Capacitance Calculator

Calculate capacitance, stored energy, and RC time constant. Enter charge and voltage, or energy and voltage — get capacitance in farads and energy in joules.

C
V

Enter your values above to see the results.

Tips & Notes

  • Capacitance units: 1 F (farad) is enormous — most capacitors are in μF (microfarad, 10⁻⁶ F), nF (nanofarad, 10⁻⁹ F), or pF (picofarad, 10⁻¹² F). A 1000 μF electrolytic is large; a 10 pF ceramic is tiny.
  • Energy stored E = ½CV². Doubling voltage quadruples stored energy — a 400 V capacitor stores 4× more energy than the same capacitor at 200 V. This is why capacitor voltage ratings must not be exceeded.
  • RC time constant τ = R × C (seconds). After one time constant, a capacitor charges to 63.2% of supply voltage; after 5τ, it is considered fully charged (99.3%). For τ = 10 ms: R = 10 kΩ, C = 1 μF.
  • Capacitors in parallel: C_total = C₁ + C₂ (capacitance adds, like resistors in series). Capacitors in series: 1/C_total = 1/C₁ + 1/C₂ (capacitance decreases, like resistors in parallel).
  • Capacitors block DC but pass AC. Capacitive reactance X_C = 1/(2πfC) — at higher frequency, reactance decreases and more current flows. At DC (f=0), X_C is infinite and no steady-state current flows.

Common Mistakes

  • Exceeding the voltage rating — capacitors have a maximum working voltage (WVDC for electrolytics). Exceeding it causes dielectric breakdown, leakage, and potentially explosive failure. Always derate by 20-30%.
  • Polarizing electrolytic capacitors incorrectly — electrolytic and tantalum capacitors are polarized; the positive lead must connect to the higher voltage. Reverse polarity causes rapid failure and can be dangerous.
  • Confusing capacitance (F) with energy (J) — a larger capacitor stores more charge per volt, but energy depends on both C and V². Two capacitors with the same capacitance at different voltages store very different energies.
  • Ignoring ESR (equivalent series resistance) in high-frequency or high-current applications — real capacitors have internal resistance that causes heating and limits performance. Electrolytic capacitors have high ESR; ceramic and film capacitors are better for high-frequency bypass.
  • Using ceramic capacitors without checking voltage coefficient — many ceramic capacitors (especially X5R, X7R types) lose 50-80% of capacitance at their rated voltage. A 10 μF ceramic at full rated voltage may measure only 3-5 μF.

Capacitance Calculator Overview

Capacitors are the fundamental energy storage elements in electronics — they store charge and release it on demand, filter noise from power supplies, set timing intervals in RC circuits, and block DC while passing AC signals. The simple formula C = Q/V underlies all capacitor calculations.

Capacitance and energy formulas:

C = Q / V | E = ½ × C × V² | Units: farads (F), coulombs (C), joules (J)
EX: 470 μF capacitor charged to 16 V → E = ½ × 0.000470 × 256 = 0.0602 J = 60.2 mJ. Camera flash: 100 μF at 300 V → E = ½ × 0.0001 × 90,000 = 4.5 J released in ~1 ms = 4,500 W peak flash power
RC time constant — charging and discharging:
τ = R × C (seconds) | V_charge(t) = V_s × (1 − e^(−t/τ)) | V_discharge(t) = V₀ × e^(−t/τ)
EX: 10 kΩ resistor, 100 μF capacitor → τ = 10,000 × 0.0001 = 1.0 s. After 1 s: 63.2% charged. After 5 s (5τ): 99.3% charged — functionally complete.
Capacitor combinations:
ConfigurationFormulaExample (100 μF + 220 μF)Voltage Rating
ParallelC = C₁ + C₂320 μFLowest of individual ratings
Series1/C = 1/C₁ + 1/C₂68.75 μFSum of individual ratings
Capacitor types — comparison:
TypeCapacitance RangeMax VoltagePolarized?Best Use
Ceramic (MLCC)1 pF – 100 μFUp to 1 kV+NoBypass, decoupling, filtering
Electrolytic (Al)1 μF – 100,000 μFUp to 450 VYesPower supply smoothing, bulk storage
Tantalum0.1 μF – 1,000 μFUp to 50 VYesMobile devices, low-profile
Film (polyester)1 nF – 100 μFUp to 600 VNoTiming, audio, precision
Supercapacitor0.1 F – 3,000 F2.3–2.7 VYesEnergy harvesting, UPS backup
Capacitors and inductors form the reactive components of electrical circuits. While resistors dissipate energy as heat, capacitors store energy in electric fields and release it — ideal for power supply filtering (smoothing voltage ripple), timing (RC delay circuits), signal coupling (blocking DC while passing AC), and energy delivery (flash photography, defibrillators). The energy calculation E = ½CV² reveals why high-voltage capacitors are dangerous even when disconnected — a 100 μF capacitor at 400 V stores 8 J, enough to deliver a potentially lethal shock.

