Capacitance Calculator
Solve C = Q/V for capacitance, charge, or voltage, or use the parallel-plate formula C = ε₀εᵣA/d to find capacitance from plate area, separation, and dielectric constant. Two 3D diagrams compare a lightly and heavily charged capacitor, and charts show how capacitance and stored energy scale.
Reviewed by the ToolNestr Editorial Team — July 2026
Two states of a charged capacitor
1. Lightly charged plates
Two parallel plates at a lower charge/voltage — the field lines between them are sparse, reflecting a weaker electric field.
2. Heavily charged plates
The same plates at a higher charge/voltage — the field lines are much denser, showing the stronger field between the plates.
Capacitance and energy graphs
How it works
The core idea in one line: Capacitance measures how much charge a component stores per volt across it — a fixed geometric property set by plate area, separation, and any dielectric material between the plates.
C = Q / V
capacitance from charge and voltage
C = ε₀ εᵣ A / d
parallel-plate capacitance; ε₀ ≈ 8.854×10⁻¹² F/m
E = ½CV² = ½QV = Q²/(2C)
energy stored in a charged capacitor
Rearranged, C = Q/V solves any variable: Q = CV and V = Q/C. For a parallel-plate capacitor, C = ε₀εᵣA/d shows that capacitance grows with plate area and dielectric constant, and shrinks as the plates move apart. Once you know C and V (or Q), the energy stored follows from E = ½CV² = ½QV = Q²/(2C) — three equivalent forms of the same result.
Worked example 1 — charge and stored energy
Given: A 100 μF (100×10⁻⁶ F) capacitor is charged to 12 V. Find the charge stored and the energy stored.
All three energy formulas (½CV², ½QV, Q²/2C) give the same 7.2 mJ result — they are algebraically equivalent.
Worked example 2 — parallel-plate capacitance
Given: Two parallel plates of area A = 0.02 m² are separated by d = 0.001 m in vacuum/air (εᵣ = 1). Find the capacitance.
Doubling the plate area or halving the separation would each double this capacitance.
Capacitance vs resistance — series and parallel behavior
Capacitors combine the opposite way resistors do: capacitances add directly in parallel, and add as reciprocals in series.
| Property | Capacitors | Resistors |
|---|---|---|
| Series combination | 1/C_total = 1/C1 + 1/C2 + ... | R_total = R1 + R2 + ... |
| Parallel combination | C_total = C1 + C2 + ... | 1/R_total = 1/R1 + 1/R2 + ... |
| Stores | Charge / energy in an electric field | Dissipates energy as heat |
| Units | Farad (F) | Ohm (Ω) |
This inverse relationship is why capacitors in parallel are used to boost total capacitance, while resistors in series are used to add up total resistance.
Where capacitance actually matters
🔋 Energy storage and smoothing
Capacitors smooth out voltage ripples in power supplies and store short bursts of energy for camera flashes, defibrillators, and backup power during brief outages, releasing their stored charge almost instantly when needed.
📱 Touchscreens
Capacitive touchscreens sense the small change in capacitance created when a conductive fingertip approaches the screen's grid of electrodes, letting the device pinpoint touch location without any moving parts.
🎚️ Timing and filter circuits
Paired with resistors, capacitors set the timing constant (RC) in oscillators, debounce circuits, and audio filters — charging and discharging at a predictable rate that engineers tune by choosing C.
⚡ Memory and DRAM cells
Each bit in a computer's dynamic RAM (DRAM) is stored as a tiny electric charge on a microscopic capacitor, which is why DRAM must be "refreshed" thousands of times per second before that stored charge leaks away.
Common misconceptions
"A bigger capacitor always stores more energy no matter the voltage."
Energy is E = ½CV², so voltage matters just as much as capacitance — a small capacitor charged to a high voltage can store more energy than a large one at a low voltage, since energy scales with the square of V.
"Capacitance depends on how much charge is on the plates."
Capacitance C = Q/V is actually a fixed geometric property of the capacitor (plate area, separation, dielectric) — it does not change as you charge or discharge it. Q and V both scale together, keeping their ratio C constant.
"Adding a dielectric always increases the voltage a capacitor can hold for the same charge."
A dielectric with εᵣ > 1 increases capacitance, which for a fixed charge Q actually decreases the voltage (since V = Q/C) — dielectrics let you store more charge at a given voltage, not more voltage at a given charge.
"Capacitors in series add like resistors in series."
They combine the opposite way: capacitors in series add as reciprocals (1/C_total = 1/C1 + 1/C2 + ...), while capacitors in parallel add directly (C_total = C1 + C2 + ...) — the reverse of how resistors combine.
Formula sources & further reading
The formulas here are standard, traceable to:
- • OpenStax, University Physics Volume 2 — §8.1 "Capacitors and Capacitance" (free, peer-reviewed). openstax.org
- • Halliday, Resnick & Walker, Fundamentals of Physics — Chapter 25, Capacitance.
- • Serway & Jewett, Physics for Scientists and Engineers — Chapter 26, Capacitance and Dielectrics.
C = Q/V and C = ε₀εᵣA/d with ε₀ ≈ 8.854×10⁻¹² F/m. Energy E = ½CV². Results are rounded for display.
How to use this calculator
Pick a mode
Use "From Q and V" for the basic C = Q/V relationship, or "Parallel-plate" for geometry-based capacitance.
Enter your known values
Use scientific notation for small values, e.g. 100e-6 for 100 μF or 1e-3 for 1 mm.
Check the energy and diagrams
Stored energy is computed automatically, and the 3D diagrams show field density at different charge levels.
Related tools
Frequently asked questions
What is capacitance?
Capacitance is a measure of how much electric charge a component can store per volt of potential difference across it: C = Q/V. It is measured in farads (F), though most practical capacitors are in the microfarad (μF), nanofarad (nF), or picofarad (pF) range.
What is the parallel-plate capacitor formula?
For two parallel conducting plates of area A separated by distance d with a dielectric of relative permittivity εᵣ between them, C = ε₀εᵣA/d, where ε₀ ≈ 8.854×10⁻¹² F/m is the permittivity of free space. Larger plates or a closer spacing both increase capacitance.
How much energy does a capacitor store?
A charged capacitor stores energy E = ½CV² = ½QV = Q²/(2C). All three forms are equivalent — pick whichever matches the variables you already know.
What does the dielectric constant εᵣ do?
εᵣ (relative permittivity) describes how much a material between the plates increases capacitance compared to vacuum (εᵣ = 1). Common dielectrics like paper, mica, or ceramic have εᵣ from about 2 to several thousand, letting capacitors pack much more capacitance into the same physical size.
Why does capacitance decrease as plate separation increases?
C = ε₀εᵣA/d is inversely proportional to the separation d. Pulling the plates apart weakens the electric field they can sustain for a given charge, so a larger voltage results from the same charge — and since C = Q/V, a bigger V for the same Q means a smaller C.