Voltage Divider Calculator

How This Voltage Divider Calculator Helps

A voltage divider is one of the simplest and most widely used circuits in electronics. It uses two resistors in series to convert an input voltage into a lower output voltage. This calculator lets you enter the input voltage and resistor values (R1 and R2) and immediately see the resulting output voltage at the junction between the two resistors.

Use this tool when you want to design simple level shifters, bias points for analog circuits, or quick reference voltages. By experimenting with different resistor ratios, you can quickly understand how each parameter affects the output.

Directly above or beside this explanation, you should see the calculator interface with three inputs:

• Input Voltage (V) – the supply or signal voltage you are dividing.
• Resistor R1 (Ω) – the resistor connected between the input voltage and the output node.
• Resistor R2 (Ω) – the resistor connected between the output node and ground (or the lower reference node).

The calculator assumes an ideal, unloaded voltage divider, which is a very common and useful approximation for many signal-level applications. The sections below explain how the divider works, show the key formulas, and outline the assumptions and limitations you should keep in mind.

How a Voltage Divider Works

A basic voltage divider consists of two resistors, R1 and R2, connected in series across an input voltage source, V_in. The output voltage, V_out, is taken from the node between R1 and R2, relative to the lower reference node (usually ground).

Because the resistors are in series, the same current flows through both of them. The total resistance seen by the source is R1 + R2. According to Ohm’s law, the current I through the series combination is:

I = V_in / (R1 + R2)

The output voltage is just the voltage drop across R2. Since the current through R2 is I, the drop across R2 is:

V_out = I × R2

Substituting the expression for I into this equation gives the classic voltage divider formula.

Voltage Divider Formula

The standard ideal voltage divider relationship is:

In plain text form:

V_out = (R2 / (R1 + R2)) × V_in

This shows that the output voltage depends only on the ratio of R2 to the total resistance, multiplied by the input voltage. If R2 is small compared with R1, the output will be a small fraction of V_in. If R2 is large compared with R1, the output will be close to V_in.

The same expression also gives you the series current:

I = V_in / (R1 + R2)

Once you know V_out and I, you can work out power dissipation in each resistor using:

• P_R1 = I² × R1
• P_R2 = I² × R2

These relationships are useful when you want to ensure that your chosen resistor values will not overheat in operation.

Interpreting the Calculator Results

After entering V_in, R1, and R2 into the calculator and running the computation, you will typically see at least one primary result:

• V_out – the voltage at the junction between R1 and R2, measured relative to the lower reference node.

Conceptually, you can think of the divider in either of two ways when you interpret this number:

1. Fraction of the input – V_out is some fraction k of V_in, where k = R2 / (R1 + R2). This is useful when you start from a given supply and target output fraction.
2. Series current perspective – the same current flows through both resistors, and V_out is simply the drop across R2 caused by that current.

If you increase R1 while holding R2 and V_in constant, the denominator (R1 + R2) increases, so the ratio R2 / (R1 + R2) decreases. The calculator will show a lower V_out. Conversely, increasing R2 while keeping R1 constant will increase V_out, bringing it closer to V_in.

When you interpret the result, it is essential to remember that the formula assumes the output is feeding a very high-impedance input (that is, almost no current is drawn from V_out). The Assumptions and Limitations section below expands on this point.

Worked Example: 5 V Down to 3 V

This example mirrors a common microcontroller scenario: scaling a 5 V signal down to approximately 3 V for an input that cannot tolerate the full 5 V level.

1. Set V_in = 5 V in the calculator.
2. Choose R1 = 2 kΩ (enter 2000 if the form expects ohms).
3. Choose R2 = 3 kΩ (enter 3000 for ohms).
4. Run the calculation to obtain V_out.

Using the formula:

V_out = (R2 / (R1 + R2)) × V_in = (3000 / (2000 + 3000)) × 5 V

The sum R1 + R2 is 5000 Ω, so:

V_out = (3000 / 5000) × 5 V = 0.6 × 5 V = 3 V

The calculator should show an output close to 3 V. This is suitable for many devices that expect a nominal 3.3 V logic level, provided their absolute maximum rating is at or above 3 V and other design constraints are satisfied.

If you keep V_in = 5 V and R2 = 3 kΩ but change R1 to 1 kΩ (1000 Ω), then:

V_out = (3000 / (1000 + 3000)) × 5 V = (3000 / 4000) × 5 V = 0.75 × 5 V = 3.75 V

By comparing these two cases, you can see how the R2 / (R1 + R2) ratio controls the output voltage. Small changes in the ratio lead to proportional changes in V_out.

