Voltage Divider Calculator

Calculate the output voltage of a resistor voltage divider circuit instantly.

📖 What is a Voltage Divider?

A voltage divider is one of the most fundamental and widely used circuits in electronics. It consists of two resistors (R1 and R2) connected in series between an input voltage source (Vin) and ground. The output voltage (Vout) is taken from the junction between the two resistors.

The circuit "divides" the input voltage proportionally based on the relative values of R1 and R2. If R1 = R2, exactly half the input voltage appears at the output. If R2 is twice R1, two-thirds of Vin appears at the output.

Voltage dividers are found in almost every electronic circuit: they set bias voltages for transistors, scale down voltages for microcontroller ADC inputs, create reference voltages, and convert between logic levels (e.g., interfacing a 5V sensor with a 3.3V microcontroller). They are passive, require no power supply of their own, and can be built with just two resistors.

The key limitation: voltage dividers only work accurately when the load connected to Vout has a much higher impedance than R2. If the load draws significant current, it lowers the effective R2 and reduces Vout - a phenomenon called "loading."

📐 Formula

Vout = Vin × R2 / (R1 + R2)

To find R2 given Vout:

R2 = R1 × Vout / (Vin − Vout)

To find R1 given Vout:

R1 = R2 × (Vin − Vout) / Vout

Current through divider:

I = Vin / (R1 + R2)

📖 How to Use This Calculator

Select what to find: Vout (given Vin, R1, R2), R2 (given Vin, Vout, R1), or R1 (given Vin, Vout, R2).

Enter the known values. Resistances in ohms - use 1000 for 1kΩ, 10000 for 10kΩ.

Click Calculate - the result, ratio, and current/power are shown.

💡 Example Calculations

Example 1 - 5V from 12V supply

Vin = 12V, want Vout = 5V. Choose R1 = 10kΩ. Find R2:

R2 = 10,000 × 5 / (12 - 5) = 10,000 × 5/7 = 7,143 Ω (use 6.8kΩ or 7.5kΩ standard value)

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Example 2 - Level shifting 5V → 3.3V

Vin = 5V, want Vout = 3.3V. R1 = 6.8kΩ.

R2 = 6800 × 3.3 / (5 - 3.3) = 6800 × 3.3/1.7 = 13,200 Ω (use 13kΩ or 12kΩ)

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Frequently Asked Questions

What is a voltage divider?+

A voltage divider is a simple circuit with two resistors in series connected across a voltage source. The output voltage (Vout) is taken across the lower resistor (R2). It divides the input voltage proportionally based on the ratio of R2 to the total resistance (R1 + R2).

When should I use a voltage divider?+

Voltage dividers are used to: scale down a voltage for ADC (analogue-to-digital) inputs, create a reference voltage, convert between logic levels (e.g. 5V to 3.3V), and bias transistor bases. They are not suitable for supplying current to loads - use a voltage regulator for that.

What happens if the load resistance is too low?+

A low load resistance (RL) in parallel with R2 reduces the effective bottom resistance and pulls Vout below the calculated value. For accurate results, RL should be at least 10× greater than R2. This is called 'loading effect'.

How do I choose R1 and R2 values?+

First, determine the required Vout/Vin ratio. Then Vout = Vin × R2/(R1+R2). Choose R2 to be significantly smaller than your load impedance but not so small that it wastes current. Common starting values: 10kΩ and 20kΩ for a 1/3 division.

What is a loaded voltage divider?+

A loaded voltage divider has a load (RL) connected in parallel with R2. The effective bottom resistance becomes R2||RL = (R2×RL)/(R2+RL). Recalculate with this as the bottom resistor. The Thevenin equivalent is often used for loaded divider analysis.

What is the voltage divider formula?+

Vout = Vin x R2 / (R1 + R2). R1 is the top resistor (connected between Vin and Vout) and R2 is the bottom resistor (connected between Vout and ground). Example: Vin = 12V, R1 = 10k ohms, R2 = 5k ohms. Vout = 12 x 5000 / (10000 + 5000) = 12 x 5/15 = 4V. This is useful for creating reference voltages, scaling sensor signals, and setting bias points in analog circuits.

Can I use a voltage divider to power a circuit?+

A voltage divider is not suitable for powering circuits that draw significant current. As load current is drawn from Vout, the effective R2 decreases (load in parallel with R2), lowering Vout below the calculated value. For stable voltage under varying load, use a voltage regulator (LM7805, LM317, etc.) instead. Voltage dividers are best used for sensing and reference applications where the load draws minimal current (much less than the divider current I = Vin / (R1+R2)).

How does load resistance affect voltage divider output?+

Connecting a load in parallel with R2 reduces the effective resistance at that node, which lowers the output voltage below the unloaded value. The loaded output voltage is Vout = Vin x (R2 || Rload) / (R1 + R2 || Rload). For the load to have minimal effect, the load resistance should be at least 10x greater than R2 (a rule of thumb called the 10:1 loading rule). If load resistance is similar to or less than R2, the divider is heavily loaded and output regulation is poor - a voltage regulator or buffer amplifier is needed instead.

What is the loading effect in a voltage divider?+

When you connect a load resistance across the output of a voltage divider, it appears in parallel with R2, lowering the effective bottom resistance and changing the output voltage. This is the loading effect. To minimize it, choose R1 and R2 much smaller than the load resistance (at least 10x). This calculator assumes no load; for loaded dividers, use R2 in parallel with the load as the effective bottom resistance.