Tire Size Chart: Overall Diameter, Width and Sidewall Height by Wheel Size
A tire size chart turns the code on a sidewall into the measurements you actually use: overall diameter, section width and sidewall height. Those figures help you compare replacement options, check a plus-size idea or see how a taller truck tire changes things. This guide explains how the code works and how the chart's numbers are calculated. It then walks through common sizes by wheel diameter so you can compare options with confidence.
How to Read a Tire Size Before You Use the Chart
Take 225/60R17 as an example. The first number is the section width in millimeters, 225 here. The 60 is the aspect ratio, meaning sidewall height equals 60 percent of that width. R stands for radial construction, and 17 is the wheel diameter in inches. You will find this string molded into the sidewall. The original-equipment size is also printed on the tire placard, usually on the driver's door jamb. Start from the placard size, because it is the size the vehicle was engineered and tested around.
Overall diameter is never printed on the sidewall. You have to calculate it or look it up, which is the main reason a size-to-diameter chart exists. Also read the full marking, not just the numbers. A P prefix marks a P-metric passenger tire. No prefix usually means a Euro-metric tire, and LT marks a light-truck tire. These designations use different load formulas, and LT tires can measure slightly differently. Check the load index and speed rating that follow the size too.
What the Tire Size Chart Shows and How It Is Organized
Each row in the chart lists a tire size, its section width in millimeters and inches, its sidewall height, and its overall diameter in inches. Some charts also give approximate revolutions per mile, which depends on tire circumference. Rows are grouped by wheel diameter. That lets you scan every common width and aspect ratio that fits your rim size without jumping around. Every row comes from the same two formulas. Sidewall height equals width times aspect ratio divided by 100. Overall diameter equals wheel diameter plus two sidewall heights converted to inches.
These are nominal design values, not measurements of a mounted tire. Actual width and diameter vary by manufacturer, tread depth and pattern, the width of the rim the tire sits on, and inflation pressure. Two tires with identical size codes can differ by a noticeable fraction of an inch. Use the chart to narrow your choices. Before buying, check the specific tire's published specifications, which list its measured diameter, approved rim width range and load rating.
15-Inch Tire Sizes: Diameter and Width Reference
The 15-inch group spans compact cars, older sedans, minivans and small trucks. A 185/60R15 comes out to about 23.7 inches tall, and a 195/65R15 to roughly 25.0 inches. A 215/70R15 is near 26.9 inches, and a 235/75R15 reaches about 28.9 inches. All four fit the same wheel diameter, but aspect ratio creates a spread of more than five inches. That is because a taller sidewall adds height twice, once above the rim and once below it.
Flotation sizes appear here too. A 31x10.50R15 is about 31 inches tall and 10.5 inches wide, and a metric 265/75R15 comes out at roughly 30.7 inches, so the two land near each other. In the passenger range, several sizes cluster tightly. The 215/60R15 is about 25.2 inches tall, the 205/65R15 about 25.5 and the 195/70R15 about 25.7. Grouping like this shows you realistic alternatives at a glance when your original size is hard to find.
17-Inch and 18-Inch Tire Sizes: Diameter and Width Reference
The 17-inch and 18-inch groups cover sedans, crossovers, SUVs and half-ton pickups. A 215/45R17 is only about 24.6 inches tall, while a 265/70R17 reaches roughly 31.6 inches. The tall end suits trucks, and the short end suits compact performance cars. In the 18-inch group, a 225/55R18 is about 27.7 inches tall and a 235/60R18 about 29.1 inches. A 265/65R18, common on trucks, comes out near 31.6 inches.
Compare the two rim sizes and plus sizing becomes clear. A 225/60R17 at about 27.6 inches nearly matches a 225/55R18 at 27.7 inches. Likewise, 235/65R17 and 235/60R18 both land close to 29 inches. The larger wheel gets a shorter sidewall, so overall height stays nearly the same. Matching diameter is only the first check, though. The tire also has to suit the wheel's rim width, and it needs a load index and speed rating at least equal to the original equipment's.
Converting Metric Tire Sizes to Inches
To convert a metric size to inches, divide the section width by 25.4. Next, multiply the width by the aspect ratio to get sidewall height in millimeters, then divide that by 25.4 as well. Double the sidewall height and add the wheel diameter to get the overall height. Take 265/70R17 as a worked example. The width is 265 ÷ 25.4, or about 10.4 inches. The sidewall is 185.5 mm, or about 7.3 inches. So 17 plus 14.6 gives about 31.6 inches overall.
Flotation sizes skip the math, because the numbers are already in inches. A 31x10.50R15 is 31 inches tall and 10.5 inches wide, and it mounts on a 15-inch wheel. You can repeat the metric steps for any size a chart leaves out. Expect small differences from real tires, though. Aspect ratios are rounded to steps of five, and manufacturers shape treads and shoulders differently. A calculated figure can therefore differ slightly from the diameter a manufacturer publishes.
Finding Sizes With the Same Overall Diameter
Overall diameter sets how far the vehicle travels per wheel revolution. The speedometer, odometer, transmission shift logic, and stability and traction systems are all calibrated to that distance. A taller tire makes the speedometer read lower than your true speed and slightly lengthens effective gearing. It also reduces clearance in the wheel well. To find an alternative size, look for sizes with a nearly identical diameter, either on your current rim diameter or across rim sizes.
To judge how much a new size changes things, subtract the original diameter from the new one, then divide by the original. Going from 225/60R17 to 235/60R18 raises diameter about 1.5 inches, roughly 5 percent, which is a big change. Moving to 225/55R18 instead changes it well under 1 percent. The chart cannot show clearance at full steering lock or over bumps, rim width compatibility, load capacity or manufacturer approvals. Confirm those with your vehicle documentation or a tire professional.
Light-Truck Tire Sizes: LT-Metric and Flotation Chart
Light-truck sizes carry the LT prefix. An LT265/75R16 is about 31.6 inches tall, and an LT285/70R17 is about 32.7 inches. On larger wheels, an LT275/70R18 measures roughly 33.2 inches and an LT275/65R20 about 34.1 inches. Flotation sizes run from about 31 to 37 inches. For example, a 35x12.50R17 sits near 35 inches, while a metric 315/70R17 calculates to about 34.4 inches. That makes them close, but the two are not identical.
For towing and hauling, load capacity matters more than dimensions. Load range letters such as C, D and E indicate progressively higher load capacity at higher inflation pressures. A tire that is dimensionally correct but underrated for the load is the wrong tire. Taller truck tires make speedometer error and clearance problems worse. Lift or leveling kits change the fitment limits without making them go away. Use the placard and owner documentation to confirm approved sizes, and get a professional fitment check before going beyond them.