How the Tire Size Calculator works.
diameter = rim + 2 × width × aspect ÷ 25.4 ÷ 100; compare new diameter with a reference tire derived from its own sidewall dimensionsEnter values in the units named in each field. The calculator retains precision during the calculation and rounds displayed currency and measurements for readability.
Worked example for the Tire Size Calculator.
For this Tire Size Calculator example, with New tire section width (mm): 225; New tire aspect ratio (%): 45; New tire rim diameter (in): 17; Reference tire section width (mm): 225; Reference tire aspect ratio (%): 45; Reference tire rim diameter (in): 17, the calculator returns 24.97 in diameter. Calculated tire geometry compared with the entered reference tire; verify fitment with vehicle and tire professionals.
Read the sidewall numbers correctly
A size such as 225/45R17 means a nominal 225 mm section width, a sidewall height equal to 45% of that width, radial construction, and a 17-inch rim. The calculator uses the width, aspect ratio and rim diameter as arithmetic inputs; the printed size is not a direct measurement of the mounted tyre.
How diameter and circumference are estimated
FigureNest converts the sidewall height from millimetres to inches, doubles it for both sides of the wheel, and adds the rim diameter to estimate overall tyre diameter. Circumference is then estimated from diameter × π. Real tyres can differ because of manufacturer tolerances, tread depth, pressure, load and wheel width.
Interpret the speedometer difference
Changing overall diameter changes how far the vehicle travels for each wheel revolution. A larger replacement tyre generally makes actual road speed higher than the speed indicated for the original calibration; a smaller tyre generally does the opposite. Treat the displayed percentage as a geometric comparison, not as proof that a specific vehicle calibration will behave identically.
Fitment needs more than matching diameter
Two sizes with similar calculated diameter can still differ in width, sidewall shape, load rating, speed rating and clearance. Steering lock, suspension travel, wheel offset, rim-width approval, brake clearance, bodywork, chains, TPMS and manufacturer requirements are outside this calculation. Confirm approved fitment before installing a different size.
Check the source size before comparing
Use the complete size from the tyre sidewall or vehicle placard and keep the original and proposed sizes in the correct columns. Do not substitute wheel diameter for overall tyre diameter or treat the aspect ratio as millimetres. If the sidewall marking uses a different sizing system, convert or use a tool designed for that system first.
What this calculator does not include.
This calculator compares labelled tyre geometry only. It does not verify actual mounted dimensions, wheel offset or rim-width compatibility, load or speed ratings, brake and suspension clearance, body clearance through steering and suspension travel, TPMS behavior, ABS or stability-control calibration, gearing, manufacturer approval, inspection rules, or legal roadworthiness. Confirm the complete replacement specification for the specific vehicle and wheel before fitting tyres.
Reference guidance and methodology.
Tire Size Calculator questions.
What does 225/45R17 mean?
225 is the nominal section width in millimetres, 45 is the sidewall height as a percentage of section width, R denotes radial construction, and 17 is the rim diameter in inches.
Does the calculator measure my actual tyre diameter?
No. It estimates geometric diameter from the labelled size. Mounted diameter can vary with brand, tread, pressure, load, wheel width and manufacturing tolerances.
Why does tyre size affect the speedometer?
The vehicle estimates road speed from wheel rotation using its calibration. A different rolling circumference changes the distance travelled per revolution, so indicated and actual speed can diverge.
Is a small diameter difference automatically safe to fit?
No. Diameter is only one fitment factor. Width, wheel offset and rim width, load and speed ratings, suspension and body clearance, brake clearance, TPMS and manufacturer approvals also matter.
Can I use this result to choose a legal replacement size?
Not by itself. The calculator provides geometry only. Check the vehicle placard/manual, tyre and wheel specifications, inspection or registration rules, and a qualified tyre professional where required.