Voltage Divider Calculator

What the voltage divider calculator does

A voltage divider is two resistors in series that split an input voltage, and this voltage divider calculator solves the output taken from the junction between them. The voltage divider formula scales Vin by the ratio of the lower resistor to the total, and the resistor divider calculator does that math for you. Enter Vin, R1, and R2, and the potential divider calculator returns the output voltage, the current through the chain, and the ratio, so the voltage divider equation never has to be worked out on paper.

The voltage divider formula and equation

The voltage divider formula scales the input by the lower resistor over the total resistance:

That is the voltage divider equation, sometimes written as the voltage divider rule formula. The same current flows through both resistors, so the current divider formula follows from Ohm's law across the total:

The voltage divider calculator evaluates both, which is why the potential divider formula and the current divider formula give consistent results here.

Voltage divider formula example

With a 12 V input, R1 = 1000 Ω and R2 = 2000 Ω, apply the voltage divider formula:

The voltage divider calculator then gives the current from the current divider formula, I = 12 ÷ 3000 = 4 mA, and a ratio of 0.667. Enter these three values and the resistor divider calculator returns exactly this.

How to use the voltage divider calculator

Enter three values: the input voltage across both resistors, the top resistor R1, and the bottom resistor R2. The output is measured across R2, and the voltage divider calculator applies the voltage divider formula to return it instantly. As a resistor divider calculator it also shows the current, and as a potential divider calculator it reports the division ratio so you can confirm the voltage divider equation at a glance.

Reading the voltage divider calculator result

The output voltage is what a meter reads across R2 with nothing else connected. Connect a load in parallel with R2 and the effective resistance drops, so the real output sits below what the voltage divider formula predicts; keep the load resistance well above R2 for accuracy. The current from the current divider formula tells you the power the divider wastes and the drain it puts on a battery, which is why the resistor divider calculator reports it alongside the voltage.

When to use the voltage divider calculator

• Setting a reference or bias voltage with the voltage divider formula
• Scaling a high voltage down for an ADC using the potential divider calculator
• Reading thermistors and LDRs where the resistor divider calculator gives the output
• Checking divider current with the current divider formula before sizing parts

Common voltage divider formula mistakes

• Ignoring the load. Reading the unloaded voltage divider formula result, then connecting a low-resistance load, gives a much lower real output.
• Resistors too small. The current divider formula shows the wasted current; tiny resistors drain batteries needlessly.
• Swapping R1 and R2. The output is across R2, the lower resistor, so swapping them inverts the ratio in the voltage divider equation.

Tips for accurate results

• Keep the load resistance at least 10× R2, or buffer the output, so the voltage divider formula holds.
• Use standard E12 or E24 values close to the ratio from the resistor divider calculator.
• Check the current from the current divider formula against your battery budget before building.

Related voltage divider calculators

The voltage divider formula is a direct application of Ohm's law, so the Ohm's law calculator solves the voltage, current and resistance behind the divider. Once the current divider formula gives you the chain current, the watts to amps calculator converts that current to power. For a divider feeding a long cable run, the voltage drop calculator extends the same resistance math along the conductor.

Understanding the voltage divider formula

The voltage divider formula is simple to write but its limits decide whether a design works. It assumes an ideal, unloaded output, and the moment you draw current from the midpoint the lower leg effectively drops in value and the output sags. This loading effect surprises most beginners, and the voltage divider calculator gives you the unloaded figure so you can judge how much margin you have. The working rule is to keep the load resistance at least ten times R2, so the output stays within a few percent of what the voltage divider equation predicts; where that is impossible, an op-amp buffer isolates the divider from the load entirely.

Choosing resistor scale with the current divider formula

Power is the other axis. Small resistors give a stiff output that tolerates loading but waste current continuously, which matters in battery devices, and the current divider formula shows exactly how much. Large resistors sip almost nothing but are easily disturbed by loading and noise. Picking the resistor scale is a balance between stability and efficiency, and the current the resistor divider calculator reports is your guide. For a three-resistor chain the same voltage divider formula applies to each tap, using the resistance below the tap over the total, so the potential divider calculator idea extends naturally to multiple outputs.

Voltage divider formula chart

Common R1 and R2 pairs and the output the voltage divider formula gives for a 12 V input:

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