Tire Size Conversion Calculator

Convert standard metric tire size codes (such as 225/45R17 or 265/70R16) into physical dimensional specifications including sidewall height, total dia

Conversions

Tire Section Width (mm)

First number in tire code (e.g. 225 in 225/45R17).

Aspect Ratio (%)

Second number in tire code (sidewall height as % of width).

Wheel / Rim Diameter (in)

Third number in tire code (wheel rim diameter in inches).

Formula & Methodology

Sidewall = Width × (Aspect ÷ 100) | Diameter = Rim + (2 × Sidewall ÷ 25.4) | Circumference = π × Diameter

Tire section width is measured in millimeters. Multiplying width by the aspect ratio percentage gives sidewall height in millimeters (divided by 25.4 to convert to inches). Total diameter adds the wheel rim diameter to twice the sidewall height. Circumference equals π times overall diameter, and revolutions per mile divides 63,360 inches (one mile) by tire circumference.

Overall Tire Diameter

24.97 in

Overview

Convert standard metric tire size codes (such as 225/45R17 or 265/70R16) into physical dimensional specifications including sidewall height, total diameter, rolling circumference, and revolutions per mile. Automotive tire codes combine millimeters (width), percentages (aspect ratio), and inches (rim diameter) into a single composite string.

Worked Examples

Calculate 225/45R17 Performance Tire

Section width 225 mm, aspect ratio 45%, 17-inch wheel rim.

Compute sidewall height: 225 mm × 0.45 = 101.25 mm.
Convert sidewall to inches: 101.25 ÷ 25.4 = 3.986 inches.
Compute total diameter: 17 in + (2 × 3.986 in) = 24.97 inches (634.3 mm).
Compute rolling circumference: π × 24.972 in = 78.45 inches (1,992.7 mm).
Compute revolutions per mile: 63,360 in/mi ÷ 78.45 in = 807.6 revs/mile.

Final Answer: Diameter: 24.97 in (634.3 mm) | Sidewall: 3.99 in | 807.6 revs/mile

Calculate 265/70R16 All-Terrain Truck Tire

Section width 265 mm, aspect ratio 70%, 16-inch wheel rim.

Compute sidewall height: 265 mm × 0.70 = 185.5 mm (7.30 inches).
Compute total diameter: 16 in + (2 × 7.303 in) = 30.61 inches (777.4 mm).
Compute rolling circumference: π × 30.606 in = 96.15 inches (2,442.3 mm).
Compute revolutions per mile: 63,360 ÷ 96.15 = 659.0 revs/mile.

Final Answer: Diameter: 30.61 in (777.4 mm) | Sidewall: 7.30 in | 659.0 revs/mile

Calculate 205/55R16 Standard Passenger Tire

Section width 205 mm, aspect ratio 55%, 16-inch wheel rim.

Compute sidewall height: 205 mm × 0.55 = 112.75 mm (4.439 inches).
Compute total diameter: 16 + (2 × 4.439) = 24.88 inches (631.9 mm).
Compute circumference: π × 24.878 = 78.16 inches (1,985.2 mm).
Compute revolutions per mile: 63,360 ÷ 78.16 = 810.7 revs/mile.

Final Answer: Diameter: 24.88 in (631.9 mm) | Sidewall: 4.44 in | 810.7 revs/mile

How to Use This Calculator

Find your tire size stamped on your tire sidewall (format: 225/45R17).

Enter section width in millimeters (the first number).

Enter aspect ratio percentage (the second number).

Enter wheel rim diameter in inches (the number following the R).

Review the calculated overall diameter, sidewall height, rolling circumference, and revolutions per mile.

Reference Table

Tire Size	Overall Diameter	Sidewall Height	Circumference	Revs per Mile
195/65R15	25.0 in (634.5 mm)	5.0 in (126.8 mm)	78.5 in (1,993 mm)	807.0 revs/mi
205/55R16	24.9 in (631.9 mm)	4.4 in (112.8 mm)	78.2 in (1,985 mm)	810.7 revs/mi
225/45R17	25.0 in (634.3 mm)	4.0 in (101.3 mm)	78.5 in (1,993 mm)	807.6 revs/mi
225/65R17	28.5 in (724.3 mm)	5.8 in (146.3 mm)	89.6 in (2,275 mm)	707.3 revs/mi
245/40R18	25.7 in (653.2 mm)	3.9 in (98.0 mm)	80.8 in (2,052 mm)	784.0 revs/mi
265/70R16	30.6 in (777.4 mm)	7.3 in (185.5 mm)	96.2 in (2,442 mm)	659.0 revs/mi
275/55R20	31.9 in (810.5 mm)	6.0 in (151.3 mm)	100.2 in (2,546 mm)	632.1 revs/mi

Frequently Asked Questions

What do the numbers on a tire size code mean?

In a tire code like 225/45R17: 225 is the section width in millimeters across the widest point of the tire; 45 is the aspect ratio (the sidewall height is 45% of the 225 mm width); R signifies radial construction; and 17 is the wheel rim diameter in inches.

How does changing tire diameter affect the speedometer?

A larger diameter tire travels further per revolution, causing the speedometer to read slower than actual ground speed. Conversely, a smaller diameter tire rotates faster, causing the speedometer to read higher than actual speed.

What is the maximum recommended difference when changing tire sizes?

Automotive engineers recommend keeping replacement tire overall diameter within ±3% of the original factory tire diameter to prevent speedometer inaccuracies, ABS sensor malfunctions, and transmission shift point errors.

How do you calculate revolutions per mile for a tire?

Divide 63,360 (the number of inches in one statute mile) by the tire’s rolling circumference in inches: Revs per Mile = 63,360 ÷ (π × Diameter).

Does tire pressure affect overall tire diameter?

Under-inflation decreases the "loaded rolling radius" (the distance from the wheel hub to the road surface under vehicle weight), increasing revolutions per mile and reducing fuel economy.