Capacitor Calculator

Dielectric preset:

Choose Custom to edit the dielectric constant directly.

Relative permittivity:

Use 1 for air/vacuum; higher values increase capacitance linearly.

Plate area:

Enter one plate area and select the area unit.

Plate separation:

Use the physical dielectric thickness between the plates.

Voltage:

Set the voltage across the plates.

Fringe correction: {{ fringe_correction_pct }}%

Use only when you have an empirical correction for edge effects.

Breakdown strength:

Enter 0 to skip the breakdown margin audit.

kV/mm

Check capacitor inputs

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Quantity	Value	Formula readout	Copy
{{ row.quantity }}	{{ row.value }}	{{ row.readout }}

Check	Status	Engineering note	Copy
{{ row.check }}	{{ row.status }}	{{ row.note }}

Gap scenario	Capacitance	Electric field	Stored energy	Voltage margin	Copy
{{ row.scenario }}	{{ row.capacitance }}	{{ row.field }}	{{ row.energy }}	{{ row.margin }}

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Parallel-plate capacitance describes how much charge two facing conductors can store for each volt applied across them. A larger plate area, a smaller separation, and a stronger dielectric material all raise capacitance, which is why thin insulating films and stacked foils are common in real capacitors.

The estimate is useful when a design sketch needs a quick electrical scale before a part is chosen or a prototype is built. It can show whether a geometry is likely to land in picofarads, nanofarads, or higher, and it links that capacitance to charge, stored energy, and electric field stress at the selected voltage.

Parallel plate capacitor diagram showing plate area, dielectric gap, voltage, capacitance, charge, energy, and field stress relationships

The model is an idealized geometry check. It assumes flat, opposing plates with a uniform dielectric gap, so it cannot replace a datasheet for a wound, multilayer, electrolytic, or high-voltage capacitor. It is best used to understand scale, compare geometry changes, and catch voltage-stress problems early.

Voltage does not change the capacitance of an ideal linear capacitor. It changes the charge stored, the energy stored, and the electric field across the dielectric. That distinction matters because a geometry can have a reasonable capacitance value and still be unsafe at the applied voltage.

Technical Details:

A parallel-plate capacitor is governed mainly by geometry and permittivity. Area gives the plates more surface for charge separation, separation sets the distance across the dielectric, and relative permittivity measures how strongly the dielectric polarizes compared with vacuum.

The ideal result ignores edge fields unless a separate fringing adjustment is added. That simplification is reasonable for teaching, first-pass sizing, and comparisons where the plate dimensions are large compared with the gap. It becomes less reliable when the gap is large, the plates are small, edges dominate, or the dielectric is not uniform.

Formula Core

The core capacitance equation uses vacuum permittivity, relative permittivity, plate area, and separation. Charge and stored energy then follow from the selected voltage.

\begin{array}{lcl} C & = & \frac{ε_{0} ε_{r} A}{d} \times (1 + fringe) \\ Q & = & C V \\ U & = & \frac{1}{2} C V^{2} \\ E & = & \frac{| V |}{d} \end{array}

Here, C is capacitance in farads, Q is charge in coulombs, U is stored energy in joules, and E is electric field. The value of A is converted to square meters, d is converted to meters, and the displayed field is converted to kV/mm for voltage-stress review.

Inputs and how they affect the capacitor estimate
Input	Accepted values	Effect on the estimate
Relative permittivity	Greater than zero, or filled by a dielectric preset	Raises capacitance linearly. Doubling this value doubles capacitance, charge, and stored energy at the same voltage.
Plate area	cm², mm², m², or in²; greater than zero	Raises capacitance linearly through the one-plate area.
Plate separation	mm, um, m, or in; greater than zero	Lowers capacitance as the gap grows and raises field stress as the gap shrinks.
Voltage	Any numeric voltage, positive or negative	Changes charge sign, stored energy, and electric field. Capacitance itself stays geometry-based.
Fringe correction	0% to 25%	Adds a percentage uplift to ideal capacitance for edge-effect estimates when an empirical adjustment is available.
Breakdown strength	0 or higher, in kV/mm	When greater than zero, creates a voltage-margin check from dielectric strength and separation.

