Voltage Divider Calculator
Calculate divider output from Vin, R1 and R2, or solve R2 for a target voltage, with load resistance, tolerance worst case and best E24/E96 resistor pairs.
The interactive tool loads here when JavaScript runs.
About this toolWhat is the Voltage Divider Calculator?
Two resistors in series across a voltage split it in proportion to their values. This voltage divider calculator gives the output voltage from Vin, R1 (the top resistor) and R2 (the bottom resistor), or turns the problem around and finds the R2 you need for a target output with the R1 you already have.
Real dividers feed something, so you can add a load resistance across the output and see how far it pulls the voltage down from the ideal figure. The result panel also shows the current through the divider, the power in each resistor, the output (Thevenin) resistance and the worst-case output range when both resistors sit at opposite ends of a 0.1%, 1% or 5% tolerance.
Below the result, a table lists the standard E12, E24 or E96 resistor pairs that land closest to the target, with their error, so you can pick parts from the drawer instead of a calculated value like 19.41 kΩ. One click copies a pair back into the calculator.
- Find Vout, or solve R2 for a target voltage
- Optional load resistance with the error it causes
- Worst-case output at 0.1%, 1% or 5% tolerance
- Best standard E12, E24 and E96 resistor pairs, ranked by error
- Current, resistor power and Thevenin output resistance
- Schematic that updates with your values, and a shareable link
How to use the Voltage Divider Calculator
- Choose Find Vout to analyse a divider, or Find R2 to design one for a target output.
- Enter Vin, and R1 with its unit. In Find Vout mode also enter R2; in Find R2 mode enter the target output.
- If the output drives something, choose Resistive load and enter the input resistance of that circuit.
- Set the resistor tolerance to see the worst-case output range, and choose E12, E24 or E96 for the standard-pair table.
- Read Vout or the required R2, the current, the resistor powers and the output resistance. Click Use on any standard pair to analyse it.
Worked examples
12 V down to about 3.8 V
Input
Vin 12 V, R1 10 kΩ, R2 4.7 kΩ, no load
Output
Vout 3.8367 V, 816.3 µA, output resistance 3.197 kΩ
The ratio is 4.7 ÷ 14.7 = 0.3197. The divider draws under 1 mA and each resistor dissipates a few milliwatts.
The same divider with a 100 kΩ load
Input
Vin 12 V, R1 10 kΩ, R2 4.7 kΩ, RL 100 kΩ
Output
Vout 3.7179 V, 3.10% below the unloaded 3.8367 V
R2 in parallel with 100 kΩ is 4.489 kΩ. Even a load about thirty times the output resistance moves the voltage by 3%, which matters for a reference or an ADC reading.
Level-shifting 5 V logic to 3.3 V
Input
Find R2: Vin 5 V, target 3.3 V, R1 10 kΩ, E24
Output
R2 = 19.412 kΩ; best E24 pair 4.7 kΩ / 9.1 kΩ gives 3.2971 V (−0.09%)
With 10 kΩ on top the ideal bottom resistor is not a stock value. 10 kΩ over 20 kΩ gives 3.333 V; 4.7 kΩ and 9.1 kΩ are closer. For fast signals use lower values or a proper level shifter, since the divider and pin capacitance form a low-pass filter.
Battery monitor on a 24 V system
Input
Find R2: Vin 24 V, target 3.3 V, R1 10 kΩ, E24
Output
Best pairs 4.7 kΩ / 750 Ω (+0.08%) and 27 kΩ / 4.3 kΩ (−0.09%)
The second pair draws about 0.77 mA instead of 4.4 mA, and its 3.7 kΩ output resistance suits most ADC inputs. Choose the target a little below the ADC full scale so a charging battery does not overrange it.
Background and insight
The same current flows through R1 and R2 when nothing else is connected, so I = Vin ÷ (R1 + R2) and the output across R2 is Vout = Vin × R2 ÷ (R1 + R2). Solving that for R2 gives R2 = R1 × Vout ÷ (Vin − Vout). The ratio is all that matters for the voltage; the absolute values set the current, the power wasted and how stiff the output is.
