Voltage divider calculator

How does a voltage divider work?

Two resistors in series split the input voltage in proportion to their values: Vout = Vin × R2 ÷ (R1 + R2). 12 V with R1 = 1 kΩ and R2 = 2 kΩ gives 8 V across R2.

How much current flows?

I = Vin ÷ (R1 + R2): 12 V ÷ 3 kΩ = 4 mA in the example.

How much power do the resistors use?

P = I² × R for each: 16 mW in R1 and 32 mW in R2. Choose resistors rated for more than that.

Why does my measured voltage differ?

Anything connected to the output draws current and lowers the voltage. The formula assumes no load; keep the load resistance much larger than R2.

Can I use a divider to power a device?

Usually not — the output voltage changes with the load current. A voltage regulator is the usual choice; dividers suit sensing and reference voltages.

How do I choose R1 and R2 for a target output voltage?

Start from the ratio: R1 ÷ R2 = (Vin − Vout) ÷ Vout. For 5 V from 12 V that is 7 ÷ 5 = 1.4. Then pick R2 so the divider current, Vin ÷ (R1 + R2), is acceptable; with R2 = 10 kΩ, R1 should be 14 kΩ. That value is not a standard one, and the neighboring E24 values give 5.22 V with 13 kΩ and 4.80 V with 15 kΩ. Two resistors in series or parallel, or a trimmer, can get closer.

How much does a load change the output?

A load RL in parallel with R2 replaces R2 in the formula with R2 ‖ RL. A 100 kΩ and 100 kΩ divider on 10 V gives 5.00 V with no load, but a meter with a 1 MΩ input, such as the one in Analog Devices' ADALM1000 lab note, reads 4.76 V, 4.8% low. With 10 kΩ resistors the same meter reads 4.98 V.

Why the formula works and where it stops

The two resistors form a series circuit, so the same current I = Vin ÷ (R1 + R2) flows through both, and the voltage across R2 is that current times R2. Analog Devices' teaching note states that the divider equation applies only to series circuits, so the output should draw almost no current. Anything connected to the output, from a meter to an ADC input, is in parallel with R2 and pulls the voltage down. The divider's own output resistance is R1 and R2 in parallel, so smaller resistors are stiffer but waste more current.

Worked example

12 V across R1 = 1 kΩ and R2 = 2 kΩ gives 8 V across R2 and 4 mA through both, which is 16 mW in R1 and 32 mW in R2. If both resistors have a 1% tolerance, the worst combinations, R1 low and R2 high or the reverse, move the output to between 7.95 V and 8.05 V, about 0.7% either way. A divider of larger resistors draws less current but is loaded more easily: 100 kΩ and 100 kΩ on 10 V should give 5.00 V, yet a 1 MΩ meter reads 4.76 V.

Common mistakes

The first is expecting the unloaded value from a loaded divider. A load ten times R2, here 1 MΩ on 100 kΩ, still costs 4.8%, and one a hundred times R2 costs 0.5%. The second is reversing the resistors: R2 is the one the output is taken across, so swapping them gives the complement, Vin − Vout. A third is ignoring resistor power; the example's resistors dissipate only milliwatts, but a divider from a higher voltage or with small resistances can heat them.

Sources: Analog Devices Wiki, University Program, ADALM1000 activity “Voltage Dividers and Lab Equipment” (the divider equation, the series-only restriction, instrument input impedance loading the circuit); Vishay, “E-Series Values,” document 28372 (IEC 60063). The loaded-divider and tolerance figures follow from Ohm's law. Related: Ohm's law calculator, series and parallel calculator, LED resistor calculator, resistor color code calculator. How we check our calculators.