How to use it
- Choose What to find: Output voltage when you know both resistors, or R2 for a target when you need a particular voltage.
- Type the Input voltage. In output mode, type Resistor R1 and Resistor R2; in target mode, type the Target output voltage and, if you have it, Resistor R1.
- Add a Load resistance only if something draws current from the output. Read Output voltage and Fall in output caused by the load.
- In target mode, use Nearest E24 value of R2 or E24 pair for this ratio to choose standard resistors you can buy.
How the output is calculated
Two resistors in series across a voltage share it in proportion to their resistances. The output is taken across the lower resistor R2, so it is the input times R2 divided by the total resistance. This holds only while nothing else draws current from the output.
Vout = Vin × R2 ÷ (R1 + R2)
with a load RL across R2:
R2 ∥ RL = R2 × RL ÷ (R2 + RL)
Vout = Vin × (R2 ∥ RL) ÷ (R1 + R2 ∥ RL)
for a target output:
R2 = R1 × Vout ÷ (Vin − Vout)- The output is never higher than the input, and it falls to zero when R2 is zero.
- Every load lowers the output, because it puts another path in parallel with R2. The lower the load resistance, the bigger the fall.
- The E24 values are the standard steps of the E24 series, 24 values per decade (IEC 60063, as listed on Wikipedia). The nearest one is chosen on a logarithmic scale.
Choosing the resistors
Keep the divider current well above the current the load takes, otherwise the load changes the output. Smaller resistors waste more power but are less sensitive to a load. The calculator shows the current drawn from the input, so you can judge this for your case.
A divider only sets a fixed fraction of the input. It does not regulate the output: if the input voltage changes, the output changes in the same proportion.
What this calculator does not cover
- Resistor tolerance and temperature drift, which move the output a little. Use the tolerance of your parts to estimate the spread.
- Power in each resistor. Work it out with the Ohm's law calculator on this site before you choose the wattage.
- Reactive loads and signals with a frequency: a capacitor in the load changes the division.
Worked example: 12 V through 10 kΩ and 10 kΩ
With R1 = 10 kΩ and R2 = 10 kΩ on 12 V, Vout = 12 × 10 k ÷ (10 k + 10 k) = 6 V, a ratio of 50 %. The current from the input is 12 V ÷ 20 kΩ = 600 µA.
Add a load of 10 kΩ across R2. The parallel value is R2 ∥ RL = 10 k × 10 k ÷ (10 k + 10 k) = 5 kΩ, so Vout = 12 × 5 k ÷ (10 k + 5 k) = 4 V. The load pulls the output down by 33.33 %, from 6 V to 4 V.
For a 3.3 V output from 12 V with R1 = 10 kΩ: R2 = 10 k × 3.3 ÷ (12 − 3.3) = 3.793 kΩ. The nearest E24 value is 3.9 kΩ, which gives 3.367 V.
Frequently asked questions
What is a voltage divider used for?
It makes a lower voltage from a higher one, for example to bring a 5 V signal down to the 3.3 V a sensor accepts or to read a battery voltage with a microcontroller pin. It sets a fixed fraction, not a regulated voltage.
Why does my output fall when I connect a load?
The load is connected in parallel with R2, which lowers the resistance that R2 presents to the divider. The output follows R2 ∥ RL, so a smaller load resistance pulls the voltage down more.
What is the E24 series?
E24 is a set of 24 standard resistor values per decade, such as 4.7 kΩ and 5.1 kΩ. Commercial resistors come in these steps, so a calculated value is rounded to the nearest one.
Which resistor should be R1 and which R2?
R1 is the resistor on the input side and R2 the one on the 0 V side; the output is taken across R2. Swapping them gives the complementary fraction, so check the drawing before you solder.
Can a voltage divider power a circuit?
It can supply only very small currents without the output changing. Use it to set a level for an input that draws almost nothing, such as an ADC pin; for a real supply use a regulator.
How much current does the divider waste?
The current is the input voltage divided by R1 plus R2, and it flows all the time. The calculator shows it, so you can compare it with what your circuit needs.
Sources
- Wikipedia: Voltage divider (Vout = R2/(R1+R2) × Vin)
- Wikipedia: E series of preferred numbers (E24 values, IEC 60063)
- Wikipedia: Series and parallel circuits (parallel resistance)
Figures and text last checked on Tuesday, November 10, 2026.