Voltage Divider Calculator

Calculation goal:

Choose the fastest path for your current bench question.

Analyze R1/R2 Solve R1 Solve R2

Input voltage:

Set the voltage across the full divider.

Target output voltage:

Enter the desired loaded output at the divider node.

R1 top resistor:

Top resistor value in the selected unit.

R2 bottom resistor:

Bottom resistor value in the selected unit.

Load model:

Model the input resistance of the circuit connected to Vout.

Custom load resistance:

Use the minimum expected input resistance for conservative sag estimates.

Build values:

Preferred series rounding applies to the divider resistors, not the load model.

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Display precision:

Set the number of decimal places used in labels, tables, and JSON strings.

Target tolerance:

Acceptable loaded-output error around the target voltage.

Resistor tolerance:

Use the tolerance printed on the actual divider resistors.

Power derating:

50% means choose a resistor rated at least twice the modeled dissipation.

Chart density:

Higher density gives a smoother output curve without changing the selected result.

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Voltage dividers show up anywhere a circuit needs a predictable fraction of a supply or signal. Two resistors in series can turn 12 V into a safe microcontroller measurement range, bias an amplifier input, set a regulator feedback point, or create a simple reference node. The basic idea is familiar, but the useful result depends on more than the nominal resistor ratio.

The upper resistor, R1, sits between the input rail and the output node. The lower resistor, R2, sits between that node and ground. In the ideal case, the output is set only by the ratio between those two resistors. In real work, Vout is usually connected to something: a voltmeter, microcontroller analog-to-digital converter (ADC), op-amp input, sensor board, feedback pin, or bias network. That connected input has resistance of its own, so it becomes a second path from Vout to ground beside R2.

Common voltage divider uses and the main design concern for each
Use case	What the divider is doing	Main concern
Battery or sensor measurement	Scales a higher voltage into an ADC-safe range.	ADC input impedance, sampling behavior, and calibration error.
Bias or reference node	Creates a steady fraction of a supply voltage.	Noise pickup, load current, and drift from resistor tolerance.
Power-supply feedback	Sets a regulator feedback ratio.	Feedback-pin current, layout, power loss, and stability.
Bench or education checks	Shows how series resistance divides voltage.	Forgetting that the measuring device also loads the node.

Loaded voltage divider with R1 above Vout, R2 below Vout, and the load resistance in parallel with R2.

Choosing resistor values is a tradeoff between accuracy, current draw, heat, noise, and how much the next circuit disturbs the node. Smaller resistors make a stiffer output and reduce load sag, but they consume more current whenever the input is present. Larger resistors save power but raise the Thevenin output resistance, which makes Vout easier to move with leakage, input bias current, PCB contamination, and coupled noise.

A voltage divider is a scaling and biasing technique, not a regulated supply. It is usually appropriate when the load is predictable and high impedance compared with the divider. When the output has to drive variable current, charge a fast ADC input, reject switching noise, or stay accurate across temperature and production tolerance, a buffer, regulator, reference IC, or calibrated measurement path is often the better design.

How to Use This Tool:

Pick the calculation path that matches the bench question before entering values. The result will update around the selected goal, load model, and preferred resistor series.

Choose Calculation goal. Use Analyze R1/R2 for a known pair, Solve R1 when R2 is fixed, or Solve R2 when R1 is fixed and you need a target output.
Enter Input voltage and Target output voltage. The target must be greater than zero and lower than the input voltage because a passive divider cannot boost voltage.
Fill the visible resistor fields using ohm, kohm, or Mohm units. R1 is the upper resistor from input to Vout; R2 is the lower resistor from Vout to ground.
Select Load model. Choose No external load for the ideal divider, a preset for a meter, ADC/op-amp, or module input, or Custom load resistance when you know the connected input resistance.
Set Build values. Exact calculated / entered values keeps the mathematical result, while E96, E24, and E12 round R1 and R2 to nearby preferred resistor values.
Open Advanced when fit tolerance, resistor tolerance, power derating, display precision, or chart density matters. These settings affect badges, tolerance windows, recommended wattage, labels, and curve sampling.
Review Divider Snapshot first, then check Load & Power Audit and Output Transfer Curve before copying the values into a schematic or parts list.

If an input warning appears, fix the named value before relying on the output. The most important solve warning occurs when Solve R2 cannot reach the target with the selected load; raise the load impedance, lower R1, or add a buffer so the node is no longer pulled down by the load.

Interpreting Results:

The headline voltage is the loaded output when a load is selected, or the ideal no-load output when no external load is modeled. Read it together with target fit, load sag, load ratio, and resistor power because a divider that lands on the desired voltage can still be weak, hot, or too sensitive to a real input.