Seen from the output, a divider looks like a voltage source of Vin × R2 ÷ (R1 + R2) with a series resistance of R1 ∥ R2 (Thevenin's theorem). Any load resistance RL forms a second divider with that resistance, which is why connecting a load lowers the output. A load ten times larger than R1 ∥ R2 costs roughly 9% of the output; one hundred times larger costs about 1%. The calculator models the load as a resistor in parallel with R2: R2' = R2 × RL ÷ (R2 + RL).
Choosing the values is a trade-off. Low resistances waste current (a 1 kΩ + 1 kΩ divider on 12 V draws 6 mA continuously, a real drain on a battery) but give a stiff output that is little affected by the load or by noise. High resistances in the hundreds of kilohms save power but are easily disturbed by the input current of an ADC or op-amp and pick up noise. For ADC inputs on microcontrollers, keeping R1 ∥ R2 below the source impedance the datasheet recommends (often around 10 kΩ) avoids sampling errors.
Common use cases
Reading a battery or supply voltage with an ADC
Scale 12 V, 24 V or 48 V down to the 3.3 V or 5 V range of a microcontroller input.
Setting a regulator output
Adjustable regulators such as the LM317 or a buck converter use a divider on the feedback pin to set the output voltage.
Bias points and references
Set a transistor base bias or a comparator threshold, and check how much the next stage loads it.
Slow logic level shifting
Drop a 5 V signal to 3.3 V for an input pin when speed is low and the signal only goes one way.
Tips and limitations
- A divider is not a power supply. The output voltage collapses as soon as a load draws real current; use a regulator to power anything.
- Keep the load at least 10 times R1 ∥ R2, and 100 times for about 1% accuracy, or buffer the output with an op-amp follower.
- For an ADC input, add a small capacitor (10 to 100 nF) from the output to ground to steady the reading, and check the ADC's maximum source impedance.
- Watch power in the top resistor on high voltages: on a 400 V DC bus even 1 MΩ dissipates 0.16 W, and the voltage rating of the resistor body matters too. Split R1 into several resistors in series for high voltages.
- Tolerances add up. Two 5% resistors can move the output by nearly 5% in the worst case; use 1% or 0.1% parts when the ratio matters.
Frequently asked questions
What is the voltage divider formula?
Vout = Vin × R2 ÷ (R1 + R2), where R1 connects to the input and R2 to ground, and the output is taken across R2. Rearranged, R2 = R1 × Vout ÷ (Vin − Vout) and R1 = R2 × (Vin − Vout) ÷ Vout.
Why is my voltage divider output lower than calculated?
Something connected to the output is loading it. That load acts as a resistor in parallel with R2, lowering the effective bottom resistance. Even a multimeter with 10 MΩ input resistance shifts a divider made of megohm resistors. Enable the load option to model it.
How do I choose resistor values for a voltage divider?
Pick the ratio from the formula, then choose the scale. Values from about 1 kΩ to 100 kΩ suit most circuits: low enough that the load and noise matter little, high enough that the divider does not waste much current. Then pick a standard pair from the table.
Can I use a voltage divider to power a device?
No. The output voltage depends on the current drawn, so it changes as soon as the device draws current, and most of the power is wasted as heat. Use a linear regulator or a buck converter to power a load, and keep dividers for signals and reference voltages.
What is the output impedance of a voltage divider?
R1 and R2 in parallel, R1 × R2 ÷ (R1 + R2). It is the resistance a load sees looking back into the output, and it decides how much the load pulls the voltage down.
How do I step 5 V down to 3.3 V with resistors?
Use a ratio of 3.3 ÷ 5 = 0.66. 10 kΩ on top with 20 kΩ to ground gives 3.333 V, and 4.7 kΩ with 9.1 kΩ gives 3.297 V. This works for slow, one-way signals; for fast or bidirectional buses such as I²C use a level-shifter chip.
How much current does a voltage divider draw?
Vin ÷ (R1 + R2) with no load. A 10 kΩ and 4.7 kΩ divider on 12 V draws 0.82 mA all the time, which adds up in battery-powered equipment over months.
Is there an in-depth guide to the Voltage Divider Calculator?
Yes. “Voltage Divider Design: Ratios, Loading and Picking Real Resistors” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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