MileCheckHow US interstate highways are numbered
The interstate numbering system isn't random — once you know the pattern, you can tell a highway's direction and roughly where it is in the country just from its number.
Odd vs even: direction
Odd-numbered interstates run north–south (I-5, I-95). Even-numbered interstates run east–west (I-10, I-90). It's the opposite of the US Highway grid in some regions, but the interstate rule is consistent nationwide.
Low to high: location
Numbers increase from west to east and from south to north. So I-5 hugs the West Coast and I-95 runs up the East Coast; I-10 crosses the southern states and I-90 crosses the north. A one- or two-digit number tells you roughly which part of the country a route is in.
Three-digit interstates: loops and spurs
A three-digit interstate is an auxiliary route tied to a two-digit "parent." The first digit tells you which: an even first digit means a loop or beltway that connects to the parent at both ends (like I-495 around Washington, DC), while an odd first digit means a spur that connects at just one end (like I-395). The last two digits are the parent route.
Mile markers reset at the state line
On every interstate, mile markers count up from 0 at the state's southern or western edge and reset at each state line — so the same mile number appears in every state the highway passes through. That's why a location is always given as route plus mile plus state.
Know exactly where you are — live
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Questions, answered
Are odd interstates north-south or east-west?
Odd-numbered interstates run north–south (like I-5 and I-95). Even-numbered interstates run east–west (like I-10 and I-90).
What do 3-digit interstate numbers mean?
Three-digit interstates are auxiliary routes off a two-digit parent. An even first digit is a loop or beltway (connects at both ends); an odd first digit is a spur (connects at one end).
Why does I-95 have a high number and I-5 a low one?
Interstate numbers increase from west to east. I-5 runs up the West Coast (low number) and I-95 up the East Coast (high number). Even numbers work the same south-to-north.
What is the difference between a loop and a spur interstate?
A loop (or beltway) rejoins its parent interstate at both ends and usually circles a city. A spur connects to the parent at only one end and dead-ends into a destination.