Tire Size Calculator

How to use this calculator

Enter the three numbers from your tire size: section width in millimetres, aspect ratio as a percentage, and rim diameter in inches. Or choose a common preset from the dropdown to auto-fill all three fields. Select whether you want results in imperial (inches and rev/mile) or metric (millimetres and rev/km), then click Calculate.

The calculator returns overall diameter, sidewall height, circumference, revolutions per mile, and revolutions per kilometre. Below the result card, you will find an annotated tire cross-section diagram and a plus-sizing table showing how one-size and two-size rim upgrades affect overall diameter.

Tire Width (mm): The section width, which is the first number in the tire size. Example: in 245/65R17, the width is 245 mm.

Aspect Ratio (%): The second number in the tire size. It represents sidewall height as a percentage of section width. In 245/65R17, the aspect ratio is 65, meaning the sidewall is 65% of 245 mm = 159.25 mm tall.

Rim Diameter (inches): The number following the R in the tire size. In 245/65R17, the rim diameter is 17 inches.

Common Presets: Click the dropdown to select a popular size. All three input fields will fill automatically. You can then modify any value and recalculate.

Understanding the tire size notation

The sidewall of every modern passenger tire carries a standardized size code. Knowing what each number means helps you compare sizes, understand what changes when you switch tires, and calculate whether a replacement tire will fit your vehicle.

The full breakdown

Consider the tire size 225/50R17 94V:

• 225: Section width in millimetres. Measured across the widest point of the inflated tire when mounted on the specified rim width. Note that section width is not the same as contact patch width, which depends on the tire’s construction and inflation pressure.

• 50: Aspect ratio. The sidewall height divided by the section width, expressed as a percentage. A 50 aspect ratio on a 225mm wide tire gives a sidewall height of 225 × 0.50 = 112.5 mm. Lower numbers mean shorter, stiffer sidewalls.

• R: Construction. R = radial, which is used on essentially all modern passenger and light truck tires. Radial construction means the cord plies run perpendicular (radially) to the direction of travel, rather than at a bias angle. Radial tires offer better fuel economy, handling, and tread life than older bias-ply designs.

• 17: Rim diameter in inches. The only imperial measurement in the metric code. This must match your wheel’s bead seat diameter exactly.

• 94: Load index. A two- or three-digit number indicating the maximum load capacity per tire. A load index of 94 equals 1,477 lbs. The full load index table is available from tire manufacturers.

• V: Speed rating. The letter code indicates the maximum sustained speed for which the tire is designed. V = 149 mph (240 km/h). Common speed ratings: S = 112 mph, T = 118 mph, H = 130 mph, V = 149 mph, W = 168 mph, Y = 186 mph.

P-metric prefix

A P before the size (like P225/50R17) denotes a tire made to the Passenger vehicle specification, which originated in North America. The difference from a metric tire is a calculation factor called the Load Range Service Description. When comparing load capacity between P-metric and metric tires of the same size, the P-metric tire has slightly lower load capacity (divide by 1.10 for single-wheel applications). For most daily driving this difference is irrelevant.

LT prefix

LT before the size (LT245/75R16) means Light Truck specification. These tires have heavier construction and higher load capacity than P-metric equivalents. They are required on some trucks and trailers where the load ratings of P-metric tires are insufficient.

Why overall diameter matters

The overall tire diameter is the single most important number when evaluating a tire size change. It affects:

Speedometer and odometer accuracy: Both are calibrated for a specific tire circumference. Change the diameter and they read incorrectly, proportionally to the diameter difference. A 3% larger tire causes a 3% speedometer read-low and a 3% odometer undercount. See the Speedometer Error Calculator for the full calculation.

Ground clearance: A larger tire raises the vehicle’s ride height. On trucks and SUVs, this is often a desired outcome of upgrading to larger tires. On passenger cars, larger tires can create clearance issues with fenders, inner fender liners, and suspension components, especially at full steering lock.

Gear ratio effect: A larger tire effectively changes your final drive ratio. A vehicle with a 3.73 axle ratio and 28” tires has an effective final drive of 3.73. Fit 33” tires and the effective final drive becomes 3.73 × (28/33) = 3.17. The engine has to work harder (more torque required) to move the vehicle, particularly at low speeds and when towing. This is why many tire upgrades are accompanied by a re-gear of the axle.

Braking distance: Taller tires increase the effective lever arm length between the brake rotor center and the road contact point. At the same brake clamping force, a larger tire requires more force to slow the vehicle. Modern ABS systems calibrate from wheel speed data and partially compensate, but significant size changes can affect braking feel and distance.

Speedometer calibration reference: Every calculation in tire sizing starts with the overall diameter, which is why this calculator puts it front and center.

The math of tire size dimensions

All tire size calculations derive from five numbers: section width (W), aspect ratio (A), rim diameter (R), and the constants for unit conversion (25.4 mm/inch, π for circumference, and 63,360 inches/mile).

The factor 63,360 is the number of inches in one mile (5,280 feet × 12 inches/foot). The factor 1,000,000 is the number of millimetres in one kilometre.

Plus sizing explained

Plus sizing refers to upgrading to a larger diameter rim while reducing the tire’s aspect ratio to keep the overall diameter close to the original. It is a common modification aimed at improving handling, fitting larger brakes, or achieving a visual change.

