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
MileCheck shows your nearest mile marker in real time as you drive, plus live DOT alerts and the nearest camera on your route. Free to start, works offline, runs on CarPlay and Android Auto.
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.