What Is Capacitance?
Capacitance is the ability of an object — typically a capacitor — to store an electric charge. It depends only on the capacitor's geometry and the dielectric material placed between its plates. In most cases, capacitance does not depend on the potential difference or the amount of charge stored.
A parallel plate capacitor is the simplest and most widely studied capacitor design. It consists of two flat conductive plates separated by a dielectric (an electrically insulating material). When voltage is applied, opposite charges accumulate on the plates, creating an electric field between them and storing energy.
Parallel plate capacitors are used in electronic circuits, energy storage systems, signal filtering, timing circuits, and countless other applications in both everyday electronics and industrial equipment.
Capacitance Equation
The capacitance of a parallel plate capacitor is given by:
Where:
- C — Capacitance, measured in farads (F)
- ε₀ — Permittivity of free space ≈ 8.854187817 × 10−12 F/m
- εr — Relative permittivity (dielectric constant) of the material between the plates; dimensionless. For vacuum/air: 1.0
- A — Overlapping area of the two parallel plates (m²)
- s — Separation distance between the plates (m)
The combined permittivity is ε = ε₀ × εr, measured in farads per meter (F/m). By default, this calculator uses ε₀ = 8.854 pF/m — the permittivity of vacuum. To change the dielectric, enter the appropriate εr value in the field.
How to Use the Parallel Plate Capacitor Calculator
- Choose unit system — select Metric (SI) for mm/cm/m units, or American for inches/feet.
- Enter plate area (A) — the overlapping area of the two parallel plates. Select your preferred unit from the dropdown (mm², cm², m², in², ft²).
- Enter plate separation (s) — the distance between the two plates. Select the unit (mm, cm, m, in, ft).
- Enter dielectric constant (εr) — use 1.0 for air or vacuum. Enter a higher value for other dielectric materials (see the table below).
- Optionally enter voltage — if a voltage is applied across the plates, the calculator will also compute the energy stored (E = ½CV²) and the charge on the plates (Q = CV).
- Click Calculate Capacitance to see the results instantly.
Dielectric Materials
The dielectric constant (relative permittivity εr) determines how much the capacitance increases compared to vacuum. Common materials and their εr values:
| Material | εr (approx.) |
|---|---|
| Vacuum / Air | 1.0 |
| Polyethylene (PE) | 2.2 – 2.4 |
| Rubber | 2 – 3 |
| Paper | 3.0 – 3.5 |
| Mica | 3 – 6 |
| Glass | 5 – 10 |
| Silicon | 11.7 |
| Water (pure) | ~80 |
| Barium Titanate (BaTiO₃) | 1200 – 10000 |
Unit Systems: Metric SI vs American
The calculator supports both unit systems:
- Metric (SI): mm², cm², m² for area; mm, cm, m for separation. Standard in science and engineering worldwide.
- American/Imperial: in², ft² for area; in, ft for separation. Used in the United States.
All inputs are internally converted to SI meters (m) and square meters (m²) before the calculation, so results are always consistent regardless of the unit system chosen.
Energy and Charge (Optional Voltage Input)
When a voltage V is applied across the capacitor, the calculator also computes:
- Energy stored (E): E = ½ × C × V² (in joules, mJ, µJ, etc.)
- Charge stored (Q): Q = C × V (in coulombs, µC, nC, pC, etc.)
Capacitors in Series and Parallel
Parallel Connection
When capacitors are connected in parallel, the total capacitance is the sum of individual capacitances:
Parallel connection increases total capacitance. Each capacitor stores charge independently.
Series Connection
When capacitors are connected in series, the reciprocals of capacitances add:
Series connection decreases total capacitance. For two equal capacitors in series: Ctotal = C / 2.
What Next?
Once you know your capacitance, you may want to explore related calculators:
- Spherical Capacitor Calculator — for two concentric spherical conducting shells
- Capacitor Energy Calculator — calculate energy stored given capacitance and voltage
- Capacitive Reactance Calculator — find how a capacitor responds to AC signals (X = 1/(2πfC))
- RC Circuit Calculator — time constant, cutoff frequency, and charging time
- RLC Circuit Calculator — resonant frequency, Q-factor, and bandwidth
Frequently Asked Questions
What is capacitance measured in?
Capacitance is measured in farads (F), named after physicist Michael Faraday. One farad is a very large unit — most practical capacitors have capacitances in the range of picofarads (pF, 10−12 F), nanofarads (nF, 10−9 F), or microfarads (µF, 10−6 F).
How does plate area affect capacitance?
Capacitance is directly proportional to the plate area (A). Doubling the plate area doubles the capacitance. Larger plates can store more charge for a given voltage, which is why high-capacitance capacitors often use very large total plate areas, achieved by rolling or stacking thin sheets.
How does plate separation affect capacitance?
Capacitance is inversely proportional to the plate separation (s). Halving the separation doubles the capacitance. This is why thin dielectric layers are used in high-capacitance capacitors — but there is a limit, as reducing the gap too much causes dielectric breakdown.
What is dielectric breakdown?
Every dielectric material has a dielectric strength — the maximum electric field it can withstand before it breaks down and conducts electricity. Breakdown voltage depends on the material and the gap thickness. Exceeding this voltage destroys the capacitor. Always check the voltage rating of a real capacitor.
What is the permittivity of vacuum?
The permittivity of free space (vacuum) is ε₀ = 8.8541878128 × 10−12 F/m (farads per meter). This is a fundamental physical constant. For air, the permittivity is virtually identical to ε₀ (εr ≈ 1.0006), so air is often treated as vacuum in calculations.
Why does a capacitor block DC but pass AC?
A capacitor charges up quickly when a DC voltage is applied, and once fully charged, no further current flows. However, with an alternating (AC) voltage, the capacitor continuously charges and discharges, allowing current to flow effectively. This makes capacitors essential in AC signal coupling, filtering, and timing applications.