A tire size calculator helps you compare two different tire sizes and understand how swapping tires will affect your vehicle's performance. By entering the width, aspect ratio, and rim diameter of both your original and new tires, you can instantly see the difference in overall diameter, circumference, sidewall height, and how many revolutions your tires make per mile.
This is useful when upgrading to larger wheels, fitting aftermarket tires, or checking if a replacement tire is a safe match for your vehicle.
Formulas Used
Sidewall Height
The sidewall height is the distance from the rim edge to the outer tread.
Sidewall Height (mm) = Section Width × (Aspect Ratio / 100)
Sidewall Height (inches) = Sidewall Height (mm) / 25.4
Overall Diameter
The total outer diameter of the tire from top to bottom.
Overall Diameter (inches) = Rim Diameter + (2 × Sidewall Height in inches)
Circumference
The total distance the tire covers in one full revolution.
Circumference (inches) = π × Overall Diameter
Revolutions per Mile
How many times the tire rotates to cover one mile.
Revs per Mile = 63,360 / Circumference
Section Width (in inches)
Section Width (inches) = Section Width (mm) / 25.4
Diameter Difference (%)
Difference (%) = ((New Diameter − Old Diameter) / Old Diameter) × 100
A general rule in the automotive industry is that your new tire's overall diameter should not differ by more than 3% from the original tire size recommended by the manufacturer.
Going beyond 3% can cause:
If the diameter difference exceeds 3%, always consult a professional mechanic before fitting the new tires.