Capacitance
The capacitance value of any capacitor is a measure of the amount of electric charge stored per unit of potential difference between the plates. The basic unit of capacitance is a farad; however, this unit has been too large for general use until the invention of the double-layer capacitor, so microfarad (μF, or less correctly uF), nanofarad (nF) and picofarad (pF) are more commonly used.
Many conditions determine a capacitor's value, such as the thickness of the dielectric and the plate area. In the manufacturing process, electrolytic capacitors are made to conform to a set of preferred numbers. By multiplying these base numbers by a power of ten, any practical capacitor value can be achieved, which is suitable for most applications.
Passive electronic components, including capacitors, are usually produced in preferred values (e.g., IEC 60063 E6, E12, etc. series).
The capacitance of aluminum electrolytic capacitors tends to change over time, and they usually have a tolerance range of 20%. Some have asymmetric tolerances, typically −20% but with much larger positive tolerance as many circuits merely require a capacitance to be not less than a given value; this can be seen on datasheets for many consumer-grade capacitors. Tantalum electrolytics can be produced to tighter tolerances and are more stable.
Read more about this topic: Electrolytic Capacitor