Clay illustration of a tire sidewall under a magnifying glass

How to read a tire size: the complete breakdown

The string of numbers and letters on your tire sidewall isn't random. Every character follows the ISO Metric (ETRTO) or P-metric standard and encodes something specific. Here's the full decode, no guessing.

Breaking down 215/55R17

Let's take 215/55R17 as our worked example — one of the most common fitments in North America.

215 — Section width (mm)

The first three digits are the nominal section width in millimeters, measured from sidewall to sidewall when the tire is inflated to the recommended pressure on the spec'd rim width. A 215 is 215 mm wide.

Important nuance: this is the nominal width. Actual width varies by rim width. The same tire can measure 210 mm on a narrow rim or 220 mm on a wide one.

55 — Aspect ratio (%)

This is the sidewall height expressed as a percentage of the section width. A 55 aspect ratio means the sidewall is 55% of 215 mm = 118.25 mm tall.

A lower number (like 35 or 40) means a shorter sidewall — the "low-profile" look. A higher number (65, 70) is a taller, more cushioned ride more common on trucks and crossovers.

R — Radial construction

Nearly every modern tire is radial (cord plies run perpendicular to the tread). You'll occasionally see B (bias-ply) or D (diagonal) on older vehicles. R is the standard.

You may also see a speed rating letter before the R, like 215/55ZR17 — the Z indicates the tire is rated above 149 mph (240 km/h). On modern tires the actual speed rating is still in the service description (see below).

17 — Rim diameter (inches)

The last number before the service description is the rim diameter in inches. This tire fits a 17-inch wheel. Tire and rim must match — a 215/55R17 will not seat on a 16" or 18" rim.

Overall diameter (the derived number)

The size doesn't state overall diameter directly — you compute it:

sidewall_mm  = section_width × (aspect_ratio / 100)
            = 215 × 0.55 = 118.25 mm

rim_mm       = rim_in × 25.4
            = 17 × 25.4 = 431.8 mm

OD_mm        = rim_mm + 2 × sidewall_mm
            = 431.8 + 2(118.25) = 668.3 mm ≈ 26.3 inches

The TreadSize decoder does this automatically for any size.

Service description: load index and speed rating

After the rim diameter, a full tire marking includes a two or three-digit number and a letter — e.g., 91V in a marking like 215/55R17 91V.

Load index

Maps to a maximum load in kilograms. A few key values:

Index	Load (kg)	Load (lbs)
85	515	1,135
91	615	1,356
95	690	1,521
100	800	1,764
110	1,060	2,337

When replacing tires, the load index of the new tire must be equal to or higher than the OEM spec.

Speed rating

Letter	Max speed (km/h)	Max speed (mph)
H	210	130
V	240	149
W	270	168
Y	300	186

The new tire's speed rating must meet or exceed the OEM rating.

Other sidewall markings

P prefix (P215/55R17): P-metric designation used primarily in North America. Slight load differences vs. Euro-metric.
LT prefix: Light truck. Higher ply rating, different load standards.
M+S or M/S: Mud and snow rated — meets light winter traction standards.
Three-peak mountain snowflake (3PMSF): Meets more rigorous winter traction tests. Genuine winter/all-season capable.
DOT code: Department of Transportation approval + 4-digit manufacturing date (WWYY) at the end.
UTQG (Uniform Tire Quality Grading): Treadwear / Traction / Temperature grades required for passenger tires in the US.

Use the decoder

Type any size into the TreadSize decoder and instantly see the full dimensional breakdown — sidewall height, overall diameter in inches and mm, circumference, and revolutions per mile — free, no sign-up.

All formulas follow ETRTO standards. Always verify fitment with your vehicle's door-placard and a qualified tire professional before purchasing.