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 powerRC 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:
| Configuration | Formula | Example (100 μF + 220 μF) | Voltage Rating |
|---|---|---|---|
| Parallel | C = C₁ + C₂ | 320 μF | Lowest of individual ratings |
| Series | 1/C = 1/C₁ + 1/C₂ | 68.75 μF | Sum of individual ratings |
| Type | Capacitance Range | Max Voltage | Polarized? | Best Use |
|---|---|---|---|---|
| Ceramic (MLCC) | 1 pF – 100 μF | Up to 1 kV+ | No | Bypass, decoupling, filtering |
| Electrolytic (Al) | 1 μF – 100,000 μF | Up to 450 V | Yes | Power supply smoothing, bulk storage |
| Tantalum | 0.1 μF – 1,000 μF | Up to 50 V | Yes | Mobile devices, low-profile |
| Film (polyester) | 1 nF – 100 μF | Up to 600 V | No | Timing, audio, precision |
| Supercapacitor | 0.1 F – 3,000 F | 2.3–2.7 V | Yes | Energy harvesting, UPS backup |
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.