Voltage divider outputs and how to interpret them
Output	Meaning	Use it to check
Output voltage	Modeled Vout after the selected resistor values and load model are applied.	Whether the circuit is close enough to the desired loaded voltage.
No-load reference	The ideal divider voltage before the external load is connected.	How much the connected input changes the divider behavior.
Target fit	The voltage and percent error against the target output. Within target means the error is inside the selected tolerance window.	Whether E-series rounding or the chosen fixed resistor moved the result too far.
Load ratio	Load resistance divided by Thevenin output resistance. Values at 100x or higher are usually gentle loading, 10x to 100x is noticeable, and below 10x is heavy loading.	Whether a buffer or lower divider impedance is likely needed.
Tolerance window	Worst-case output range from opposite resistor tolerance directions.	Whether the result still fits after real resistor tolerance is included.
Recommended rating	Minimum resistor wattage after the selected derating margin is applied to the hottest divider resistor.	Which physical resistor power class to round up to before layout and thermal review.

Do not overread a clean target badge. Recheck the attached device's input resistance, the actual resistor tolerance, the expected input-voltage range, and any ADC or feedback-node datasheet limits before treating the divider as build-ready.

Technical Details:

An unloaded divider is a series circuit, so the same current flows through R1 and R2 and the output voltage is the fraction of the input that appears across R2. A loaded divider changes the lower leg because the connected input resistance is in parallel with R2. That lower equivalent resistance sets the loaded output voltage.

The Thevenin equivalent is a compact way to judge the load problem. Seen from Vout, the unloaded divider behaves like its no-load voltage source in series with R1 parallel R2. A load much larger than that resistance causes little sag. A load near that resistance becomes a significant part of the circuit and must be included in the calculation.

Formula Core:

The loaded calculation replaces R2 with the effective lower resistance, then applies the normal divider ratio. With no external load, the effective lower resistance is just R2.

\begin{array}{rcl} R_{low} & = & R_{2}, with no external load \\ R_{low} & = & \frac{R_{2} R_{load}}{R_{2} + R_{load}}, when a load is connected \\ V_{out} & = & V_{in} \frac{R_{low}}{R_{1} + R_{low}} \\ R_{th} & = & \frac{R_{1} R_{2}}{R_{1} + R_{2}} \\ P & = & I^{2} R or \frac{V^{2}}{R} \end{array}

Solving for a missing resistor starts from the target loaded output. Solving R1 is direct once the effective lower resistance is known. Solving R2 with a load has an extra boundary because R2 in parallel with the load can never be greater than the load itself.

\begin{array}{rcl} R_{1} & = & R_{low} (\frac{V_{in}}{V_{target}} - 1) \\ R_{low,needed} & = & \frac{R_{1} V_{target}}{V_{in} - V_{target}} \\ R_{2} & = & R_{low,needed}, without a load \\ R_{2} & = & \frac{R_{low,needed} R_{load}}{R_{load} - R_{low,needed}}, when R_{load} > R_{low,needed} \end{array}

Voltage divider terms and calculation notes
Term	Meaning	Calculation note
R1	Upper resistor from input voltage to Vout.	Increasing R1 lowers Vout when the other values stay fixed.
R2	Lower resistor from Vout to ground.	Increasing R2 raises Vout in the unloaded divider.
R_load	Input resistance of the circuit connected to Vout.	Modeled in parallel with R2, not in series with the divider.
R_th	Thevenin output resistance, equal to R1 parallel R2.	Used to judge how much the load will disturb the output node.
Load sag	No-load output minus loaded output.	Zero only when no external load is modeled.
E12, E24, E96	Preferred value families with 12, 24, or 96 nominal steps per decade.	Rounding applies to the divider resistors, while the load model stays as selected.

Preferred-value rounding is a parts-selection step, not a different electrical law. A solved 26.10 kohm resistor may stay close in E96 or round to 27 kohm in E24, and that shift can move the target error. Display precision changes labels and copied values only; it does not change the underlying arithmetic.

The output curve sweeps R2 around the selected value on a logarithmic scale. A shallow curve means nearby stocked values change Vout gently. A steep curve or a wide gap between loaded and unloaded curves means the chosen load and resistor family deserve closer review.

Boundary Rules:

Voltage divider status boundaries and validation rules
Rule	Boundary	Practical meaning
Passive target	0 V < target < Vin	A two-resistor passive divider can only reduce a positive input voltage.
Within target	\|target error %\| ≤ selected target tolerance	The loaded output is inside the chosen fit window.
Near target	selected target tolerance < \|target error %\| ≤ 2x selected target tolerance	The result is close, but a tighter series or adjusted resistor may be needed.
Heavy load	load resistance < 10x Thevenin output resistance	The load is a major part of the divider and a buffer or lower divider impedance is usually cleaner.
Solve R2 load limit	required lower resistance < load resistance	If the required lower leg is equal to or above the load, no positive R2 can reach the target.

Accuracy Notes:

The calculation is a DC or low-frequency resistive model. It is useful for first-pass sizing, parts comparison, and bench checks, but it does not replace circuit simulation, datasheet limits, layout review, or measurement on the actual hardware.

