Is That a 33-Inch Tire? Calculate Diameter From the Sidewall Size

An 18-inch wheel does not mean an 18-inch-tall tire. The wheel sits inside the tire, with a sidewall above it and another below it. To work out whether a metric size is roughly 33 inches tall, you need all three numbers—not just the one after the R.

This calculation is useful for understanding a specification or a walkaround description. It estimates nominal diameter from the size code; it is not a measurement of the fitted tire or permission to install a different size.

Read the three numbers first

Michelin Canada’s sidewall guide explains that a metric size gives nominal width in millimetres, sidewall height as a percentage of width, and wheel diameter in inches. R identifies radial construction.

For the example 275/70R18:

• 275 is the nominal width in millimetres.
• 70 means the sidewall height is 70% of that width—not 70 mm.
• 18 is the wheel diameter in inches—not the outside diameter of the tire.

The calculation below uses those nominal dimensions. Record the full code from your tire or the exact manufacturer specification. Do not fill in a missing number from another trim’s equipment list.

The formula

Approximate diameter in inches = wheel diameter + [2 × width in mm × (aspect ratio ÷ 100) ÷ 25.4].

There are 25.4 mm in 1 inch. Multiply sidewall height by 2 because the overall diameter includes rubber on both sides of the wheel.

Worked example: 275/70R18

• One sidewall: 275 × 0.70 = 192.5 mm.
• Two sidewalls: 192.5 × 2 = 385 mm.
• Convert to inches: 385 ÷ 25.4 = 15.16 inches, rounded.
• Add the wheel: 15.16 + 18 = 33.16 inches, approximately.

A clear description is “275/70R18 tires, approximately 33.2 inches in nominal diameter.” That keeps the exact size and the calculated shorthand together. It does not claim that a tape measure across every brand’s tire will show precisely 33.16 inches.

Two more examples—and a common trap

285/70R17: 285 × 0.70 × 2 ÷ 25.4 + 17 = 32.71 inches, approximately.

315/70R17: 315 × 0.70 × 2 ÷ 25.4 + 17 = 34.36 inches, approximately.

These are arithmetic examples, not recommended sizes for a particular vehicle. Notice that both use 17-inch wheels but have different overall diameters. Also, a 70-series sidewall is not a fixed height: 70% of 315 mm is greater than 70% of 285 mm.

Do not round 34.36 up to 35 merely because “35-inch tires” sounds familiar. If the tire or vehicle manufacturer uses a marketed size description, identify that separately from the calculated metric diameter.

Use product specifications when precision matters

For a particular tire, look up the exact brand, product line and size in the manufacturer’s specification table. Use its stated overall diameter and measurement conditions rather than presenting the formula as a laboratory result. A calculator based only on the code cannot inspect tread wear, inflation, the fitted wheel or the loaded tire.

When reading a review, distinguish the sidewall size, the manufacturer’s published diameter and a calculated estimate. Only an actual measurement should be labelled “measured diameter.”

A similar diameter does not establish compatibility

Michelin’s size-change guidance warns that changing size can affect the speedometer, odometer and vehicle-control systems. It recommends the manufacturer’s specified size or an approved alternative. Similar overall diameter alone does not settle load capacity, wheel fit, clearance or other requirements.

Do not use a generic percentage-tolerance rule as approval for your vehicle. Confirm permitted fitments with the vehicle manufacturer and a qualified tire professional. Keep load and speed requirements separate from the diameter arithmetic.

For a replacement purchase, give the supplier the full size, tire model, vehicle details and placard information. “I need 33s” is a useful conversation starter, not a complete order specification.

Canadian manufacturer guidance checked November 1, 2026. Calculations use nominal metric dimensions and rounded results.