Capacitor codes

It's essential to be able to read capacitor codes in order to correctly recognise the correct part and as the schematic, BOM and component supplier might use pF, nF or µF interchangeably to be able to convert between these.

EIA scheme
The Electronic Industry Alliance (EIA) has standardised some of the marking schemes.

Leaded
On larger capacitors their value is printed on the case or encapsulation. The values of small capacitors are indicated in an alphanumeric code. With older capacitor values were sometimes indicated by a colour code.

With small general purpose capacitors where values range from a few pF to less than 1 µF the convention arose that whole numbers designate values in picofarads while decimal fractions designates values in microfarads.

Often p, n and u are substituted for the decimal point. For example 8p2 for 8.2pF, n51 for 0.51nF or 4u7 for 4.7µF.

Working voltage
Where there is space the working voltage is given. For smaller capacitors this needs to be found from the data sheet.

Polarity
Care must be taken to insert polarised capacitors in the correct orientation. The stripe on radial or arrow on axial aluminium electrolytic types indicates the negative lead. On tantalum types a + sign indicates the positive lead.

Stripe
If there is a stripe on a non-polarised capacitor, it might indicate which lead is connected to the outermost foil. To connect the capacitor connect this lead to the lowest impedance (usually the ground) side of the circuit. Many modern capacitors are unmarked, and often when a capacitor does have a stripe, it’s not related to the outside foil marking.

Small values
For small value capacitors the letter R is used to denote a decimal point.

Abbreviated marking codes
A newer way to mark values is with three numbers on the body of the capacitor with the value in picofarads. The first two digits are the significant figures and the third the multiplier (i.e. number of zeros). So for example 151 indicates a capacitance of 150 pF.

SMD
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