Comparison of Common Divider Setups

The table below summarizes some typical voltage divider configurations to illustrate how different resistor ratios affect the output voltage. All examples assume an ideal, unloaded divider.

These examples highlight two important design levers:

• The ratio of R2 to (R1 + R2) determines V_out as a fraction of V_in.
• The absolute magnitude of R1 and R2 together shapes the current draw and power dissipation. Larger resistances reduce current, while smaller resistances increase current and power but may improve noise performance and reduce sensitivity to loading.

Typical Applications

Voltage dividers are used across many areas of electronics. Common applications include:

• Level shifting for digital inputs – converting higher logic voltages (such as 5 V) down to a level safe for 3.3 V or lower inputs.
• Setting bias points – creating stable DC bias voltages for transistor bases, MOSFET gates, or op-amp inputs in analog circuits.
• Generating reference voltages – deriving mid-supply or other reference points for analog-to-digital converters (ADCs) and comparators.
• Monitoring supply rails – scaling down higher supply voltages (for example, 12 V or 24 V rails) to fit within the measurable range of a low-voltage ADC.

In all of these cases, the divider is usually feeding a high-impedance input (for instance, the input pin of an ADC or op-amp), so that the loading on the divider is minimal and the ideal formulas remain accurate.

Assumptions and Limitations

The calculator is based on an ideal model of a voltage divider. When you interpret the results and apply them to real circuits, it is important to understand the assumptions behind the math:

• Ideal resistors – R1 and R2 are treated as exact, linear resistances. Real resistors have tolerances (for example, ±1% or ±5%) and may change value slightly with temperature.
• No load on V_out – the formula assumes the output is connected to a very high-impedance input that draws negligible current. If the load draws significant current, it effectively appears in parallel with R2 and alters the divider ratio.
• DC or slowly varying signals – the basic equation assumes purely resistive behavior. At high frequencies, stray capacitances and inductances can change the effective impedance and the division ratio.
• Safe voltage and power levels – the calculation does not check whether the resistors can safely dissipate the power or whether insulation and clearance distances are adequate for high-voltage applications.

One of the most important practical limitations is loading. If you connect a low-impedance load (such as a motor or another resistor of similar magnitude to R2) to V_out, then R2 and the load form a parallel combination. The effective resistance seen in the divider formula becomes smaller than R2 alone, which reduces V_out compared with the ideal prediction.

Because of this, voltage dividers are generally not recommended for powering devices that draw substantial current, such as motors, LEDs without series resistors, or entire boards. They are best reserved for signal-level tasks, where currents are tiny and the loading of V_out is negligible.

For robust designs, always check:

• That the nominal V_out from the calculator remains within acceptable limits even when resistor tolerances and supply variations are taken into account.
• That the calculated current through the divider and resulting power dissipation in each resistor remain below their rated limits, with adequate margin.

Practical Design Notes

When choosing resistor values for a real-world voltage divider:

• Start from the ratio – decide the desired fraction k = V_out / V_in. Then choose a convenient resistor pair such that R2 / (R1 + R2) ≈ k.
• Control current draw – check I = V_in / (R1 + R2). For battery-powered systems, you often want I to be small (for example, tens of microamps or less) to conserve energy.
• Check power ratings – evaluate P = I² × R for each resistor. Standard small resistors are often rated for 0.125 W, 0.25 W, or 0.5 W. Keep actual dissipation comfortably below these limits.
• Consider noise and input impedance – very large resistor values reduce current but can make the node more susceptible to noise and input bias currents of amplifiers or ADCs.

The calculator gives you the ideal V_out. Combine that result with these checks to produce a design that is both accurate and reliable.

Frequently Asked Questions

Can I use a voltage divider as a power supply?

In most cases, no. A voltage divider is poor at supplying power to loads that draw significant current because the output voltage will drop as the load current increases. Use a regulator or dedicated power supply instead, and reserve voltage dividers for low-current signal applications.

How do resistor tolerances affect the output?

Resistor tolerances cause the actual values of R1 and R2 to deviate from their nominal values. This changes the ratio R2 / (R1 + R2) and therefore shifts V_out. For precision applications, use tighter tolerance parts (for example, 1% metal film resistors) and, if necessary, account for worst-case combinations in your design calculations.

How do I enter kiloohms or megaohms into the calculator?

Unless the interface specifically supports suffixes, enter all resistor values in ohms as plain numbers. For example, 2 kΩ becomes 2000, 47 kΩ becomes 47000, and 1 MΩ becomes 1000000. The electrical behavior depends only on the numeric ratio, not the units, as long as both R1 and R2 are expressed in the same unit.