Capacitor Calculator Guide: Basics, Formulas & Circuit Design
Quick Answer
- *Charge: Q = C × V. Energy: E = ½CV².
- *RC time constant: τ = R × C. Full charge/discharge at ~5τ.
- *Parallel: capacitances add. Series: reciprocals add (opposite of resistors).
- *Units: 1F = 1,000,000 μF = 10&sup9; nF = 10¹² pF.
How Capacitors Work
A capacitor is two conductive plates separated by an insulator (dielectric). Apply voltage and electrons accumulate on one plate and deplete from the other, creating an electric field that stores energy. Remove the voltage source and the capacitor holds its charge until a discharge path is provided.
Capacitance depends on three physical properties: plate area (bigger = more capacitance), plate separation (closer = more), and dielectric material (higher permittivity = more). This is why electrolytic capacitors, which use an extremely thin oxide layer as the dielectric, achieve large capacitance in a small package.
Essential Formulas
| Formula | Variables | Use |
|---|---|---|
| Q = C × V | Q = charge (coulombs), C = capacitance (farads), V = voltage | Charge stored at a given voltage |
| E = ½ × C × V² | E = energy (joules) | Energy stored in capacitor |
| τ = R × C | τ = time constant (seconds), R = resistance (Ω) | RC circuit timing |
| Xc = 1 / (2πfC) | Xc = capacitive reactance (Ω), f = frequency (Hz) | AC impedance of capacitor |
Series and Parallel Combinations
Parallel: Capacitances simply add. Two 100μF capacitors in parallel give 200μF. This is the most common way to increase total capacitance. The voltage rating stays the same as the lowest-rated capacitor.
Series: 1/C_total = 1/C1 + 1/C2. Two 100μF capacitors in series give 50μF. But the voltage rating effectively doubles. Series connections are used when you need to handle higher voltages than a single capacitor can withstand.
RC Time Constants
The RC time constant (τ) determines how fast a capacitor charges or discharges through a resistor. At 1τ, the capacitor reaches 63.2% of the final voltage. At 3τ: 95%. At 5τ: 99.3% (considered fully charged).
Example: 10kΩ resistor with 100μF capacitor. τ = 10,000 × 0.0001 = 1 second. Full charge takes about 5 seconds. This timing behavior is the basis for LED flashers, debounce circuits, and audio filters.
Capacitor Types
| Type | Range | Voltage | Best For |
|---|---|---|---|
| Ceramic (MLCC) | 1 pF – 100 μF | 6–100V | Decoupling, high-frequency filtering |
| Electrolytic (aluminum) | 0.1 – 47,000 μF | 6–450V | Power supply filtering, bulk storage |
| Tantalum | 0.1 – 1,000 μF | 4–50V | Stable, compact, low ESR |
| Film (polyester/polypropylene) | 100 pF – 10 μF | 50–2000V | Audio, timing, snubbers |
| Supercapacitor | 0.1 – 3,000 F | 2.5–5.5V | Energy backup, pulse current |
Reading Capacitor Values
Small ceramic capacitors use a 3-digit code. The first two digits are the value; the third is the number of zeros to add, giving the value in picofarads. “104” = 10 + 0000 = 100,000 pF = 100 nF = 0.1 μF. “472” = 47 + 00 = 4,700 pF = 4.7 nF.
Electrolytic capacitors print the value and voltage directly on the body: “470μF 25V”. The negative terminal is marked with a stripe. Polarity matters — reversing an electrolytic capacitor can cause it to vent or explode.
Calculate capacitance, charge, and RC timing
Use our free Capacitor Calculator →Frequently Asked Questions
What is a capacitor and what does it do?
A capacitor stores electrical energy in an electric field between two conductive plates. It charges when voltage is applied and discharges into a load. Used for filtering, timing, coupling, and energy storage.
How do you calculate capacitance?
C = Q/V (charge divided by voltage). For parallel plates: C = ε•A/d. Measured in farads, though practical values are in μF, nF, or pF.
How do capacitors combine in series and parallel?
Parallel: capacitances add (C_total = C1 + C2). Series: reciprocals add (1/C_total = 1/C1 + 1/C2). Opposite of resistors.
What is an RC time constant?
τ = R × C. At 1τ, the capacitor is 63.2% charged. At 5τ (99.3%), it’s considered fully charged. A 10kΩ + 100μF circuit has τ = 1 second.
How much energy does a capacitor store?
E = ½ × C × V². Energy scales with the square of voltage. A 1000μF cap at 12V stores 0.072 joules. High-voltage caps can be dangerous even at small capacitance.