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Voltage Divider Calculator

Find output voltage, current, and power for a two-resistor voltage divider network

Frequently Asked Questions

What is the voltage divider formula?

Vout = Vin × R2 / (R1 + R2). R1 is the top resistor connected to the input and R2 is the bottom resistor connected to ground, with the output tapped at their junction.

Why does my real output voltage read lower than calculated?

That is the loading effect. Any load connected to the tap sits in parallel with R2, lowering the effective resistance and pulling Vout down. Keep the load at least 10× R2, or buffer the output with an op-amp follower.

Can I use a voltage divider as a power supply?

No. A divider only works for high-impedance references like an ADC input or transistor bias. It cannot source meaningful current without the output sagging, and it wastes power continuously. Use a regulator instead.

Should I use large or small resistor values?

Large values (tens of kΩ) waste less power but are more sensitive to loading and bias current. Small values stiffen the output but burn more power. Choose the ratio for the voltage and the magnitude for the current budget.

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Estimates for informational purposes only.

Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator Works

A voltage divider is two resistors in series across a source. The output is tapped from the junction between them. Enter the input voltage V_in and the two resistor values R1 (top, connected to V_in) and R2 (bottom, connected to ground). The output follows V_out = V_in × R2 ÷ (R1 + R2). The same current I = V_in ÷ (R1 + R2) flows through both resistors, and each dissipates P = I²R. This calculator reports the output voltage, the divider current, and the power burned in each resistor so you can confirm both parts stay within their wattage rating.

The Loading Effect

The clean V_out equation only holds when nothing draws current from the tap. As soon as you connect a load R_L across R2, R2 and R_L form a parallel combination, pulling V_out lower than calculated. The rule of thumb: keep the load resistance at least 10× R2 so the error stays under about 10%, or buffer the divider with an op-amp follower. A divider is a poor power supply - it works for biasing high-impedance inputs (ADC reference, transistor base, sensor signal conditioning) but not for sourcing real current.

Choosing Resistor Values

Pick the ratio R2 ÷ (R1 + R2) to set the voltage, then pick the magnitude to set the current. Larger resistors (tens of kΩ) waste less power and extend battery life, but make the divider more sensitive to loading and to input bias current. Smaller resistors stiffen the output but burn more power continuously. For a 3.3 V tap from 5 V battery monitoring, values like R1 = 1.5 kΩ and R2 = 3 kΩ balance accuracy and drain. Use 1% resistors when the exact ratio matters.

Limitations & Related Tools

This calculator assumes ideal resistors, a stiff source, and no load current. Real designs must account for resistor tolerance, temperature coefficient, and the loading effect described above. For level-shifting that must drive a load, use an op-amp or a dedicated reference IC instead. Related tools: LED series resistor calculator, Ohm's law calculator, and the 555 timer calculator for timing circuits that often pair with dividers.

Loading Effects and Impedance Matching

The unloaded voltage divider produces V_out = V_in x R2 / (R1 + R2). When a load resistance R_load is connected across R2, the effective lower resistance becomes R2 in parallel with R_load. This parallel combination is always less than R2 alone, so the output voltage drops below the unloaded value. The loaded output is V_out,loaded = V_in x (R2 || R_load) / (R1 + (R2 || R_load)). The error increases as R_load approaches R2 and becomes negligible when R_load is much larger than R2.

Thevenin Equivalent

Any resistive voltage divider can be replaced by its Thevenin equivalent: a voltage source V_th equal to the open-circuit output voltage, in series with a resistance R_th equal to R1 in parallel with R2 (written R1 || R2). For a standard divider, V_th = V_in x R2 / (R1 + R2) and R_th = R1 || R2 = R1 x R2 / (R1 + R2). The Thevenin resistance is the output impedance of the divider. For the loaded output voltage to stay close to V_th, the load must be large compared to R_th, which leads to the practical rule of thumb that R_load should be at least 10 times R2 to keep the output error below about 10%.

Op-Amp Buffer

When the divider must drive a low-impedance load without sagging, a unity-gain op-amp voltage follower (buffer) solves the problem completely. The op-amp input draws essentially no current (typically nano-amperes for CMOS inputs), so it does not load the divider. The op-amp output can source tens or hundreds of milliamps depending on the device, presenting a low-impedance source to the load. A single-supply op-amp (such as the LM358 or TLV271) powered from the same supply as the divider is all that is needed. The buffer adds one IC but eliminates the need to calculate loading effects or use impractically low resistor values.

Practical Applications of Voltage Dividers

Voltage dividers appear in virtually every electronic circuit, from the simplest hobby project to sophisticated measurement systems.

Level Shifting for ADC

Microcontrollers with 3.3 V ADC inputs cannot directly measure 5 V signals. A resistor divider with R1 = 47 kΩ and R2 = 100 kΩ gives a ratio of 100 / 147 = 0.68, which maps a 5 V signal to 3.4 V, just within the ADC input range. The ADC input impedance of most microcontrollers is high enough (megaohms) that it does not significantly load the divider at these values. For battery voltage monitoring, a divider with a low quiescent current (using megaohm values) allows continuous measurement without significant battery drain.