Breakdown margin is a simple stress ratio. The estimated breakdown voltage is dielectric strength multiplied by the gap thickness. The margin is that voltage divided by the absolute applied voltage. A margin below 1 means the entered voltage exceeds the entered dielectric-strength estimate.

Voltage margin status bands
Voltage margin	Status shown	How to read it
Not set	Not checked / limit not set	No dielectric-strength value was entered, so the stress audit reports field only.
< 1	Over limit	The applied voltage is above the estimated breakdown voltage.
1 to < 1.5	Thin margin	The voltage is below the estimate, but the buffer is small.
1.5 to < 3	Working margin	The entered assumptions leave a moderate stress buffer.
≥ 3	Comfortable margin	The entered breakdown-strength estimate is several times above the applied voltage.

A 25 cm² polypropylene plate pair with a 0.5 mm gap and 12 V gives about 97.396 pF before fringe adjustment. With a 10% fringing uplift, the same geometry becomes about 107.136 pF, while the electric field remains 0.0240 kV/mm because voltage and gap did not change.

Everyday Use & Decision Guide:

For a first pass, choose the closest Dielectric preset, then enter measured plate area, physical dielectric thickness, and applied voltage. Use Custom when you have a material datasheet value for relative permittivity or when the preset is only a loose teaching match.

Leave Fringe correction at 0% unless you already have a measured or simulated correction. Raising it can be useful for comparing an ideal estimate against a known prototype, but it should not be used to make an uncertain design look better than the plate geometry supports.

Capacitance Ledger is the audit view for capacitance, charge, energy, surface charge density, energy density, field, and fringe adjustment.
Voltage Stress Audit is the place to check breakdown voltage, voltage margin, polarity, and the area-to-gap geometry driver.
Plate Gap Sensitivity shows how capacitance changes as the current gap is swept from half to twice the entered value.
Gap Sweep Table gives the same sweep as rows, including capacitance, electric field, stored energy, and voltage margin.
JSON preserves the current inputs, modeled values, tables, sweep data, and validation messages.

Use the gap sweep before committing to a thickness. Halving the separation roughly doubles capacitance, but it also doubles electric field at the same voltage. That tradeoff is easy to miss when only the headline capacitance looks attractive.

Stop and revise the inputs when the summary says Check values, when the voltage margin is Over limit, or when the field is much higher than the dielectric and manufacturing process can safely support. The result is a geometry estimate, not a certified capacitor rating.

Step-by-Step Guide:

Work from material choice to geometry, then verify voltage stress before exporting values.

Pick Dielectric preset or choose Custom. The preset fills relative permittivity, and it can fill Breakdown strength if that field is still 0.
Enter Relative permittivity if Custom is active or if the preset needs a datasheet adjustment. The summary badge should reflect the entered epsilon r value.
Enter Plate area and its unit. The Capacitance Ledger formula readout converts that area to square meters.
Enter Plate separation and its unit. Check the field badge and Plate Gap Sensitivity when a thinner gap is being considered.
Enter Voltage. The summary line updates stored charge and stored energy, and Voltage Stress Audit reports polarity and field stress.
Open Advanced only when you need Fringe correction or Breakdown strength. A breakdown strength of 0 intentionally skips the margin audit.
If the error panel appears, fix the field named in the message. Relative permittivity, plate area, and plate separation must be greater than zero; fringe correction and breakdown strength cannot be negative.
Review Capacitance Ledger and Voltage Stress Audit before copying rows, downloading CSV, exporting DOCX, saving the chart image, or using the JSON record.

Interpreting Results:

Read the headline capacitance together with the field and voltage margin. Capacitance tells you the storage scale, while the stress outputs tell you whether the same geometry is being pushed close to the entered dielectric-strength limit.

The sign of voltage affects stored charge sign, but stored energy and electric field use magnitude. A negative voltage therefore does not make capacitance negative and does not make the voltage-stress check disappear.

