Capacitance Calculator
Calculate capacitance from charge and voltage (C = Q/V) or from plate geometry and dielectric constant (C = ε₀εᵣA/d). Determine the capacitance of parallel plate capacitors and stored charge. See also our Capacitor Energy Calculator and RC Circuit Calculator.
How to Calculate Capacitance
Capacitance is the ability of a component to store electrical charge. It is measured in farads (F), where one farad equals one coulomb of charge stored per volt of potential difference. In practice, most capacitors are measured in microfarads (µF), nanofarads (nF), or picofarads (pF) because one farad is an extremely large capacitance.
There are two fundamental ways to calculate capacitance: from the charge-voltage relationship (C = Q/V), which is the definition of capacitance, and from the physical geometry of a parallel plate capacitor (C = ε₀εᵣA/d), which relates capacitance to plate area, separation distance, and the dielectric material between the plates.
Capacitance Formulas
Definition of Capacitance:
C = Q / V
Where: C = capacitance (F), Q = charge (C), V = voltage (V)
Parallel Plate Capacitor:
C = ε₀ × εᵣ × A / d
Where: ε₀ = 8.854×10⁻¹² F/m, εᵣ = relative permittivity
A = plate area (m²), d = plate separation (m)
Energy Stored:
E = ½CV² = ½QV = Q²/(2C)
Capacitors in Parallel:
Ct = C1 + C2 + C3 + ...
Capacitors in Series:
1/Ct = 1/C1 + 1/C2 + 1/C3 + ...
Example Calculation
A capacitor stores 0.001 coulombs of charge at 5 volts:
C = Q/V = 0.001C / 5V = 0.0002 F = 200 µF
Energy stored = ½CV² = ½ × 0.0002 × 25 = 0.0025 J = 2.5 mJ
For a parallel plate capacitor with same capacitance:
If εᵣ = 1000 (ceramic), d = 0.01mm = 10⁻⁵m:
A = C×d/(ε₀×εᵣ) = 0.0002×10⁻⁵/(8.854×10⁻¹²×1000)
A = 0.000226 m² = 2.26 cm² (small ceramic capacitor)
Dielectric Constants Reference Table
| Material | Relative Permittivity (εᵣ) |
|---|---|
| Vacuum | 1.0 |
| Air | 1.0006 |
| Paper | 2.0-4.0 |
| Polyester (Mylar) | 3.2 |
| Polypropylene | 2.2 |
| Mica | 5.0-8.0 |
| Glass | 4.0-10.0 |
| Ceramic (Class I) | 6-200 |
| Ceramic (Class II) | 200-14000 |
| Tantalum oxide | 27 |
| Aluminum oxide | 8.5 |
| Water (pure) | 80 |
Frequently Asked Questions
What is capacitance?
Capacitance is the ability to store electrical energy in an electric field. A capacitor with 1 farad of capacitance stores 1 coulomb of charge when 1 volt is applied. Capacitance depends on the physical construction (plate area, separation, dielectric) and is independent of the applied voltage or stored charge.
What is a dielectric and why does it matter?
A dielectric is an insulating material placed between capacitor plates. It increases capacitance by a factor equal to its relative permittivity (εᵣ). The dielectric polarizes in the electric field, effectively reducing the field strength and allowing more charge to be stored at the same voltage. Higher εᵣ materials enable smaller capacitors for the same capacitance.
How do I convert between µF, nF, and pF?
1 F = 10⁶ µF = 10⁹ nF = 10¹² pF. To convert: 1 µF = 1000 nF = 1,000,000 pF. For example, 0.1 µF = 100 nF = 100,000 pF. Electrolytic capacitors are typically in µF (1-10,000 µF), ceramic capacitors in pF to µF, and film capacitors in nF to µF.
Why are large capacitance values difficult to achieve?
Large capacitance requires large plate area, small separation, and high dielectric constant. Physical size limits plate area, and thin dielectrics have lower breakdown voltage. Supercapacitors achieve farads by using porous carbon electrodes (enormous surface area) with nanometer-scale separation, but are limited to low voltages (2.5-3V per cell).
How does temperature affect capacitance?
Temperature stability varies by capacitor type. NP0/C0G ceramics are extremely stable (±30 ppm/°C). X7R ceramics vary ±15% over -55 to +125°C. Y5V ceramics can lose 80% of capacitance at temperature extremes. Electrolytic capacitors lose capacitance at low temperatures and have shorter life at high temperatures. Choose the appropriate type for your temperature range.
What determines a capacitor's voltage rating?
The voltage rating is determined by the dielectric breakdown strength of the insulating material and its thickness. Exceeding the rated voltage causes dielectric breakdown (short circuit), permanently damaging the capacitor. Always select a capacitor with voltage rating at least 50% above the maximum expected voltage for reliability. Derating is especially important for ceramic capacitors, which lose capacitance under DC bias.
Types of Capacitors
Different capacitor technologies offer different trade-offs between capacitance, voltage, size, stability, and cost. Ceramic capacitors (pF to 100µF) are small and inexpensive but vary with voltage and temperature. Electrolytic capacitors (1µF to 100,000µF) offer high capacitance but are polarized and have limited life. Film capacitors (100pF to 100µF) offer excellent stability and self-healing properties. Tantalum capacitors (0.1µF to 1000µF) are compact but can fail catastrophically if overvoltaged.
Practical Applications
- Power supply filtering: Large capacitors smooth rectified AC into DC
- Decoupling: Small capacitors near ICs filter high-frequency noise from power rails
- Timing circuits: RC time constant determines oscillator frequency and delay times
- Energy storage: Supercapacitors provide backup power and regenerative braking
- Signal coupling: Capacitors pass AC signals while blocking DC bias
- Power factor correction: Capacitor banks compensate inductive reactive power