Understanding a tyre marking makes it easier to choose compatible car tyres, compare alternatives, and avoid ordering the wrong size. This practical tyre size calculator guide explains what each number means, how to estimate overall diameter, and which fitment details must be checked before you buy tyres online.
Overview
A marking such as 225/45 R17 94W contains several separate measurements and ratings:
- 225 is the nominal tyre width in millimetres.
- 45 is the aspect ratio. The sidewall height is 45% of the tyre width.
- R identifies radial construction, which is used by most modern car tyres.
- 17 is the wheel diameter in inches.
- 94 is the load index, indicating the maximum load assigned to one tyre when used under the specified conditions.
- W is the speed rating, identifying the maximum speed category for the tyre.
The first four parts describe the tyre's physical size. The load index and speed rating describe its operating capability. A replacement tyre should normally meet or exceed the vehicle manufacturer's required load and speed ratings, but a higher number is not automatically suitable if the tyre's dimensions, construction, clearance, or vehicle approval do not match.
For a complete refresher on markings and ratings, see our tyre size guide. This article focuses on using those markings as a repeatable fitment check.
How to estimate
You can estimate a tyre's overall diameter with three inputs: width, aspect ratio, and wheel diameter. Use these formulas:
- Sidewall height in millimetres = width × aspect ratio ÷ 100.
- Wheel diameter in millimetres = wheel diameter in inches × 25.4.
- Overall tyre diameter = (sidewall height × 2) + wheel diameter in millimetres.
To estimate circumference, multiply the overall diameter by 3.1416. To compare two sizes, use:
Percentage difference = (new diameter − original diameter) ÷ original diameter × 100.
This calculation is useful when considering an approved alternative size, but it is not a complete fitment decision. A tyre can have a similar overall diameter and still be unsuitable because it is too wide, has the wrong load rating, contacts the suspension, or does not fit the wheel correctly.
A simple tyre size calculator can therefore be used in four steps:
- Read the original marking from the tyre sidewall, vehicle handbook, door-jamb label, or manufacturer's fitment information.
- Calculate the original diameter and record the complete specification, including load index and speed rating.
- Calculate the proposed size using the same method.
- Compare the result with the vehicle's approved sizes and have a qualified tyre professional confirm clearance and compatibility.
Do not use the calculator to replace the vehicle handbook or a confirmed fitment lookup. It is a comparison tool, not permission to install any size that appears mathematically close.
Inputs and assumptions
Width and aspect ratio
In 205/55 R16, the sidewall is 55% of 205 mm: 112.75 mm. In 225/45 R17, it is 45% of 225 mm: 101.25 mm. A wider tyre may change steering feel, road noise, fuel use, and clearance. A lower aspect ratio usually means a shorter sidewall, which can affect ride comfort and wheel protection.
Wheel diameter and construction
The number after R refers to the wheel diameter, not the tyre's total diameter. A 17-inch tyre must be fitted to a 17-inch wheel. The wheel also needs a compatible width and profile. Do not fit a tyre simply because its diameter calculation looks similar.
Load index
The load index is a code rather than a weight measurement printed directly on the sidewall. Use a current tyre load rating chart or a reputable fitment database to convert the code into its assigned load capacity. Check the complete marking, because a tyre with the correct width but an insufficient load index may not be suitable.
Pay particular attention to SUVs, 4x4s, vans, towing vehicles, and cars carrying frequent heavy loads. Some vehicles require reinforced or extra-load tyres. Run-flat tyres may also require compatible wheels, pressure-monitoring equipment, and vehicle approval.
Speed rating
The speed rating is shown by a letter. Common examples include H, V, and W, but the exact requirement depends on the vehicle and market. Use the rating specified by the manufacturer as the baseline. Speed rating is not a recommendation to drive at that speed; it describes a certified capability under defined test conditions.
Other fitment inputs
Before ordering, confirm the wheel's width, bolt pattern, centre bore, and offset. The PCD, or pitch circle diameter, describes the bolt-hole arrangement. Wheel offset, often shown as ET, describes the wheel's mounting position relative to its centreline. These details matter when buying a wheel and tyre package. Our guide to wheel offset and clearance explains why a small offset change can affect suspension and body clearance.
Also check whether the vehicle uses a staggered setup, in which the front and rear axles use different sizes. Such vehicles may have limited rotation options, as explained in our staggered tyre setup guide.
Worked examples
Example 1: 205/55 R16
For a 205/55 R16 tyre:
- Sidewall height = 205 × 55 ÷ 100 = 112.75 mm.
- Wheel diameter = 16 × 25.4 = 406.4 mm.
- Overall diameter = (112.75 × 2) + 406.4 = approximately 631.9 mm.
This gives an estimated circumference of approximately 1,985 mm before allowing for tyre deformation under load. The result can be used to compare another approved size, but it does not confirm that the alternative will fit the vehicle.
Example 2: 225/45 R17
For a 225/45 R17 tyre:
- Sidewall height = 225 × 45 ÷ 100 = 101.25 mm.
- Wheel diameter = 17 × 25.4 = 431.8 mm.
- Overall diameter = (101.25 × 2) + 431.8 = approximately 634.3 mm.
If these two sizes are being compared, the estimated diameter difference is about 0.4%. That is a close numerical match, but the widths differ by 20 mm and the wheel diameters differ by one inch. The vehicle must specifically approve the proposed size, and the wheel must be suitable for its tyre width.
Example 3: comparing load and speed markings
Suppose two tyres share the same 225/45 R17 dimensions but one is marked 91Y and the other 94W. They are not identical specifications. The load index differs, as does the speed category. Select the version that meets the vehicle's required specification and intended use. For everyday driving, the best tyres are not simply those with the highest letter or number; they are tyres that match the vehicle, load, climate, and driving conditions.
When to recalculate
Revisit your tyre size calculation whenever you change wheel diameter, consider a non-standard size, replace only two tyres, or buy a wheel and tyre package. Recheck the complete marking if a replacement tyre looks different from the other tyres, if the vehicle has been modified, or if you are moving between summer tyres, winter tyres, and all season tyres.
Before placing an order, use this checklist:
- Confirm the vehicle's make, model, year, engine or trim, and drive configuration.
- Read the exact size, load index, and speed rating from the vehicle label or handbook.
- Check whether the vehicle has different front and rear specifications.
- Compare the proposed overall diameter using the formulas above.
- Confirm wheel width, PCD, centre bore, offset, and clearance if changing wheels.
- Check that the tyre type, such as run-flat, reinforced, SUV, 4x4, or van, matches the vehicle's requirements.
- Ask a qualified fitter to confirm the final combination before installation.
Tyre size calculations are most valuable when used alongside regular inspections. Check inflation pressures against the vehicle label rather than the maximum pressure printed on the tyre, and inspect tread and sidewalls for damage. Our tyre maintenance checklist covers those routine checks. If a tyre has a bulge, deep cut, or other sidewall damage, do not rely on a size comparison; arrange an inspection and replacement decision promptly.
