Introduction
Capacitance is the ability to store charge per unit voltage, in farads (F). Most practical capacitors are microfarad (µF), nanofarad (nF), or picofarad (pF). A 1 F capacitor is huge — only modern supercapacitors approach this. Power-supply filter caps are usually 100-10,000 µF; bypass caps are 0.1 µF ceramic; RF and timing circuits use pF. Capacitance spans twelve orders of magnitude in the parts catalog — from sub-pF pcb traces to kilofarad ultracapacitor banks — which is why the prefix ladder matters more in capacitance than almost any other electrical quantity.
Why capacitance units exist and how they diverged
The farad (F) is the SI unit — 1 coulomb stored per volt. A 1 µF cap at 5 V holds 5 µC of charge, which is 3.12 × 10¹³ electrons more on one plate than the other. Capacitor types: ceramic (small, cheap, pF-µF; C0G/NP0 for precision, X7R/Y5V for bulk), electrolytic aluminum (polarized, 1 µF-10 mF, short life in heat), tantalum (precise, polarized, fails short-circuit if overvoltaged), polymer (long life, low ESR), film (polyester, polypropylene — precision and audio), supercapacitor (1 F+, low voltage, slow charge/discharge). Each type has different ESR, voltage rating, temperature coefficient, and DC-bias behavior.
The impedance of a capacitor at frequency f is Z = 1/(2πfC). At DC (f=0) a cap is an open circuit; at infinite frequency it's a short. This frequency-dependent behavior is why caps couple AC but block DC, decouple power rails, and make up the C half of every RC filter and LC tuned circuit. Time constant τ = RC — a 10 kΩ-10 µF pair charges to 63% in 100 ms, 95% in 300 ms. Familiar ratios make circuit timing predictable.
How to convert capacitance
µF to F: divide by 1,000,000. nF to µF: divide by 1,000. pF to nF: divide by 1,000. Capacitors in parallel add: C_total = C1 + C2 + C3. In series, inverse sums (opposite of resistors): 1/C_total = 1/C1 + 1/C2. Energy stored: E = ½CV². A 10,000 µF cap at 50 V holds 12.5 J — enough to cause a painful shock after a power supply is turned off. Charge: Q = CV. Time constant: τ = RC in seconds with C in farads, R in ohms.
Units supported by this capacitance calculator
- Farad (F)
- Millifarad (mF)
- Microfarad (µF)
- Nanofarad (nF)
- Picofarad (pF)
- Kilofarad (kF)
- Abfarad
- Statfarad (esu)
Common capacitance conversion mistakes
- Units scale. 1 F = 10⁶ µF = 10⁹ nF = 10¹² pF. Easy to misread a component label. Surface-mount caps use 3-digit codes — '104' means 10 × 10⁴ pF = 100 nF = 0.1 µF. '472' is 4,700 pF = 4.7 nF. Knowing the code saves reading glasses.
- Voltage rating. Exceeding a cap's voltage rating fails it, sometimes explosively (electrolytics vent; tantalums can ignite). Always pick caps rated at 2-3× expected voltage. On a 5 V rail, use 10 V or 16 V parts; on 12 V, use 25 V; on 48 V, use 63 V+.
- Polarity. Electrolytic and tantalum caps are polarized. Reversing polarity destroys them — electrolytic venting, tantalum sometimes igniting. The negative lead is marked on the can (electrolytics) or the body (tantalums).
- DC bias derating (ceramics). High-K ceramic dielectrics like X7R and Y5V lose capacitance under DC bias. A '10 µF 16 V' Y5V ceramic at 12 V DC bias can drop to 2-3 µF actual. For power filtering, use polymer or electrolytic, or oversize the ceramic and check the datasheet curve.
- ESR and ripple current. High ESR in switching power supplies causes heating and premature failure. Ripple current rating is not a 'nice to have' — aluminum electrolytics have life halved for every 10°C rise above spec.
- Parasitic capacitance. Every wire, every PCB trace, every pin has capacitance — typically 1-10 pF. For RF or high-speed digital, these parasitics matter as much as lumped components.
Real-world capacitance examples
- Smoothing cap in power supply: 1,000-10,000 µF at 25-35 V electrolytic.
- Output cap in DC-DC converter: 22-100 µF polymer or ceramic.
- Bypass cap near an IC: 0.1 µF (100 nF) ceramic X7R, one per power pin.
- Bulk bypass on a board: 10-47 µF tantalum or polymer.
- AC coupling in audio: 1-10 µF film or bipolar electrolytic.
- RC timing for 1 s blink (10 kΩ ladder): 100 µF.
- Tuning cap in old radios: 10-365 pF variable.
- Crystal load capacitance: 12-22 pF C0G ceramic.
- Supercapacitor in backup memory: 1-5 F at 2.5-5 V.
- EV 'ultracapacitor' banks: 1,000+ F combined at 48-400 V.
- Human body capacitance (standing): ~150 pF.
- PCB trace (10 cm long, 0.2 mm wide, over ground plane): ~10 pF.
Tips for accurate capacitance conversion
- For filtering, µF range. For bypass, 0.1 µF ceramic close to the IC.
- For RF circuits, pF and nF. Always ceramic C0G/NP0 or mica — low loss, stable over temperature.
- For precision timing, film caps (polypropylene) hold value to 100 ppm.
- Bleed resistors across large electrolytics discharge them safely after power-off.
- For ceramics, derate voltage to prevent capacitance loss under DC bias.
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