How to interpret major capacitor outputs
Output	Best first use	Verify before trusting
Capacitance	Check whether the geometry is in pF, nF, uF, or mF scale.	Area and gap units, especially cm² versus mm² and mm versus um.
Stored charge	Estimate charge at the entered voltage.	Voltage sign if the downstream circuit cares about polarity.
Stored energy	Compare discharge energy or pulsed-load scale.	Voltage, because energy grows with voltage squared.
Electric field	Check stress across the dielectric thickness.	The real dielectric thickness and any local field concentration near edges or defects.
Voltage margin	Compare applied voltage with the entered dielectric-strength estimate.	The material datasheet, temperature, manufacturing tolerance, aging, and safety factor required for the application.

A comfortable modeled margin does not certify a safe capacitor. It means only that the entered dielectric strength, gap, and voltage produce a favorable ratio. Check datasheets, creepage, contamination, electrode shape, surge conditions, and applicable standards before treating a real assembly as safe.

Worked Examples:

Small film-capacitor sketch

A polypropylene estimate with Plate area set to 25 cm², Plate separation set to 0.5 mm, and Voltage set to 12 V produces a headline near 97.396 pF. The Capacitance Ledger also shows about 1.169e-9 C stored charge and 7.013e-9 J stored energy. With no Breakdown strength entered, Voltage Stress Audit reports that the limit is not set.

Ceramic gap check with margin

For a ceramic class 2 estimate with relative permittivity 1200, 10 cm² plate area, 0.1 mm separation, 50 V, and 8 kV/mm breakdown strength, the headline is about 106.250 nF. The Voltage Stress Audit shows a 0.5000 kV/mm electric field and a 16.00x voltage margin, which lands in the comfortable-margin band for the entered assumptions.

High-voltage paper case near the limit

A paper estimate with relative permittivity 3.5, 100 cm² plate area, 0.05 mm separation, 1000 V, and 16 kV/mm breakdown strength produces about 6.198 nF. The field is 20.0000 kV/mm, and Voltage Stress Audit reports a margin of 0.80x, so the run is over the entered breakdown limit even though the capacitance value may look useful.

Input error recovery

If Plate separation is set to 0, the summary changes to Check values and the error panel says Plate separation must be greater than zero. Enter the actual dielectric thickness before reading Plate Gap Sensitivity or exporting the tables, because capacitance and electric field both depend directly on the gap.

FAQ:

Why does reducing the gap raise capacitance?

The ideal formula divides by plate separation, so a smaller gap raises capacitance. The same smaller gap also raises electric field at the same voltage, which is why the voltage-stress outputs should be checked with the headline value.

Does voltage change the capacitance result?

No. In this ideal linear model, voltage changes stored charge, stored energy, electric field, and voltage margin. Capacitance comes from relative permittivity, plate area, separation, and any fringe correction.

When should I enter breakdown strength?

Enter it when you have a credible dielectric-strength value in kV/mm and want the Voltage Stress Audit to calculate breakdown voltage and margin. Leave it at 0 when the value is unknown.

Why did the calculator show Check values?

One or more inputs failed validation. Relative permittivity, plate area, and plate separation must be greater than zero, while fringe correction and breakdown strength cannot be negative.

Can this replace a capacitor datasheet?

No. The estimate is for an ideal parallel-plate geometry with optional fringe uplift. Real capacitor ratings also depend on construction, electrode shape, dielectric losses, temperature, aging, tolerances, ripple current, surge conditions, and manufacturer testing.

Glossary:

Capacitance: Charge stored per volt across the plates, reported in farads and scaled to pF, nF, uF, or mF.
Dielectric: The insulating material between the plates that affects capacitance and voltage withstand.
Relative permittivity: The dielectric constant used to scale capacitance compared with vacuum.
Fringe correction: An optional percentage uplift for edge fields not included in the ideal parallel-plate equation.
Electric field: Voltage divided by separation, reported here in kV/mm for dielectric stress review.
Breakdown strength: The entered field limit, in kV/mm, used to estimate breakdown voltage and voltage margin.
Voltage margin: The estimated breakdown voltage divided by the absolute applied voltage.

References:

CODATA Value: vacuum electric permittivity, National Institute of Standards and Technology.
Capacitors and Capacitance, OpenStax, 2016.
Capacitors and Dielectrics, OpenStax.