How it works

Each “plus” step increases rim diameter by 1 inch and reduces the aspect ratio to compensate:

• Plus Zero: Same rim, wider tire. Aspect ratio drops to keep the same diameter. Example: 195/65R15 to 215/60R15.
• Plus One: Rim increases by 1 inch. Example: 205/65R15 to 225/50R16.
• Plus Two: Rim increases by 2 inches. Example: 205/65R15 to 235/45R17.

The 3% rule

The general guideline is to keep the overall diameter within 3% of the original. Within that range, speedometer error is below the factory tolerance for most vehicles (typically 2-3% factory read-fast). Beyond 3%, you begin to get noticeable speedometer inaccuracy and possible clearance issues.

Why plus sizing improves handling: A lower aspect ratio sidewall deflects less under lateral load during cornering, keeping the contact patch more stable on the road. A wider contact patch increases lateral grip. The trade-off is a harsher ride because the shorter sidewall cannot absorb road impacts as effectively.

Why plus sizing does not increase braking distance (much): A wider contact patch distributes braking force over a larger area. This can improve dry stopping distances slightly. The negative effect is more rotating mass (a wider, heavier rim and tire) which slightly increases the kinetic energy to be dissipated during braking.

What to watch for

Before buying a plus-sized set, verify:

1. Wheel offset and backspacing: A wider wheel that does not fit the hub correctly can create interference with suspension components or cause poor handling.
2. Tire speed rating: Match or exceed the original specification.
3. Load index: Never drop below the factory load index.
4. Clearance: Check for clearance at full steering lock and full suspension travel, especially if you are also running aftermarket suspension.

Section width vs rim width compatibility

Every tire has a recommended rim width range. Mounting a tire on a rim that is too narrow or too wide causes problems ranging from poor handling to bead seating failure.

Too narrow a rim: The sidewalls bow inward, narrowing the contact patch. The tire may be prone to rolling off the rim under hard cornering loads. The handling becomes vague.

Too wide a rim: The sidewall stretches outward. This can expose the bead area to damage and in extreme cases prevent proper bead seating. It also changes the aspect ratio geometry, making the sidewall stiffer than the tire was designed for.

General guideline: The minimum recommended rim width is approximately the section width divided by 25.4, then multiplied by 0.75. The maximum is approximately the section width divided by 25.4 times 0.95.

Always verify the exact recommendation in the tire manufacturer’s fitment guide for the specific tire model, as construction differences between tires affect the acceptable rim width range.

Reading all the markings on a tire sidewall

The size code gets most of the attention, but modern tire sidewalls carry a significant amount of additional information. Here is a guide to the markings you will encounter.

DOT code: The U.S. Department of Transportation compliance code, required on all tires sold in the United States. The last four digits encode the week and year of manufacture. For example, DOT 3420 means the 34th week of 2020.

UTQG (Uniform Tire Quality Grading): Three ratings required by the NHTSA: treadwear (higher = longer tread life), traction (AA, A, B, C), and temperature resistance (A, B, C). A treadwear grade of 500 means the tire should last roughly twice as long as a tire rated 250, tested on a government track under controlled conditions. Real-world results vary.

Max inflation pressure: The maximum cold inflation pressure the tire can sustain. This is not the recommended operating pressure for your vehicle, which is found on the door jamb sticker. Max pressure on the tire is a safety limit, not a target.

Max load: Listed in both pounds and kilograms. This is the maximum load per tire at max inflation.

Ply rating or load range: Indicates the number of structural plies (layers) in the tire’s sidewall. Load range E (for LT tires) = 10-ply equivalent. This relates to the tire’s maximum load and inflation capacity.

M+S marking: Mud and Snow. Indicates the tire meets a basic all-season traction standard. Does not indicate a winter tire. For genuine winter performance, look for the Three-Peak Mountain Snowflake (3PMSF) symbol, which requires passing a traction test in snow.

Rotation direction arrow: If present, the tire has a directional tread pattern and must be mounted and rotated only in the indicated direction. Directional tires cannot be rotated side-to-side (left to right) without dismounting and remounting them on the wheels.

Winter tire sizing considerations

Many drivers run a separate set of winter tires on a second set of wheels. The sizing strategy for winter tires differs from plus sizing in one important way: narrower is often better in snow and slush.

Why narrower winter tires work better in snow: A narrower tire generates higher contact pressure (force per unit area) on the snow, which helps cut through to the underlying surface. Wide summer performance tires tend to float on top of deep snow rather than biting through it.

Winter sizing strategy: For a vehicle running 245/45R18 summer performance tires, a common winter set would be 225/55R17 on 17-inch steel wheels. This keeps the overall diameter close to the original (just check it with this calculator) while narrowing the section width and using a smaller, less expensive rim.

Diameter matching: The same 3% rule applies to winter fitments. Keep the winter tire diameter within 3% of the summer tire diameter to avoid significant speedometer issues and to maintain ABS calibration.

Rim considerations: Using a less expensive steel wheel for winter service is practical. Steel wheels are more resistant to corrosion and less likely to crack from cold temperatures and winter road hazards. The weight difference compared to an alloy wheel does not noticeably affect winter performance.

Load and speed ratings: The winter tires must meet or exceed the vehicle’s minimum load index and may carry a lower speed rating. Most winter tires are rated Q (99 mph) to H (130 mph). If the factory spec is V (149 mph), installing Q-rated winter tires is acceptable as long as you observe the speed limit indicated by the tire.