Voltage Divider Calculator | Free Online Tool

Voltage Divider Calculator | ProEngCalc

⚡ Electrical Engineering

Voltage Divider Calculator

Calculate output voltage from two resistors — with current, power, and ratio breakdown

Reference: Kirchhoff's Voltage Law | V_out = V_in × R2 / (R1 + R2)

Inputs

Input Voltage Vin

Supply voltage in Volts

Resistor 1 R1

Upper resistor (series with supply)

Resistor 2 R2

Lower resistor (to ground)

Output Voltage (Vout)

—

Divider Ratio

—

Current (I)

—

Power (R1)

—

Power (R2)

—

📐 Solution Breakdown

Reference: Kirchhoff's Voltage Law | Voltage Divider Rule | IEEE Std 1291

Voltage Divider Formula

Vout = Vin × R2 / (R1 + R2)

Variable Definitions

Symbol	Variable	Unit	Notes
Vin	Input Voltage	Volts (V)	Supply voltage
R1	Upper Resistor	Ohms (Ω)	In series with supply
R2	Lower Resistor	Ohms (Ω)	Connected to ground
Vout	Output Voltage	Volts (V)	Voltage across R2
I	Divider Current	Amperes (A)	I = Vin / (R1+R2)

⚠ Assumptions & Limits

Assumes no load current — output is open circuit (infinite load impedance). Load resistance will reduce Vout.
Valid for DC circuits and resistive loads only — not for reactive components
Does not account for resistor tolerances — standard 5% tolerance can shift Vout by ±5%
For loaded dividers, calculate the parallel combination of R2 and load resistance first, then apply the formula
Power dissipation values assume steady-state DC operation — derate for elevated temperatures

What Is a Voltage Divider?

A voltage divider is one of the most fundamental and widely used circuits in electronics. Two resistors in series create a predictable output voltage that is a fraction of the input. This principle underpins countless real-world applications from sensor interfaces to microcontroller inputs to audio level controls. Understanding voltage dividers — including their limitations — is essential knowledge for every electrical and electronics engineer.

The Voltage Divider Formulas

Output Voltage

Vout = Vin × R2/(R1+R2)

Divider Ratio

k = R2/(R1+R2)

Quiescent Current

I = Vin/(R1+R2)

Vin — Input supply voltage (V)
R1 — Upper resistor between supply and output node (Ω)
R2 — Lower resistor between output node and ground (Ω)
Vout — Output voltage at the midpoint node (V)
k — Divider ratio, always between 0 and 1

Worked Examples

Example 1 — 3.3V Reference from 5V Supply

A microcontroller needs a 3.3V reference but only a 5V supply is available. Design a voltage divider using standard E24 resistor values.

Given

Vin = 5 V | Vout = 3.3 V target

Required Ratio

k = 3.3/5.0 = 0.660

Standard Values

R2 = 33 kΩ, R1 = 18 kΩ → k = 33/51 = 0.647

Actual Vout

Vout = 5 × 0.647 = 3.24 V (error: −1.8%)

1.8% error is acceptable for most reference applications. For precision applications, use a dedicated 3.3V reference IC (e.g. LM4040) instead of a resistor divider.

Example 2 — ADC Input Scaling (0–10V Sensor to 0–3.3V ADC)

An industrial pressure sensor outputs 0–10V. Your microcontroller ADC input is limited to 0–3.3V. Design the scaling divider.

Given

Vin_max = 10 V | Vout_max = 3.3 V | Load impedance ≥ 100 kΩ (ADC input)

Required Ratio

k = 3.3/10 = 0.33

Choose R2

R2 = 10 kΩ → R1 = R2(1/k – 1) = 10k × 2.03 = 20.3 kΩ → use 20 kΩ

Solution

R1 = 20 kΩ, R2 = 10 kΩ → Vout = 3.33 V at 10V input ✓

ADC input impedance (≥100 kΩ) is 10× greater than R2 (10 kΩ) — loading error is less than 10%. For better accuracy, buffer with an op-amp voltage follower.

Example 3 — NTC Thermistor Temperature Sensing

A 10 kΩ NTC thermistor (at 25°C) is used in a voltage divider with a 10 kΩ fixed resistor and 3.3V supply. What is the output voltage at 25°C and how does it change?

Given

Vin = 3.3 V | R1 = 10 kΩ (fixed) | R2 = 10 kΩ (thermistor at 25°C)

At 25°C

Vout = 3.3 × 10k/(10k+10k) = 1.65 V

At 50°C (R_NTC ≈ 4.16 kΩ)

Vout = 3.3 × 4.16k/(10k+4.16k) = 0.968 V

Solution

Voltage drops from 1.65V to 0.97V as temperature rises 25°C

The ADC reads voltage and firmware converts to temperature using the Steinhart-Hart equation or a lookup table from the thermistor datasheet. Place the NTC as R2 (lower resistor) for an output that decreases with increasing temperature.

