A standard convention reveals important dimensions about tire sizes, which are often confusing because of their variety. In order to choose the right tires, you must know how to read these sizes and convert them.
A Tire Size Converter Calculator has been created to simplify this process. It is able to convert metric and inch-based tire sizes with ease.
Calculator for converting tire sizes
Converter for tire sizes
Calculate the inch size of a metric tire
Width (mm):
Aspect Ratio (%):
Wheel Diameter (inches):
Convert
Tire Sizing Convention
The tire size is an expression of the tire’s key dimensions that consists of both letters and numbers. In a nutshell, tire size is composed of the following components:
- Section Width: The first three digits of a tire’s section width indicate the measurement in millimeters. Sidewall to sidewall, this measures the tire’s width.
- Aspect Ratio: A tire’s aspect ratio is calculated as part of its section height divided by its width. The section height of a tire in a 50 aspect ratio is equal to the section width.
- Construction Code: The type of construction indicated by this letter. Most passenger tires are radial, which is an abbreviation for radial.
- Wheel Diameter: A two-digit number that specifies how big the tire needs to be in order to fit the wheel.
- Load Index: A load index specifies how much weight a tire is capable of supporting (its load-bearing capacity).
- Speed Rating: The highest speed that a tire can reach is indicated by this letter code. S, T, H, V, W, Y, and Z are common speed ratings.
A tire with a 215mm section width, 65% aspect ratio, radial construction, fitting a 15-inch wheel, and having a load index of 95 (equivalent to 1,521 pounds) and a speed rating of H (130 mph) would have an aspect ratio of 215mm, be made of rubber, and conform to the tire specification P215/65R15 95H).
Chart for converting metric and imperial tire sizes
| Common Tire Size (P-Metric) | Width (inches) | Aspect Ratio | Wheel Diameter (inches) |
| P215/65R15 | 8.46 | 65 | 15 |
| P225/70R16 | 8.85 | 70 | 16 |
| P235/75R15 | 9.25 | 75 | 15 |
| P245/45R20 | 9.64 | 45 | 20 |
The importance of tire size
The tire dimensions must match those of your vehicle, so understanding the tire size from the manufacturer is crucial.
The handling, speedometer accuracy, and wheel wells of the vehicle can all be negatively impacted by incorrectly sized tires.
The sidewall of the tire contains information about the tire size. In order to determine what size OE (original equipment) tires the manufacturer recommends, vehicle owners should consult their owner’s manual or door jamb.
The importance of considering size conversions
The following considerations should be taken into account when converting tire sizes:
- Generally, the size and diameter of the sections can vary a little, but the wheel wells should not be affected by large variations.
- Ensure that you do not exceed the load and speed recommendation of your vehicle’s manufacturer.
- Comparing the current tires to new sizes requires measuring their actual diameter.
- Check how the dimensions of the new and old tires affect the speedometer by entering the current and new sizes. To find out whether your tires are compatible, consult an auto technician or tire retailer.
It is always best to follow the tire size recommendations of the vehicle manufacturer. The tire size converter, however, makes it easy to select the right tire and explore alternative sizes.
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Conclusion
There are several sets of conventions for tire sizing, as well as letter or number codes that can be confusing.
Converting between metric and inch-based tire sizes can be made easier with online tools (like the one I created above).
The dimensions, load capacities and speed ratings of key components must be consistent when converting sizes
