☑ Tracking Number Explained: Why Tracking Numbers Have Letters and Numbers
Tracking Number Explained: Why Tracking Numbers Have Letters and Numbers
Not decoration, not randomness — a deliberate engineering tradeoff between running out of room and being impossible to read aloud.
It's a fair question that most tracking guides skip entirely: why do tracking numbers bother mixing letters and digits at all? Why not just use numbers, the way phone numbers and zip codes do? The honest answer involves a mix of pure math, human error rates, and — for some carriers — decades of running out of room as shipping volume exploded far beyond what anyone designing the original systems expected.
The Simple Reason: Running Out of Room
The most fundamental answer is arithmetic. A tracking number needs enough unique combinations to cover every shipment a carrier will ever process without repeating — and once you're moving billions of packages a year, pure digits run out of room faster than most people assume.
An 8-digit purely numeric code offers 100 million unique combinations — sounds like a lot, until a single carrier processes that many shipments in a matter of weeks at modern e-commerce volumes. Expand that same 8 characters to include the 26 letters of the alphabet alongside the 10 digits, and the combination count jumps to roughly 2.8 trillion — a difference of more than four orders of magnitude, from the same number of characters. That's the entire case for mixing letters and numbers in one sentence: it's dramatically more efficient use of the same space.
Letters Aren't Just Extra Capacity — They're Usually Meaningful
Carriers didn't stop at "letters give us more room." Most formats put those letters to double duty: expanding capacity while simultaneously encoding something useful, like which company or account issued the number. A container's owner code, a courier's shipper block, a postal service's country code — all are letters doing identification work, not just filling space for its own sake. For the full breakdown of what specific letter segments mean across major formats, see our companion guide on how to read the numbers and letters.
Why Not All Letters, or All Numbers?
If letters buy so much more capacity, why not make tracking numbers entirely alphabetic? And if digits are simpler, why use letters at all? Both extremes run into real, practical problems.
| Approach | The Problem |
|---|---|
| All digits | Limited combinations force longer numbers or force the format to run out of unique values as volume grows |
| All letters | Harder to read aloud accurately over the phone, and many legacy systems and databases expect at least partially numeric identifiers |
| Mixed alphanumeric | The practical middle ground most carriers converge on — enough capacity, still reasonably readable and typeable |
Why Certain Letters Get Excluded
Look closely at several tracking and identification formats and you'll notice something: certain letters rarely or never appear, specifically because they're too easy to confuse with digits or with each other.
The letter O looks nearly identical to the digit 0 in many fonts. I resembles 1. S can be misread as 5, and B as 8. Some formats deliberately skip these characters entirely in certain positions, or rely on consistent formatting and check digits to catch the resulting errors when they inevitably happen anyway.
- Digits are fast to type, easy to read aloud, and familiar to older systems — but limited in raw capacity.
- Letters multiply capacity dramatically and can double as identity codes — but introduce more room for human misreading.
- Mixing them, carefully — often while avoiding the most confusable letters — is how most carriers balance both problems at once.
How This Played Out Historically
This isn't a new problem. The ISO 6346 container numbering standard was developed specifically because containerized shipping exploded through the 1960s and 70s and needed a globally unique system from the start. SCAC codes, originally two letters, expanded to three and four specifically because trucking deregulation multiplied the number of registered carriers well past what two letters alone could uniquely represent. Courier tracking numbers have followed the same pressure more recently, as e-commerce volume pushed carriers toward longer, more capacity-rich formats than their pre-internet predecessors ever needed.
Why Not Just Make Numeric Codes Longer Instead?
This is the obvious follow-up question, and it has a real answer: length alone does work, but it's a much less efficient way to buy capacity than adding letters. Going from an 8-digit to a 10-digit purely numeric code multiplies capacity by 100 — a 9-digit numeric code alone only offers 10^9, roughly 1 billion combinations. Adding just two letter positions to an 8-character code, by contrast, multiplies capacity by nearly 28,000 times over the pure-digit version, without making the number any longer at all. Every additional character costs the same in terms of typing, printing, and scanning — so a format that extracts more capacity per character is simply a better use of that cost.
There's a practical ceiling to this trade-off too: a tracking number that's too long becomes harder to type accurately, harder to fit cleanly on a small label, and more error-prone regardless of whether it's digits or letters. Carriers are effectively optimizing for the shortest number that still offers enough headroom for years of future volume growth — and mixing in letters is the most efficient lever available for hitting that target without the number becoming unwieldy.
Frequently Asked Questions
Could carriers just make numeric-only tracking numbers longer instead of adding letters?
In principle, yes — length alone can also expand capacity. But adding letters achieves a much bigger capacity increase per added character than adding another digit would, which is generally the more efficient design choice.
Do all carriers exclude the same confusable letters?
Not universally — practices vary by carrier and format, and not every system avoids every confusable character. It's a common practice rather than a strict universal rule.
Why do some tracking numbers still look purely numeric today?
Some carriers, like FedEx, have historically stuck with purely numeric formats, relying instead on sufficient length to maintain enough unique combinations rather than mixing in letters.
Will tracking number formats need to keep changing in the future?
Likely yes, in the same way they have historically — as shipping volume continues growing, formats that feel spacious today may eventually need to expand again, the same pressure that has driven every past format change.
- Mixing letters and digits dramatically increases the number of unique combinations available in the same character length — roughly 28,000 times more for an 8-character code.
- Letters usually do double duty: expanding capacity while also encoding meaningful identity information.
- Purely numeric or purely alphabetic formats each have real drawbacks — capacity limits on one side, readability and legacy-system friction on the other.
- Confusable character pairs (O/0, I/1, S/5) are often deliberately avoided or handled carefully in tracking number design.
- Format changes throughout history — ISO 6346, SCAC codes, courier tracking numbers — have all been driven by the same underlying pressure: running out of room as volume grew.
Skip the Math — Just Track the Shipment
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Track a ShipmentWritten by Track4Trace Editorial Team
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