The POSTNET (Postal Numeric Encoding Technique) barcode was invented by US Postal Office to encode ZIP information. Having your letter printing system POSTNET capable enables you to receive a discount on the postage. The POSTNET barcode can represent a five-digit ZIP Code( 32 bars),a nine-digit ZIP+4 code(52 bars), or an eleven-digit delivery point code( 62 bars).


The POSTNET barcode is always printed in a format that begins and ends with a frame bar (full or tall bar). To ensure POSTNET accuracy during mail processing, a correction character (five bars) must be included immediately before the rightmost frame bar of all POSTNET barcodes(see bellows).

Each digit is represented by a distinct pattern composed of five bars (see image on right for patterns). Basically, there are two distinct types of bars: the Full Bar and the Half Bar. A Full Bar must be between a minimum of 0.115″ and a maximum of 0.135″ tall. A Half Bar must be between a minimum of 0.04″ and a maximum of 0.06″ tall. Both the Full Bar and the Half Bar must be between 0.015″ and 0.025″ wide.

In addition to the height and width specifications, there is spacing requirement between bars; it must be between 0.012″ and 0.040″.

Specifications for POSTNET Barcode
Specification Normal Allowable Variance
Full Bar Height 0.125″ +/-0.01″
Half Bar Height 0.05″ +/-0.01″
Bar Width 0.02″ +/-0.005″
Bar Spacing 0.026″ +/-0.014
Center Line Spacing 0.0458″ +/-0.0042″

Check Digit Calculation

The correction character is always the digit that, when added to the sum of the other digits in the barcode, results in a total that is a multiple of 10. For example, the sum of the ZIP+4 barcode 12345-6789 is 45. The next higher multiple of 10 is 50, so the correction character is 5 (50 minus 45).