is useless.

Reverse Divider — pick R1 and R2 for a target Vout

The divider problem backwards: you know the voltage you need, this finds the standard resistor pairs that get you there. Only E12/E24 preferred values are searched, because those are the ones actually sold in a component kit.

Input voltage (V)

Wanted output (V)

Resistor series

Required ratio R1 : R2

Best pair — R1 (top, to Vin)

Best pair — R2 (bottom, to GND)

Actual output

Error

Divider current

R1	R2	Vout	Error	Current
180 kΩ	130 kΩ	5.0323 V	+0.65 %	38.7097 µA
51 kΩ	36 kΩ	4.9655 V	-0.69 %	137.931 µA
47 kΩ	33 kΩ	4.95 V	-1 %	150 µA
33 kΩ	24 kΩ	5.0526 V	+1.05 %	210.5263 µA
130 kΩ	91 kΩ	4.9412 V	-1.18 %	54.2986 µA
43 kΩ	30 kΩ	4.9315 V	-1.37 %	164.3836 µA

1.Given: Vin = 12 V, wanted Vout = 5 V
2.FormulaVout = Vin · R2/(R1+R2) → R1/R2 = (Vin − Vout)/VoutSubstituteR1/R2 = (12 − 5) ÷ 5ResultR1/R2 = 1.4
3.Closest E24 pair: R1 = 180 kΩ, R2 = 130 kΩ (ratio 1.3846)
4.FormulaCheck with the real valuesSubstituteVout = 12 × 130 kΩ ÷ (180 kΩ + 130 kΩ)ResultVout = 5.0323 V (+0.65 %)

Common trap: A divider is not a power supply. These values assume nothing is drawing current from the tap — connect a load and the output sags, because the load sits in parallel with R2. Keep the divider current at least ten times the load current, or use a regulator instead.

The formula

R₂ / (R₁ + R₂) = V_out / V_in

V_in: available supply voltage (volts)
V_out: voltage you want at the junction (volts)
R₁, R₂: the pair to find, chosen from the E12 or E24 series

Worked example

Get 3.3 V from a 5 V rail using E12 parts.

Required ratio: 3.3 / 5 = 0.66
Try R₁ = 10 kΩ, R₂ = 18 kΩ → 18 / 28 = 0.643
V_out = 5 × 0.643 = 3.21 V, an error of −2.7 %

10 kΩ / 18 kΩ gives 3.21 V — close enough for a sense line, not for a reference.

Where you'll use it

Level-shifting a 5 V signal into a 3.3 V input, or setting a feedback ratio. For anything needing accuracy, note that E12 resistors are ±10 % parts, so the tolerance dominates the ratio error you see here.

The laws behind it

Parts this applies to

Potentiometer / Rheostat / Trimmer Photoresistor (LDR)555 Timer IC Rectifier Diode Zener Diode Bridge Rectifier SCR (Thyristor)TRIAC

Related calculators

Voltage Divider CalculatorV_out = V_in × R₂ / (R₁ + R₂)Ohm's Law CalculatorV = I × R Zener Diode Regulator CalculatorR_s = (V_in − V_z) / (I_load + I_z(min))