Resistor tolerance is estimated with opposite-direction worst cases. Temperature coefficient, aging, leakage, solder flux, humidity, and board contamination are not included.
Power checks use modeled dissipation and the selected derating fraction. Real parts also need voltage rating, body size, ambient temperature, airflow, enclosure heat, and clearance review.
Fast ADC inputs can require lower source impedance than a simple DC load model suggests because the sampling capacitor must settle during the acquisition time.
Switching-regulator feedback dividers and high-impedance measurement nodes need layout care. Long traces or noisy boards can disturb a divider even when the resistor ratio is correct.
The calculation runs in the browser. Entered voltages, resistor values, and load settings are not submitted for the calculation.

Advanced Tips:

Use the lowest realistic Custom load resistance when the connected input impedance varies. That makes the load sag estimate conservative.
Compare Exact calculated / entered values against E96, E24, and E12 before choosing parts. Preferred-value rounding can move a solved divider from within target to near target.
For ADC work, check Load ratio and Thevenin output resistance before trusting the voltage alone. A high DC input resistance does not guarantee fast sampling-capacitor settling.
Set Resistor tolerance to the grade you will actually buy. The tolerance window assumes opposite R1 and R2 tolerance directions, which is useful for a first worst-case estimate.
Use Target tolerance for the electrical limit that matters to the next circuit, not just a round percentage. A regulator feedback divider and a rough battery monitor may need very different fit windows.
Treat Recommended rating as a minimum. Round up to a standard resistor wattage and still check voltage rating, body size, ambient temperature, and board clearance.
Increase Chart density only when the Transfer Curve needs a smoother exported sweep. It changes chart sampling, not the selected divider calculation.

Worked Examples:

A 12 V rail feeding a 3.3 V ADC input is a typical Solve R1 case. With R2 fixed at 10 kohm, a 1 Mohm load model, and E96 build values, the selected R1 is 26.1 kohm. Output voltage lands at about 3.300 V, Target fit stays inside a 2% window, Load ratio is about 138x load/Rth, and Load sag is about 0.024 V. That is usually a reasonable first pass for a slow, high-impedance measurement input.

A 5 V signal scaled near 2.5 V with R1 at 4.7 kohm and a 10 kohm custom load is much more loaded. Solve R2 with E24 values selects 9.1 kohm and the Output voltage lands near 2.517 V, but Load ratio is only about 3.23x load/Rth and Load sag is roughly 0.780 V from the no-load reference. The target may be numerically close while the divider is still too soft for a changing input.

A troubleshooting case appears when Solve R2 reports that no positive R2 can reach the target. With 5 V input, 2.5 V target, R1 at 10 kohm, and a 10 kohm load, the required lower-leg resistance equals the load resistance. A parallel R2 can only reduce that value, so the practical fixes are lower R1, higher load impedance, or a buffer.

FAQ:

Which resistor is R1?

R1 is the upper resistor between the input voltage and Vout. R2 is the lower resistor between Vout and ground. Swapping them changes the divider ratio.

Why does a load lower the output voltage?

The load is connected from Vout to ground, so it sits in parallel with R2. That lowers the effective lower resistance and pulls the loaded output below the no-load reference.

Why can Solve R2 fail?

With a load connected, R2 in parallel with the load must be smaller than the load. If the target needs an effective lower resistance equal to or greater than the load, no positive R2 can make that result.

Should I choose exact values or E-series values?

Use exact values to study the ideal math or when you already have precision parts. Use E96, E24, or E12 to see how stocked resistor values change Target fit, load sag, current, and power.

Does the model handle high-frequency signals?

It models DC or low-frequency resistive behavior. It does not include resistor capacitance or inductance, PCB parasitics, ADC sampling capacitance, cable effects, or frequency-dependent input impedance.

Where is the calculation performed?

The entered values are calculated in the browser. The resistor, voltage, and load settings are not sent away for calculation.

Glossary:

R1: Upper divider resistor from input voltage to Vout.
R2: Lower divider resistor from Vout to ground.
Load resistance: The input resistance of the circuit connected to Vout, modeled in parallel with R2.
Thevenin output resistance: The resistance seen looking back into Vout, equal to R1 parallel R2.
Load sag: The voltage drop from the ideal no-load output to the loaded output.
E-series: Preferred resistor value families such as E12, E24, and E96.
Power derating: A design margin that keeps modeled resistor dissipation below the selected fraction of the resistor rating.

References:

IEC 60063:2015 Preferred number series for resistors and capacitors, International Electrotechnical Commission, 2015-03-26.
Voltage Dividers in Power Supplies, Analog Devices, 2018-08-01.
Analysis of ADC System Distortion Caused by Source Resistance, Analog Devices, 2003-03-25.
Voltage Divider Circuits, All About Circuits.
Power Rating, Resistor Guide by EEPower.