Find The Voltage Across Each Resistor Without Calculating Current

Easily calculate the voltage drop across resistors in series using the Voltage Divider Rule, without needing to find the current first. Our Voltage Divider Calculator does the work for you.

Voltage Distribution Across Resistors

Component	Resistance (Ω)	Voltage Drop (V)
R1
R2
Total

What is a Voltage Divider Calculator?

A Voltage Divider Calculator is a tool used to determine the voltage across individual resistors connected in series to a voltage source. It applies the Voltage Divider Rule, a fundamental principle in electronics, to calculate these voltages directly from the resistance values and the total supply voltage, without needing to calculate the circuit current first. This is particularly useful for analyzing and designing circuits where a specific voltage, lower than the source voltage, is required at some point.

Anyone working with electronics, from hobbyists to students and professional engineers, can use a Voltage Divider Calculator. It simplifies the process of finding voltage drops in series circuits, which is essential for biasing transistors, setting reference voltages, or reducing a voltage to a level suitable for a particular component.

A common misconception is that the voltage divides equally regardless of resistor values. However, the voltage across each resistor is directly proportional to its resistance relative to the total resistance of the series combination. A larger resistance will have a larger voltage drop across it.

Voltage Divider Calculator Formula and Mathematical Explanation

The Voltage Divider Rule is derived from Ohm's Law (V = IR). For a series circuit with a total voltage V_s and resistors R₁, R₂, …, R_n, the total resistance R_total = R₁ + R₂ + … + R_n. The current (I) flowing through the series circuit is I = V_s / R_total.

The voltage drop across any resistor R_x is V_x = I * R_x. Substituting the expression for I, we get:

V_x = (V_s / R_total) * R_x = V_s * (R_x / R_total)

For a simple circuit with two resistors R₁ and R₂ in series with V_s:

• R_total = R₁ + R₂
• Voltage across R₁ (V₁) = V_s * (R₁ / (R₁ + R₂))
• Voltage across R₂ (V₂) = V_s * (R₂ / (R₁ + R₂))

Our Voltage Divider Calculator uses these formulas.

Variables Table

Variable	Meaning	Unit	Typical Range
Vs	Total Supply Voltage	Volts (V)	0.1 – 1000+ V
R1, R2, Rx	Resistance of individual resistors	Ohms (Ω)	1 – 1,000,000+ Ω
Rtotal	Total Series Resistance	Ohms (Ω)	Sum of individual resistances
V1, V2, Vx	Voltage drop across individual resistors	Volts (V)	0 – Vs

Practical Examples (Real-World Use Cases)

Example 1: LED Current Limiting (Conceptual)

While a voltage divider isn't ideal for directly powering an LED due to current changes, imagine you have a 12V supply and want to get a reference voltage around 3V. You could use a Voltage Divider Calculator to find suitable resistors. Let's say you choose R₁ = 9000 Ω and R₂ = 3000 Ω.

• V_s = 12 V
• R₁ = 9000 Ω
• R₂ = 3000 Ω
• R_total = 9000 + 3000 = 12000 Ω
• V₁ = 12 * (9000 / 12000) = 9 V
• V₂ = 12 * (3000 / 12000) = 3 V

The voltage across R₂ would be 3V, which could be used as a reference, though connecting a load will change this voltage.

Example 2: Setting a Reference Voltage for a Comparator

Suppose you need a 5V reference from a 9V battery for a comparator circuit. Using the Voltage Divider Calculator, you might try R₁ = 4000 Ω and R₂ = 5000 Ω.

• V_s = 9 V
• R₁ = 4000 Ω
• R₂ = 5000 Ω
• R_total = 4000 + 5000 = 9000 Ω
• V₁ = 9 * (4000 / 9000) = 4 V
• V₂ = 9 * (5000 / 9000) = 5 V

You get 5V across R₂. If the comparator has high input impedance, this voltage will remain relatively stable.