Bias Point Setting

Biasing a transistor amplifier requires setting the base voltage to a stable point between supply and ground, typically at half the supply for maximum signal swing. A voltage divider from Vcc to ground sets this DC operating point. The transistor base draws only microamperes of bias current, which is negligible compared to the divider current if the divider resistors are in the tens of kilohm range. Op-amp non-inverting inputs are biased with voltage dividers for single-supply operation, setting the input reference voltage to Vcc/2 for rail-to-rail signal processing.

Reference Voltages

Precision voltage dividers with low-tolerance, low-temperature-coefficient resistors produce accurate sub-reference voltages from a precision voltage reference IC. For example, to generate 1.65 V from a 3.3 V reference, equal resistors produce the exact midpoint. Using 0.1% tolerance, 25 ppm/°C resistors keeps the ratio accuracy within 0.2% over a wide temperature range. This approach is simpler and cheaper than adding a second voltage reference IC.

Resistive Sensors

NTC (negative temperature coefficient) thermistors change resistance with temperature and are almost always used in a voltage divider configuration. A fixed resistor in series with the thermistor converts the resistance change into a voltage change readable by an ADC. Choosing the fixed resistor equal to the thermistor resistance at the midpoint of the temperature range of interest maximizes the sensitivity of the voltage output to temperature changes. Resistance temperature detectors (RTDs), light-dependent resistors (LDRs), and strain gauges are similarly used in voltage dividers to convert resistance changes into measurable voltage signals.

Potentiometer

A potentiometer is a continuously variable voltage divider: a resistive element with a sliding wiper that can be positioned anywhere between 0 and 100% of the total resistance. Audio volume controls use potentiometers to vary the signal amplitude; the human perception of loudness follows a logarithmic scale, so audio pots use a logarithmic (audio-taper) resistance profile rather than a linear one. Position sensors (string potentiometers, rotary position encoders) use the wiper voltage as an analog measure of mechanical position. Digital potentiometers replace mechanical wipers with a resistor ladder and switches, allowing software control of the division ratio.

High-Frequency and High-Voltage Considerations

At high frequencies, every resistor has parasitic capacitance across it, and the connecting traces and wires add inductance. A simple resistive voltage divider behaves correctly only at DC and low frequencies; above a few hundred kilohertz, these parasitics cause the divider ratio to deviate from its nominal value.

An oscilloscope 10x probe is a precision compensated voltage divider. It uses a 9 MΩ resistor in the probe tip and a 1 MΩ input on the oscilloscope, giving a 10:1 voltage division ratio. To maintain this ratio across the full probe bandwidth (often 500 MHz or more), a small trimmer capacitor in parallel with the 9 MΩ tip resistor must be adjusted to exactly match the capacitance ratio of the oscilloscope input. This compensation ensures the probe has a flat frequency response: without it, high-frequency signals would be attenuated differently than low-frequency signals, causing waveform distortion.

High-Voltage Dividers

High-voltage DC measurement requires series resistors that stack the voltage drop across multiple components. A 100 kV measurement might use 100 x 1 MΩ resistors in series to keep each resistor well within its 1 kV voltage rating. The divider tap point provides a fraction of the input voltage for measurement by a standard instrument. Matched temperature coefficients are essential: if the top resistors in the stack have a different temperature response than the bottom resistors, the division ratio changes with temperature, introducing measurement error. At very high voltages, the self-capacitance of each resistor becomes significant at higher frequencies, and the design of precision HV dividers for impulse voltage measurement requires careful control of parasitic capacitances.

Power Dissipation

Total power dissipated in a voltage divider is P = V_in² / (R1 + R2), split between the two resistors in proportion to their values. Low resistor values improve divider stiffness (less sensitivity to load) but increase quiescent power drain. High resistor values reduce power draw but make the divider more susceptible to loading by the measurement circuit or connected load. For battery-powered designs, resistor values in the 100 kΩ to 1 MΩ range are typical, limiting divider current to 3-50 µA. For precision reference applications where a stiff source is needed, resistors in the 1-10 kΩ range are used, accepting the higher continuous power draw in exchange for accuracy.

Frequently Asked Questions

How do I choose R1 and R2 values?

Start by setting the desired output ratio: R2 / (R1 + R2) = V_out / V_in. This gives one equation in two unknowns, so you have freedom to choose the scale. The guiding constraints are: the divider current should be small enough not to drain the power source (so R1 + R2 should be large), but large enough that the load impedance R_load is at least 10 times R2 to minimize loading error. For general purpose bias and level-shifting circuits, the 1-100 kΩ range for R2 balances these requirements well. For battery-powered sensors with light loads, 100 kΩ to 1 MΩ is more appropriate. Always verify the power dissipation in each resistor stays within its rated wattage.

Can a voltage divider be used to step down AC voltage?

Yes, a resistive voltage divider works for AC as well as DC. The ratio V_out/V_in = R2 / (R1 + R2) applies to AC peak, RMS, or any other amplitude measure since both voltages scale together. However, for AC the situation is complicated by capacitive and inductive parasitics that change the effective impedance at higher frequencies. For power-frequency AC (50-60 Hz) measurement, a simple resistive divider works well as long as the resistors can handle the voltage and power. For galvanic isolation, use a transformer. For signals where the divider ratio must remain accurate across a wide bandwidth, use a compensated divider with matched RC time constants in each leg.