Free Capacitor Size Calculator

This capacitor size calculator is a practical online tool that helps determine the required capacitance for an application when the start‑up energy and operating voltage are known. Acting as both a capacitance calculator and a capacitor energy calculator, it also functions as a capacitor voltage calculator by allowing you to rearrange the key formulas. Whether you are designing motor start circuits, power supply filters, or timing elements, this electronics calculator simplifies the process of finding the right component.

The Meaning of Capacitance

Capacitance quantifies how much electric charge a capacitor can store per unit of voltage. The formal relationship is expressed as:

$$C = \dfrac{Q}{V}$$

where:

• $C$ = capacitance in farads (F)
• $Q$ = charge in coulombs (C)
• $V$ = voltage in volts (V)

A higher capacitance means the capacitor can hold more charge at a given voltage. Several construction parameters determine the capacitance value:

• Plate area – larger plates increase charge storage capacity.
• Plate separation – closer plates create a stronger electric field and higher capacitance.
• Dielectric material – materials with high permittivity (e.g., ceramic, tantalum) allow greater charge storage.

In most practical circuits, capacitance values are expressed in microfarads ($\mu\text{F}$), nanofarads ($\text{nF}$), or picofarads ($\text{pF}$) because the farad is extremely large.

Interpreting Capacitor Codes

Capacitors are often marked with a three‑digit code that encodes the capacitance in picofarads. The first two digits are the significant figures, and the third digit is the multiplier (power of 10). For instance, code 472 means $47 \times 10^{2} = 4700\ \text{pF}$. The following table lists common codes and their equivalents:

Code	pF	nF	μF
100	10	0.01	0.00001
220	22	0.022	0.000022
331	330	0.33	0.00033
102	1,000	1	0.001
152	1,500	1.5	0.0015
472	4,700	4.7	0.0047
562	5,600	5.6	0.0056
333	33,000	33	0.033
224	220,000	220	0.22
225	2,200,000	2,200	2.2

(For a complete list and a decoding tool, refer to a dedicated capacitor code calculator.)

How the Start‑up Energy Determines Capacitor Size

The calculator is built around the energy that a capacitor must supply during start‑up (e.g., when an electric motor begins to turn). The stored energy is given by:

$$E = \frac{1}{2} C V^{\,2}$$

which can be rearranged to solve for the required capacitance:

$$C = \frac{2E}{V^{\,2}}$$

In these formulas, $E$ is the start‑up energy in joules, $C$ is the capacitance in farads, and $V$ is the working voltage in volts. The calculator accepts the energy and voltage values and instantly returns the necessary capacitance.

Worked Example

Consider a motor that needs a start‑up energy of $64\ \mu\text{J}$ and runs at $16\ \text{V}$. Using the capacitance formula:

$$C = \dfrac{2 \times 64 \times 10^{-6}}{16^{2}} = \dfrac{128 \times 10^{-6}}{256} = 0.5 \times 10^{-6}\ \text{F} = 0.5\ \mu\text{F}$$

A capacitor of at least $0.5\ \mu\text{F}$ (with an appropriate voltage rating) should be selected.

Energy Storage and Release

Capacitors store energy by physically separating opposite charges on two conductive plates. When a voltage is applied, electrons migrate to one plate, leaving the other plate with a positive charge. This separation creates an electric field that holds energy. Upon discharge, the charge flows back, delivering the stored energy to the circuit. This behavior is used in applications ranging from simple smoothing filters to high‑energy defibrillators.

When several capacitors are combined, the total capacitance changes: series connections reduce capacitance, and parallel connections increase it. The energy‑storage formula applies individually to each capacitor; the overall circuit behavior must consider the combined capacitance and the voltage distribution.

Practical Tips

• Always choose a capacitor with a voltage rating higher than the maximum voltage it will encounter.
• Use the start‑up energy calculated from your motor’s specification or measure it to obtain accurate results.
• For series or parallel networks, first compute the equivalent capacitance, then apply the same energy formula to find the total stored energy.

By understanding these principles, you can confidently select the correct capacitor for your project, ensuring reliable operation and efficient energy use.