Calculate capacitive reactance Xc = 1/(2πfC), impedance, phase angle, and cutoff frequency. Frequency response and standard capacitor tables.
Capacitive reactance (Xc) is the opposition a capacitor presents to alternating current. Unlike resistance, it depends on frequency: Xc = 1/(2πfC). At DC (f = 0), a capacitor is an open circuit with infinite reactance. As frequency increases, reactance drops — the capacitor passes more current.
This frequency-dependent behavior is the basis of filters, coupling networks, and power factor correction. A 10 µF capacitor has 265 Ω reactance at 60 Hz but only 16 Ω at 1 kHz. Combined with resistance, the impedance magnitude is Z = √(R² + Xc²) and the current leads the voltage by the phase angle φ = arctan(Xc/R).
This calculator computes reactance, impedance, phase angle, power factor, and cutoff frequency for series RC circuits. It includes frequency response tables, standard capacitor reactance lookup, and the ability to solve for capacitance or frequency given a target reactance. Check the example with realistic values before reporting. That makes it useful both for one-off RC calculations and for scanning how the same capacitor behaves across a practical frequency range.
Capacitive reactance calculations are essential for filter design, coupling/decoupling capacitor selection, power factor correction, and impedance matching. While the formula is straightforward, frequency response analysis across a range of values and comparison with standard capacitor sizes requires tedious manual computation.
This calculator provides instant results with multiple solve modes, frequency response tables, and standard capacitor lookup — saving time for electronics engineers, audio designers, and physics students.
Xc = 1/(2πfC). Impedance: Z = √(R² + Xc²). Phase: φ = −arctan(Xc/R). Cutoff: f_c = 1/(2πRC). Current leads voltage in capacitive circuits.
Result: 15.9 Ω reactance, 1000.1 Ω impedance
At 1 kHz, a 10 µF capacitor has Xc = 1/(2π × 1000 × 10×10⁻⁶) = 15.9 Ω. With 1000 Ω series resistance, Z = √(1000² + 15.9²) = 1000.1 Ω. The phase shift is only −0.91°, and the cutoff frequency is 15.9 Hz.
An RC circuit forms a first-order filter with a −20 dB/decade rolloff. In a low-pass configuration (output across C), frequencies below f_c pass through while higher frequencies are attenuated. The transfer function is H(f) = 1/√(1 + (f/f_c)²).
In a high-pass configuration (output across R), frequencies above f_c pass through. These simple filters are the building blocks of audio crossover networks, anti-aliasing filters, and signal conditioning circuits.
When a capacitor is in series with a resistor, the impedance is a complex number: Z = R − jXc. The magnitude is √(R² + Xc²) and the phase angle is −arctan(Xc/R). This phase shift is critical in power systems — a large phase angle means poor power factor and wasted reactive power.
Power factor correction capacitors are sized to provide capacitive reactance that cancels the inductive reactance of motors and transformers, bringing the phase angle close to zero and the power factor close to 1.
Capacitors come in standard E-series values (E12, E24). When a calculation yields a non-standard value, choose the next available standard size. For filtering, it's generally better to round up (more capacitance = lower reactance = better filtering). For timing circuits, precision is more important, and multiple capacitors may need to be combined to achieve the exact value.
Capacitive reactance (Xc) is the opposition a capacitor provides to AC. It is measured in ohms and decreases with increasing frequency and capacitance: Xc = 1/(2πfC).
At higher frequencies, the capacitor charges and discharges more rapidly, allowing more current to flow. In the limit, at very high frequencies, the capacitor behaves almost like a short circuit.
Resistance dissipates energy as heat; reactance stores and returns energy. Reactance causes a phase shift between voltage and current (90° for a pure capacitor), while resistance keeps them in phase.
The cutoff frequency f_c = 1/(2πRC) is where the reactance equals the resistance (Xc = R). At this point, the output of an RC filter is −3 dB (70.7%) of the input, and the phase shift is −45°.
Reactances add in series (like resistors in series) and combine in parallel using the reciprocal formula (like resistors in parallel). But note that inductive and capacitive reactances partially cancel each other.
By convention, current leads voltage in a capacitor, producing a negative phase angle. In an inductor, current lags voltage (positive phase). At resonance, they cancel, and the circuit appears purely resistive.