Frequently Asked Questions

Capacitance C = Q / V, where Q is charge in coulombs and V is voltage in volts. The result is in farads (F). Example: a capacitor holds 0.005 C (5 mC) of charge at 10 V → C = 0.005 / 10 = 0.0005 F = 500 μF. Rearranged: Q = C × V (charge stored at given voltage) and V = Q / C (voltage from known charge and capacitance). For most practical calculations, you calculate energy storage (E = ½CV²) or RC time constants rather than directly measuring charge.

Energy E = ½ × C × V². Example: a 4,700 μF (0.0047 F) capacitor charged to 50 V → E = ½ × 0.0047 × 2,500 = 5.875 J. A camera flash capacitor: 100 μF at 300 V → E = ½ × 0.0001 × 90,000 = 4.5 J — enough to produce a bright flash. A large bank: 1 F supercapacitor at 2.7 V → E = ½ × 1 × 7.29 = 3.645 J. Despite high capacitance, supercapacitors store much less energy than batteries at the same volume due to lower voltage.

The RC time constant τ = R × C (seconds) defines how quickly a capacitor charges or discharges through a resistance. Voltage during charging: V(t) = V_supply × (1 − e^(−t/τ)). After 1τ: 63.2% charged. After 2τ: 86.5%. After 3τ: 95%. After 5τ: 99.3% (considered fully charged). Example: R = 22 kΩ, C = 10 μF → τ = 22,000 × 0.000010 = 0.22 s. The capacitor charges to ~63% in 220 ms and is fully charged in ~1.1 s. This principle governs timing circuits, filters, and power supply smoothing.

Parallel capacitors: C_total = C₁ + C₂ + ... (total capacitance increases, voltage rating is the lowest of all). Same voltage across all capacitors. Series capacitors: 1/C_total = 1/C₁ + 1/C₂ + ... (total capacitance decreases below any individual value). Voltage divides across capacitors in proportion to their inverse capacitances. Two equal capacitors in series: C_total = C/2 but voltage rating doubles. Example: 100 μF + 220 μF in parallel = 320 μF. Same values in series: 1/C = 1/100 + 1/220 = 0.01455 → C = 68.75 μF.

Ceramic (disc and MLCC): 1 pF to 100 μF, excellent high-frequency performance, low ESR, non-polarized. Used for bypass, decoupling, filtering. Electrolytic (aluminum): 1 μF to 100,000 μF, polarized, high ESR, for power supply bulk storage and smoothing. Tantalum: 0.1 μF to 1,000 μF, polarized, low profile, moderate ESR, for coupling and bypass in mobile devices. Film (polyester, polypropylene): precise values, low ESR, stable with temperature, for timing circuits and audio applications. Supercapacitor/EDLC: 0.1 F to 3,000 F, very high capacitance, low voltage (2.7 V), for energy harvesting and backup power.

Capacitive reactance X_C = 1 / (2πfC) in ohms, where f is frequency in Hz and C is capacitance in farads. X_C decreases as frequency increases — capacitors pass high frequencies and block low frequencies and DC. Example: 10 μF capacitor at 60 Hz: X_C = 1/(2π × 60 × 0.00001) = 265 Ω. Same capacitor at 10 kHz: X_C = 1.59 Ω — passes high-frequency signals easily. This frequency dependence is the basis for all RC filters, crossover networks in speakers, and coupling capacitors that block DC bias while passing AC signals.