Example 4 — Battery Voltage Monitor

Monitor a 12V lead-acid battery (range 10–14.4V) using a 3.3V microcontroller ADC. Design the divider to use the full ADC range.

Given

Vin_max = 14.4 V | Vout_max = 3.3 V | Target: use full ADC range

Required Ratio

k = 3.3/14.4 = 0.229

Standard Values

R1 = 100 kΩ, R2 = 30 kΩ → k = 30/130 = 0.231

Solution

Vout = 14.4 × 0.231 = 3.32 V at full charge ✓

High resistance values (100 kΩ + 30 kΩ) keep quiescent current to 85 µA from the battery — acceptable for a continuous monitor. Add a 100 nF bypass capacitor on the output to filter noise.

Real World Applications

🌡

Temperature Sensing

NTC/PTC thermistors paired with fixed resistors to convert temperature to ADC-readable voltage.

🔋

Battery Monitoring

Scaling battery voltage (12V, 24V, 48V) down to microcontroller ADC input range for state-of-charge monitoring.

📊

Sensor Signal Conditioning

Scaling industrial sensor outputs (0–10V, 4–20mA with shunt) to 0–3.3V or 0–5V ADC inputs.

⚡

Transistor Biasing

Setting BJT base bias voltage using a voltage divider to establish the correct operating point (Q-point).

🎚

Potentiometers

A potentiometer is a variable voltage divider — the wiper position sets the division ratio for volume controls, position sensors, and trim adjustments.

🔌

Level Shifting

Interfacing 5V logic outputs to 3.3V inputs using a simple two-resistor divider when speed requirements are modest.

Common Mistakes Engineers Make

❌ Mistake 1 — Ignoring Load Impedance

Connecting a low-impedance load to a voltage divider output drastically reduces Vout. The load appears in parallel with R2 and reduces the effective lower resistance. Rule: load impedance should be at least 10× R2 for less than 10% error. For precision applications, buffer with a unity-gain op-amp.

❌ Mistake 2 — Using as a Power Supply

A voltage divider is NOT a voltage regulator. Output voltage varies with load current. For powering circuits, use a linear regulator (LM7805, LM1117) or DC-DC converter. Use dividers only for signal-level, high-impedance applications.

❌ Mistake 3 — Not Checking Power Dissipation

With low resistor values and high supply voltages, dividers can waste significant power. P = Vin²/(R1+R2). At 12V with 1 kΩ + 1 kΩ, you’re burning 72 mW continuously — fine for bench use but a real concern in battery-powered products.

❌ Mistake 4 — Resistor Tolerance Errors

Two 5% resistors in a divider can produce up to ±10% error in Vout in the worst case. For precision reference voltages, use 0.1% or 1% tolerance resistors. For ADC scaling, tolerance errors translate directly to measurement error.

Frequently Asked Questions

How do I find R1 if I know the desired Vout?

Rearrange the formula: R1 = R2 × (Vin/Vout − 1). Choose a standard value for R2 first (typically 10 kΩ for most signal applications), then calculate the required R1. Select the nearest standard E24 or E96 value and verify the actual Vout with the standard values using this calculator.

What resistor values should I use?

For signal-level dividers connected to high-impedance ADC inputs: 10 kΩ–100 kΩ range balances low current consumption with good noise immunity. For low-impedance loads: use lower values (1 kΩ–10 kΩ) to maintain output regulation. For battery-powered circuits: use higher values (100 kΩ+) to minimize quiescent current drain.

Can I use a voltage divider for 4–20mA signals?

Yes — place a precision shunt resistor (typically 100 Ω or 250 Ω) as R2, with no R1. The current through the shunt creates a voltage: V = I × R. For 4–20 mA through 250 Ω: V ranges from 1.0V to 5.0V — perfect for a 0–5V ADC. For 0–3.3V ADC with 4–20 mA, use a 165 Ω shunt (0.66V–3.30V range).

How does temperature affect voltage divider accuracy?

Both resistors change value with temperature according to their temperature coefficient (TC). If both resistors have the same TC (matched tempco), the ratio stays constant and Vout doesn’t drift. Use matched-TC resistor pairs or resistor networks from the same manufacturer for temperature-stable dividers. Metal film resistors (±100 ppm/°C) are far more stable than carbon composition (±1500 ppm/°C).

What is a loaded voltage divider?

A loaded voltage divider has a resistive load (R_L) connected across R2. The effective lower resistance becomes R2_eff = (R2 × R_L)/(R2 + R_L), which is always less than R2. This reduces Vout below the unloaded value. The loading error percentage ≈ R2/R_L × 100%. Use this calculator’s loaded divider mode to find the actual output with your specific load.