The Interstate numbering system is a grid. Once you know a handful of rules, almost any route number tells you where it runs and what job it does. Here is the full pattern, including the exceptions.
Mainline grid
Loop vs spur
One and two-digit mainlines
Primary Interstates use one or two digits. Odd numbers run roughly north to south; even numbers run roughly east to west.
The numbers also climb across the map. Low odd numbers sit in the west (I-5 on the Pacific coast) and high odd numbers in the east (I-95 on the Atlantic). Low even numbers sit in the south (I-10 along the Gulf) and high even numbers in the north (I-90 near the Canadian border).
Major cross-country routes tend to end in 0 or 5. I-10, I-20, I-40, I-70, I-80, and I-90 are long east-west routes; I-5, I-15, I-35, I-75, and I-95 are long north-south routes.
Three-digit auxiliary routes
A three-digit Interstate borrows its last two digits from a parent route. I-495 is tied to I-95; I-275 is tied to I-75.
The first digit tells you the job. An even first digit usually means a loop or a bypass that rejoins its parent at both ends. An odd first digit usually means a spur that connects to its parent at one end only.
Because the same two-digit parent passes through many cities, the same three-digit number is reused all over the country. There are many separate I-295 and I-495 routes.
Suffixes and split routes
A few mainlines split into lettered pairs through a metro area, such as I-35E and I-35W through Dallas-Fort Worth and the Twin Cities, or I-69C, I-69E, and I-69W in south Texas.
Older suffixed routes like I-35E in those metros were kept because both branches serve large populations and renumbering would confuse millions of drivers.
The routes that break the rules
I-99 sits far east of many lower-numbered routes, breaking the west-to-east order, because its number was written directly into federal law.
I-238 in California has no two-digit parent at all, since there is no I-38. The number was the only one available when it was designated.
Duplicate two-digit numbers exist too. There are two separate I-76, I-84, I-86, I-87, and I-88 routes in different parts of the country.
Non-contiguous programs
Hawaii, Alaska, and Puerto Rico have federally recognized Interstate routes that are funded like the mainland system but use their own labels: H-1, H-2, and H-3 in Hawaii, A-1 through A-4 in Alaska, and PRI-1 through PRI-3 in Puerto Rico.
Alaska's routes are eligible for Interstate funding but are not signed with the familiar red, white, and blue shields.
Put it to the test.
Type any number into the decoder and watch the rules apply.