How to Use This Voltage Divider Calculator

1. Enter Total Voltage (V_s): Input the total DC voltage applied across the series combination of resistors.
2. Enter Resistor 1 (R₁): Input the resistance value of the first resistor in ohms.
3. Enter Resistor 2 (R₂): Input the resistance value of the second resistor in ohms.
4. Calculate: The calculator automatically updates as you type, or you can click "Calculate".
5. View Results: The calculator displays the total resistance, and the voltage drops V₁ (across R₁) and V₂ (across R₂). The results are also shown in a table and a chart for better visualization.
6. Reset: Click "Reset" to clear the inputs to default values.
7. Copy Results: Click "Copy Results" to copy the main outputs to your clipboard.

The Voltage Divider Calculator helps you quickly see how the voltage is distributed. If you need a specific voltage across one resistor, you can adjust the resistance values until you achieve the desired output.

Key Factors That Affect Voltage Divider Results

• Total Supply Voltage (V_s): The voltages across the resistors are directly proportional to V_s. If V_s doubles, V₁ and V₂ will also double, assuming resistances remain constant.
• Resistor Values (R₁, R₂): The ratio of the resistor values determines the ratio of the voltage drops. V₁/V₂ = R₁/R₂. Changing one resistor value changes both voltage drops (while their sum remains V_s).
• Number of Resistors: While this calculator focuses on two, the principle extends to more resistors in series. The voltage across one resistor is its resistance divided by the total series resistance, times V_s.
• Resistor Tolerance: Real resistors have a tolerance (e.g., ±5%). This means their actual resistance can vary, leading to slight variations in the calculated voltages compared to the measured ones.
• Load Connected: If you connect a load (another circuit or component) in parallel with one of the resistors (e.g., across R₂ to use V₂), the load's resistance will affect the equivalent resistance and thus change the voltage division. The Voltage Divider Calculator assumes no load is connected across the individual resistors.
• Temperature Changes: The resistance of most materials changes with temperature. If the resistors heat up significantly, their values might change, affecting the voltage division.

Frequently Asked Questions (FAQ)

Why is it called a "Voltage Divider"?

Because the total voltage V_s is "divided" or distributed among the series resistors, with each resistor getting a portion of the total voltage proportional to its resistance.

Can I use this Voltage Divider Calculator for AC circuits?

This calculator is primarily for DC circuits or AC circuits with purely resistive components at a given moment. For AC circuits with capacitors or inductors, impedance (Z) replaces resistance (R), and phase angles become important, making the calculation more complex (use our Impedance Calculator for that).

What if I have more than two resistors in series?

The principle remains the same. For R₁, R₂, R₃… R_n in series, R_total = R₁ + R₂ + R₃ + … + R_n, and V_x = V_s * (R_x / R_total).

Is a voltage divider an efficient way to get a lower voltage?

Not always. It's simple, but power is wasted as heat in the resistors (P = V*I). If the load connected to the divider draws significant current, the output voltage will drop, and the efficiency will be low. For power applications, voltage regulators or DC-DC converters are much more efficient.

What happens if I connect something across R2?

If you connect a load with resistance R_L across R₂, the equivalent resistance of R₂ and R_L in parallel becomes (R₂ * R_L) / (R₂ + R_L). This new value replaces R₂ in the voltage divider formula, and the voltage across it will be lower than when R_L was not present.

Why don't the voltages add up exactly to Vs sometimes?

This could be due to rounding in the display or the tolerance of the resistors if you are measuring real components. In theory, V₁ + V₂ should exactly equal V_s.

Can I input zero resistance?

The calculator accepts zero, but in a real circuit, a zero-ohm resistor is a short circuit. If R1 and R2 are both zero, it's a short across the supply. If one is zero, it gets zero volts across it.

Where is the Voltage Divider Rule used?

It's used in sensor circuits to read resistive sensors, setting reference voltages for comparators and ADCs, biasing transistors, and simple attenuation circuits.