Outward and Inward Codes Explained

Some postcode systems split the code into two parts. The first part, often called the outward code, tells the network which area to send mail to. The second part, the inward code, narrows it to a small cluster of addresses within that area.

Why split the code?

The two parts match two stages of mail handling. Between sorting centres, only the broad area matters, so the first part is enough to route the item. Once the item reaches the destination area, the second part sorts it to a delivery round or street section. This is explained further in how mail sorting uses postcodes.

What the outward code does

The outward code usually combines a letter-based area and a district number, though exact patterns depend on the country. It answers "which town or district?". Because it is short, many businesses group customers by this part alone, for example to define broad delivery zones.

What the inward code does

The inward code is short and typically identifies a sector and a small unit, covering a street or part of one. Together with the outward code, it often narrows an address to a limited number of properties. A short house number or name then completes the picture.

Typing it correctly

The two parts are normally separated by a space. Spaces matter to people reading the code but are often ignored by software. Some forms store it without the space, others with, so see spaces and hyphens in postcodes for what to do.

How common is it?

Two-part designs are used in several countries, and the same hierarchical idea underlies single-block codes too: early characters are broad, later characters are narrow. Compare with other designs in postcode structure explained.

Still an area, not a point

Even the narrowest inward code stays a delivery group rather than a precise position. Two doors sharing a code can be very different on the ground, which is why an exact-location code may be added when a driver needs the exact entrance.

Key takeaway

If you design a system that handles two-part codes, treat them as one value for storage and as two parts only when you need to group by area. Splitting on the separator is risky if users type the code without one, so normalise first and then split by the known length of the second part.