Tire Size Calculator
Take advantage of the calculators below to compute a tire’s dimensions from its code and to compare other tires that will fit the same wheel.
Tire Size Comparison Calculator
Use this calculator to compare the sizes of two tires both numerically and visually.
Tire code
Each tire is identified by an alphanumeric sequence (see the diagram). Understanding how to interpret this code is essential, especially when selecting new wheels or tires, since the code determines specifications such as size. Every segment of the sequence represents a specific tire characteristic; refer to the illustration.
Intended use: The initial segment consists of one or two letters (P, LT, ST, T) indicating the vehicle category the tire is designed for. Four classes are defined:
- P denotes passenger vehicles. Tires marked with P are suitable for cars, SUVs, crossovers, minivans, and light pickup trucks.
- LT represents light trucks. LT tires are built to handle heavier payloads and can tow trailers.
- ST indicates special‑purpose trailers. These tires are used on boat, utility, and travel trailers.
- T means temporary. T‑specified tires are typically spare units not meant for regular driving.
Nominal section width: The next segment is a three‑digit number indicating the tire’s nominal width in millimetres. This measurement corresponds to the tire’s widest point, measured from sidewall to sidewall. In the illustration, “215” signifies a width of 215 mm.
Aspect ratio: Following the slash ( / ), a two‑ or three‑digit number expresses the proportion of sidewall height to tread width. The sidewall height is measured from the rim to the outer tire edge, while the width is sidewall‑to‑sidewall. A code such as xxx/65 means the sidewall height equals 65 % of the width; e.g., 200/65 yields a height of 200 × 0.65 = 130 mm.
Internal construction: The following character, a single letter (B, D, R), specifies how the tire’s carcass is built. The carcass consists of cord plies that provide the tire’s strength and longevity. This letter is optional; if missing, the tire is presumed to be of cross‑ply construction.
- R denotes a radial build. The vast majority of passenger vehicles are equipped with radially constructed tires. In this design, the reinforcing cords are laid circumferentially, crossing the tread at right angles to the rolling direction.
- D indicates a diagonal (bias) layout. Here the ply cords are oriented around the tire at roughly a 30‑40° angle relative to the bead, creating a criss‑cross pattern between successive layers.
- B marks a bias‑belted construction. It resembles a diagonal build but adds a reinforcing belt atop the final ply. These stabilizer belts—often steel or similar cords—are laid at distinct angles, boosting rigidity and load‑bearing capacity.
- Cross-ply tires are tires whose internal construction is made of nylon cord placed diagonally across each other, usually at an angle of 55°.
Wheel diameter: This segment of the code consists of one or two digits representing the wheel’s diameter in inches—the rim size the tire is meant for. Numerous tire models exist, and a single rim may accommodate various tire dimensions provided the tire’s central section matches the rim width.
Load index: Following that, a two‑ or three‑digit figure specifies the tire’s load index, which tells how much mass the tire can support when properly inflated. As an illustration, a load index of 95 corresponds to a capacity of roughly 1,520 lb (≈690 kg) at 42 psi.
Speed rating: The final element, a single letter or a letter‑digit pair, denotes the tire’s speed rating—the highest velocity at which it can sustain its rated load. For instance, a tire rated H with a load index of 95 is approved for 1,520 lb (≈690 kg) at 42 psi and can safely travel up to 130 mph (210 km/h).
Note that tires have many additional marks or codes, such as the DOT code.
DOT code: In the U.S., the DOT marking is mandatory and encodes the manufacturer, plant, batch, mold and production date of the tire. The date is given as a four‑digit number: two digits for the production week and two for the year (e.g., “1324” indicates week 13 of 2024). Knowing this stamp is useful when buying new tires, because rubber deteriorates over time; tires older than a decade are generally unsafe, and many makers advise swapping them out after six years irrespective of mileage.
How is tire size calculated?
The dimensions computed by our calculator use the following equations:
Section Height = Aspect Ratio × Tire Width
Tire Diameter = Wheel Diameter + 2 × Section Height
The aspect ratio and the tread width are both extracted from the tire designation. The width appears as a three‑digit figure before the slash (/) and is measured in millimetres. The aspect ratio follows the slash as a two‑ or three‑digit value; multiplying these two numbers yields the section height.
After determining the section height, the overall tire diameter is found by adding twice that height to the wheel diameter. The wheel size is also encoded in the tire label, appearing as one or two digits after the internal‑construction letter (or directly after the aspect ratio when the letter is omitted). This dimension is expressed in inches, and the formula is: tire diameter = wheel diameter + 2 × section height.
What to consider when changing tire/wheel size?
Drivers alter tire or wheel dimensions for a variety of motives, frequently for visual appeal. However, resizing any component requires careful assessment; neglecting this can lead to serious drawbacks, like accelerated wear on the suspension and braking system (especially ABS), altered handling characteristics, or even transmission‑related problems.
Tire sizes vary widely, and you can mount several dimensions on one rim as long as the tire's interior width matches the rim. Many owners choose to upsize the wheels, which means the tread profile must be adjusted. Bigger wheels usually require lower‑profile tires so they clear the wheel well. As a rule of thumb, keep the overall diameter within about 3 % of the original equipment specification.
Beyond the visual appeal, drivers replace tires when they suffer punctures or reach the end of their service life. The safest approach is to fit a full set at once; if that isn’t feasible, swap the two front or the two rear tires together, depending on which axle contains the worn unit. When only a single or a few tires are changed, it’s best to choose the same model and brand, because mixing types can lead to handling problems, erroneous speedometer data, or even interference with the vehicle's stability‑control system.
Fitting different‑size wheels and tyres also alters the calibration of the speedometer and odometer, which are calibrated for the factory‑fit tires. Larger‑diameter setups increase the wheel's circumference, so the car travels farther per revolution; this makes the speedometer read lower than the true speed and the odometer under‑report mileage. Conversely, smaller diameters cause the gauges to over‑state speed and distance.