Capacitor Size Interactive Calculator

If you miss the mark on capacitor sizing—wrong voltage rating, wrong footprint, or not enough energy headroom—you’ll hit snags long before you ever build a prototype. This calculator gives you numbers for stored energy, body size, energy density, and reactance using the specs you have on hand. Practical in power supply, automotive, and industrial PCB work, where space and margins are tight and mistakes are expensive. Key formulas, example calculations, and real engineering context are below.

What is capacitor sizing?

Capacitor sizing means selecting a device with the right capacitance, voltage rating, and case outline to fit both what your circuit needs electrically and the space you actually have. Go too big and you waste board area and money; underestimate and you might see early failures.

Simple Explanation

A capacitor acts like a small bucket for electrical charge: bigger buckets store more, but also take up space you may not have available. Sizing is simply picking the smallest bucket that reliably handles the job—no extra, no shortage.

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Physical Dimensions Diagram

Capacitor Size Calculator

How to Use This Calculator

1. Pick what you want to calculate (energy, volume, required capacitance, etc.).
2. Enter the numbers you have—capacitance, voltage, dimensions, or frequency—depending on what you’re solving for.
3. If you want to see it work first, use the “Try Example” button to load a sample problem.
4. Click Calculate to get the result.

Key Equations

Here’s the core energy equation for a capacitor.

Where:

• E = Energy stored (Joules)
• C = Capacitance (Farads)
• V = Voltage (Volts)
• Q = Charge (Coulombs)
• D = Diameter (mm)
• H = Height (mm)
• r = Radius (mm)
• X_C = Capacitive reactance (Ohms)
• f = Frequency (Hz)
• ρ_E = Energy density (J/cm³)

Simple Example

A 1000 μF capacitor charged to 5V stores the following energy:

• Capacitance: 1000 μF
• Voltage: 5 V
• Energy = ½ × 0.001 F × 5² = 0.0125 J = 12.5 mJ
• Charge stored: 1000 μF × 5 V = 5000 μC

Theory & Practical Applications

Physical Package Standards and Selection Criteria

Capacitor sizes aren’t arbitrary. Vendors use package codes: aluminum electrolytics get diameter-height in mm (like 10×20), ceramic chip capacitors use EIA/metric sizing (0402, 0603, etc). The choice is rarely just about electricals—cap value, voltage, ESR, PCB area, and heat all come into play at once.

One thing many overlook: voltage derating. Run aluminum electrolytics close to their voltage limit (say, over 85%), and you’ll burn out the electrolyte much faster—sometimes down to a quarter of rated life, especially in hot places like under a vehicle’s hood. Most engineers play it safe and go for half the rated voltage—so use a 50V device for a 25V rail. This usually means a bigger cap, and higher cost.

Energy Storage Density and Technology Comparison

Energy density is mostly a function of what’s inside the cap. Aluminum electrolytics sit down at about 0.01-0.05 J/cm³—fine for power supplies with room to spare. Tantalum types go up tenfold, which is why you see them in handhelds, even though they’re costly. Ceramics (X7R/X5R) are in between, but the actual value drops off as voltage and temperature rise—so don’t count on that "10 μF" at rated voltage: at 24V you might only get 3-4 μF. Nonlinear dielectrics like C0G/NP0 are much more stable, but you don’t get big values in practical sizes. Supercapacitors go well higher (1–10 J/cm³), nearing battery levels, but their use case is different—millions of charge cycles, not hours of storage.

If you need to crunch numbers for parallel caps or power loss, check the free engineering calculators library.

Thermal Management and Package Size Interaction

How big a cap you need depends just as much on its heat as its stats sheet. Ripple current causes power loss from ESR, and that raises temperature. The hotter it runs, the shorter it lasts. A wider cap spreads the heat better (lower θ_JA). Doubling the diameter, for example, can nearly halve the temperature rise. But sheer size can't always fix things: at high ripple and modest ESR, heat rise can quickly hit levels that will kill the part unless you parallel devices, use better dielectrics (polymers with 10 mΩ ESR, for example), or add cooling. Smaller caps often mean higher prices, so these trade-offs add up fast in final BOM and layout.

Worked Example: Backup Power Capacitor Bank Sizing

Let’s say you need to ride through a 5-second power outage at 12V and 250mA, starting from 13.5V (lowest supply) to 12V. Figure out how much cap you need, the right voltage rating, and the footprint.

Step 1: Calculate total energy requirement

Energy = Power × Time = (12V × 0.25A) × 5s = 15 Joules

The voltage drops from 13.5V to 12V—so you only use that slice of voltage for storage.

Step 2: Determine required capacitance from energy equation

For a capacitor discharging from V₁ to V₂, energy delivered is:

E = ½C(V₁² - V₂²) = ½C(13.5² - 12²) = ½C(38.25)

15J = ½C(38.25)

C = 30J / 38.25 = 0.784F = 784,000μF

Step 3: Apply voltage derating and select standard values

With 50% derating for reliability, required voltage rating = 13.5V × 2 = 27V minimum. Go with a 35V cap for margin, as 16V and 25V won’t cut it. At 35V, common values are 10,000–47,000μF.

Using 22,000μF units: 784,000μF / 22,000μF = 36 caps in parallel (rounding up).

Step 4: Calculate individual capacitor dimensions and array layout

Each 22,000μF 35V aluminum electrolytic is about 18mm diameter × 40mm tall. For PCB layout, you’ll need about 441mm² per cap including spacing, so a 6×6 array covers roughly 126mm on a side.

Step 5: Verify discharge characteristics and ripple current capability

Total cap bank is about 792,000μF, and each cap sees a few mA—far below its max current limit. The time constant is long compared to 5 second ride-through, so the voltage drop calculation is reliable. The bank gets just beneath 12V at the end, so you may want to upsize by one more cap.

Step 6: Iteration—increase capacitance to meet specification

A little more capacitance (eight-hundred thirty thousand μF total, 38 caps) squares the math, so you don’t sag below 12V. At $1.20–1.60 per cap, your total cost is $45–60, and board area is in the 130mm × 150mm range.

PCB Layout Considerations for Large Capacitor Arrays

Big electrolytic caps can break at their leads from vibration, especially in automotive. The taller and heavier the part, the more strain you get—far beyond what the solder joint can handle after millions of cycles unless you brace it well. Through-hole mounting is far more robust than SMD for big caps. For high-vibration situations, glue the base with silicone: this spreads out the force, raising durability several times over.

Frequency-Dependent Impedance and Multi-Capacitor Networks

Every real cap has stray inductance, so above a certain frequency, its impedance rises instead of falls—it acts more like an inductor. For a 470μF electrolytic with 20nH of leads, this turnover happens in the tens of kHz. That’s why you mix cap types: electrolytics for low frequencies, ceramics close to the load for high frequency. Larger caps are more inductive, so keep high-frequency ceramics as close as you can; don’t rely on a big aluminum can for your MHz bypassing.

Frequently Asked Questions

📹 Video Walkthrough — How to Use This Calculator

📹 Video Walkthrough — How to Use This Calculator