Capacitor Size
Calculate the required capacitor size (capacitance) from start-up energy and voltage using C = 2E / V². Free online capacitor sizing calculator with instant results, step-by-step breakdowns, and interactive charts for electronics and physics.
About This Calculator
The Capacitor Size Calculator helps you determine the exact capacitance (capacitor size) required for any application where a capacitor must store a specific amount of start-up energy at a known voltage. It is an essential tool for electronics engineers, hobbyists, physics students, and anyone designing circuits that use capacitors for energy storage, filtering, timing, or power conditioning.
The calculator uses the fundamental capacitor energy formula E = ½ × C × V², rearranged to solve for capacitance: C = 2E / V². Enter the required start-up energy (in J, mJ, µJ, nJ, or pJ) and the operating voltage (in V), and the calculator instantly computes the required capacitance in farads, along with the stored charge Q = C × V. Results are displayed with an interactive breakdown and charts showing the relationship between capacitance, charge, energy, and voltage.
When selecting a real-world capacitor, choose the nearest standard value above the calculated capacitance to ensure your circuit meets its energy requirements. Common applications include sizing the capacitor for a motor start circuit, a camera flash unit, a defibrillator, a power supply smoothing capacitor, or an energy storage element in a switched-mode power supply.
Regional Notes
This calculator uses SI units (joules, volts, farads) which are standard worldwide. No region-specific adjustments are needed. The results apply equally in India, the United States, the United Kingdom, and all other countries using the International System of Units.
Frequently Asked Questions
How do I calculate the capacitor size I need?
Enter the start-up energy in joules (or mJ, µJ, nJ, pJ) and the voltage in volts. The calculator uses the formula C = 2E / V² to find the required capacitance. For example, if you need 64 µJ of energy at 16 V, you need a 0.5 µF capacitor.
What is the capacitor size formula?
The capacitor size formula derives from the energy stored in a capacitor: E = ½ × C × V². Rearranging gives C = 2 × E / V², where C is capacitance in farads, E is energy in joules, and V is voltage in volts.
What units are used for capacitor sizing?
Capacitance is measured in farads (F) with common sub-units: millifarads (mF = 10⁻³ F), microfarads (µF = 10⁻⁶ F), nanofarads (nF = 10⁻⁹ F), and picofarads (pF = 10⁻¹² F). Energy can be entered in joules (J), millijoules (mJ), microjoules (µJ), nanjoules (nJ), or picojoules (pJ).
Can I calculate both capacitor size and stored energy?
Yes. Given energy and voltage, the calculator determines the required capacitance, stored charge Q = C × V, and confirms the stored energy. All results include interactive charts and a step-by-step formula breakdown.
What applications use capacitor sizing?
Capacitor sizing is essential for power supply filtering, motor start-up circuits, defibrillators, camera flashes, snubber circuits, energy storage systems, and any electronic design where a capacitor must store a specific amount of energy at a given voltage.
Is this calculator free to use?
Yes, the Capacitor Size Calculator is completely free to use with no registration or hidden charges. You can bookmark or share your calculation via the URL.
How accurate are the capacitor size results?
Results are computed using the exact formula C = 2E / V² and displayed with 4 significant figures in scientific notation. For practical component selection, choose the nearest standard capacitor value available in your required dielectric type.
What is the difference between capacitor size and capacitor energy?
Capacitor size refers to the capacitance value (in farads) needed for a given application, while capacitor energy is the amount of electrical energy (in joules) stored in the capacitor. They are related by E = ½CV² — given any two values, you can calculate the third.