How do I use the voltage divider calculator?
This is a voltage divider calculator – a comprehensive but simple tool that helps you evaluate the output signal (i.e., voltage) that we obtain in a single voltage divider, often used in voltage regulators. Using it is easy:
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Select the divider type. The calculator covers the three basic passive dividers:
- Resistive divider (R–R) — for DC and AC circuits;
- Capacitive divider (C–C) — for AC circuits; and
- Inductive divider (L–L) — for AC circuits.
- Choose your unit system — metric (SI) or American. Electrical quantities use the same SI base units worldwide (volts, ohms, farads, henries), so both systems share identical formulas; the drop-down selectors let you enter convenient sub-units such as kV, mV, kΩ, MΩ, μF, nF, pF, mH and μH.
- Enter the input voltage Vin and the two component values (Z1 and Z2).
- Click Calculate. The tool returns the output voltage Vout, the division ratio (as a decimal, a percentage and in decibels), and — for the resistive divider — the current and the power dissipated in each resistor.
Read on to learn what a voltage divider is, find out the basic voltage divider formula, and how it extends to various equations for different voltage divider types. 🙋 You may also be interested in using our voltage regulation calculator.
What is a voltage divider?
A simple voltage divider is a part of a linear circuit that changes an input voltage (V1) into an output voltage (V2), which is a different value. Since the circuit is a passive one, the ratio V2/V1 is never greater than 1.
A generic voltage divider consists of two impedances, Z1 and Z2, connected in series across the input voltage; the output is taken across Z2. The impedances can be related to the resistance R, the capacitance C, or the inductance L. We can distinguish a few basic types of voltage dividers for which we can apply the voltage divider rule:
- Resistive divider, RR;
- Capacitive divider, CC;
- Inductive divider, LL;
- RC and CR dividers (also known as RC, CR filters);
- RL and LR dividers (also known as RL, LR filters); and
- CL and LC dividers (also known as CL, LC filters).
Voltage divider rule
The principle behind voltage dividers is that the current passing through several elements connected in series is constant, but the voltage divides somehow between them. We need to apply Ohm's law to our circuit to find the exact values. Before doing that, we must state one crucial point:
The RR composition is the only one applicable to DC (direct current) circuits. In these cases, we can treat any inductance as a wire with zero resistivity, and capacitances work as a gap in a circuit, so they have infinite resistance. For the rest, they are all used with AC (alternating current) circuits, and the voltage divider rule is applicable for the maximum value of the potential difference. It may also be helpful to find the phase shift for these voltages.
Voltage divider formula
The general voltage divider equation (or formula) for impedances is as follows:
where:
- Vin — the input (source) voltage;
- Vout — the output voltage measured across Z2;
- Z1 — the first (series) impedance; and
- Z2 — the second impedance, across which the output is taken.
Resistive voltage divider
For a resistive divider, the impedances are simply resistances, so the voltage divider rule becomes:
Because the same current I = Vin / (R1 + R2) flows through both resistors, the total power drawn from the source is P = Vin × I, and each resistor dissipates P = I²R. Example: with Vin = 12 V, R1 = 10 kΩ and R2 = 10 kΩ, the output is Vout = 12 × 10/(10 + 10) = 6 V, exactly half of the input.
Capacitive and inductive voltage divider equations
For a capacitive divider the reactance of a capacitor is XC = 1/(2πfC), so it is inversely proportional to the capacitance. The voltage therefore divides inversely with capacitance — the output taken across C2 is:
For an inductive divider the reactance XL = 2πfL is proportional to the inductance, so the formula mirrors the resistive case:
RC and CR filters
When you combine a resistor and a capacitor, you get a frequency-dependent divider — a filter. In an RC filter (resistor first, output across the capacitor) the output falls as frequency rises: it is a low-pass filter. In a CR filter (capacitor first, output across the resistor) the output rises with frequency: it is a high-pass filter. The corner (cutoff) frequency where the response is −3 dB is fc = 1/(2πRC).
RL and LR filters
Replacing the capacitor with an inductor gives an RL/LR filter. With the output across the resistor it behaves as a low-pass filter; across the inductor it behaves as a high-pass filter. The cutoff frequency is fc = R/(2πL).
CL and LC as resonant circuits
A divider made of a capacitor and an inductor (CL or LC) is special: at the resonant frequency f0 = 1/(2π√(LC)) their reactances cancel. Such a network is the basis of tank (resonant) circuits used in oscillators, radio tuners and band-pass/band-stop filters, where the divider's output peaks sharply around resonance.
Usage of a voltage divider in more complex circuits
Voltage dividers rarely appear alone. They set the reference voltage of a voltage regulator, bias the base of a transistor, define the gain-setting feedback network of an op-amp, and scale a high voltage down to a level a microcontroller's ADC can safely read. Whenever you need "some fraction of" a voltage, a divider is the simplest answer — apply the voltage divider rule and you obtain that fraction of the input voltage directly.
Applications of voltage dividers
- Reading sensors such as potentiometers, thermistors and photoresistors;
- Level-shifting and attenuating signals before an analog-to-digital converter;
- Setting the output of adjustable voltage regulators and references;
- Biasing transistor amplifier stages;
- Building simple RC, RL and LC filters; and
- Probing high voltages safely with scaled-down measurements.
Pros and cons of voltage dividers
Pros:
- Extremely simple and inexpensive — just two passive components;
- Predictable, easy to calculate with the voltage divider rule; and
- No power supply needed (passive).
Cons:
- The output voltage sags when a load draws current (the load appears in parallel with Z2);
- Resistive dividers waste power as heat — poor for high-current or battery designs; and
- They attenuate only (passband gain ≤ 1) — they cannot amplify a signal.
Current divider
The dual of the voltage divider is the current divider. Where a voltage divider splits voltage between series elements, a current divider splits current between parallel elements. For two parallel resistors the current through R1 is I1 = Itotal × R2/(R1 + R2) — note that the resistor ratio is "swapped" compared with the voltage divider, because more current flows through the path of lower resistance.
FAQs
- What is the basic voltage divider formula?
- For a resistive divider, Vout = Vin × R2/(R1 + R2), where the output is taken across R2. More generally, replace the resistances with impedances Z1 and Z2.
- Can the output be larger than the input?
- No. A passive voltage divider can only attenuate, so Vout/Vin ≤ 1. To increase a voltage you need an active circuit such as an op-amp amplifier or a boost converter.
- Why does my divider output drop when I connect a load?
- The load resistance appears in parallel with Z2, lowering the effective lower impedance and therefore the output. Keep the divider resistances much smaller than the load (a factor of 10 or more) to minimise this loading effect.
- Does the unit system change the result?
- No. Electrical engineering uses SI units (V, Ω, F, H) worldwide, so the American and metric systems give the same output voltage. The calculator converts your chosen sub-units (kΩ, μF, mH, …) to base SI units automatically.
- How do I pick resistor values for a given output?
- Choose the ratio R2/(R1 + R2) to match the fraction of Vin you want, then scale both resistors up or down together to trade lower power consumption (large values) against stiffer output under load (small values).