This free tire size calculator is your complete tire dimension calculator and tire diameter calculator. Enter any tire code (like 225/45R17) and this tire code calculator instantly converts it into real-world measurements: overall diameter, sidewall height, circumference, and revolutions per mile. Use the Compare Sizes tab as a tire size conversion calculator to check whether a different tire measurement will affect your speedometer accuracy. Perfect for figuring tire size for upgrades, replacements, or winter tire fitment.
Calculator Mode
Input Method
Tire Details
Overall Diameter
24.97″
634.30 mm
101.25 mm
Sidewall
78.45″
Circumference
807.61
Rev / Mile
8.86″
Section Width
Your 225/45R17 tire has an overall diameter of 24.97 inches (634.30mm), a circumference of 78.45 inches, and turns 807.61 times per mile.
Getting started
How to read a tire size code
Every tire sold in North America carries a standardized size code stamped on the sidewall. It looks like this: 225/45R17. These three numbers are all you need to calculate every physical dimension of the tire.
225
Section Width
Section width in millimeters. This is the widest point of the tire from sidewall to sidewall.
45
Aspect Ratio
The sidewall height is 45% of the 225mm width.
17
Rim Diameter
The tire fits a 17-inch wheel rim. The 'R' means Radial construction.
The calculation
Step-by-step: how tire dimensions are calculated
1
Calculate sidewall height
Sidewall Height = Width x Aspect Ratio / 100
= 225mm x 45% / 100
= 101.25mm (3.99in)
This is the height of the rubber between the wheel rim and the road contact patch.
2
Calculate overall tire diameter
Diameter = Rim x 25.4 + 2 x Sidewall
= 17in x 25.4 + 2 x 101.25mm
63,360 inches = 1 mile. This value calibrates speedometers and odometers.
Formula
How the tire diameter calculator formula works — tire measurement calculator explained
The overall tire diameter is the total height from the ground to the top of the tire. Since the rim sits in the center and sidewalls wrap both top and bottom, the formula is:
Diameter (mm) = Rim Diameter (in) x 25.4 + 2 x Sidewall Height (mm)
Sidewall Height = Width x Aspect Ratio / 100. For 225/45R17: sidewall = 225 x 0.45 = 101.25mm. Diameter = 17 x 25.4 + 2 x 101.25 = 431.8 + 202.5 = 634.3mm = 24.97 inches.
Background Information
Revolutions per mile: why it matters
Your vehicle's ECU (engine control unit), ABS system, and speedometer all use wheel revolution counts to determine speed and distance. The calibration assumes a specific tire diameter - typically your vehicle's stock tire size.
Formula: Rev/Mile = 63,360 / Circumference (inches). If you install a tire with a larger diameter, the wheel spins fewer times per mile. Since the speedometer expects more rotations, it will display a speed lower than your actual speed.
Any size change beyond ±3% from stock is generally considered significant enough to require recalibration. Use the Compare mode to check any two sizes before buying.
Interpreting
Common tire fitment scenarios
Plus-sizing
Going from a 17" to 18" or 19" rim while reducing the aspect ratio to keep the same overall diameter. For example, 225/45R17 (24.97") and 225/40R18 (25.12") are close in diameter and are a common sport upgrade.
Winter tires
Many drivers run a slightly narrower winter tire on the same rims. The narrower width cuts through snow better, and the higher aspect ratio compensates to maintain a similar overall diameter.
Truck/SUV upsizing
Mud terrain or all-terrain tires are often 1-3 sizes larger in diameter. Vehicles with a 2-3 inch lift can usually clear these without fender rubbing, but a speedometer recalibration is typically required.
It lists the exact OEM tire size, recommended inflation, and rim size. Don't guess - this is the definitive source for your vehicle's required specification.
Keep diameter within ±3% of stock.
Beyond ±3%, speedometer error exceeds typical legal tolerance and traction control calibration becomes unreliable. Use Compare mode to check any two sizes.
Load and speed ratings must meet OEM specs.
A visually identical tire with a lower load index can fail under vehicle weight. Verify the full specification number, not just the physical size.
Common tire size mistakes to avoid
Matching only the section width
Width is the first number in the tire code - but rim diameter and aspect ratio determine whether the tire physically fits the wheel and the overall diameter. Two tires can share the same width but differ by 2 inches in overall diameter.
Not recalibrating after a plus-size upgrade
Plus-sizing keeps overall diameter similar but rarely identical. Even a 1.5% increase in diameter adds nearly 1 mph of speedometer error at 60 mph. Use Compare mode to check the speedometer error percentage.
FAQ
Frequently asked questions
Q
How do I read a tire size code like 225/45R17?
The three numbers tell you everything about the tire's dimensions. The first number (225) is the section width in millimeters - measured across the widest point of the tire. The second number (45) is the aspect ratio - the sidewall height as a percentage of the section width. So the sidewall is 45% x 225mm = 101.25mm tall. The R means radial construction. The last number (17) is the rim diameter in inches that the tire fits. This calculator converts those three numbers into the actual diameter, circumference, and revolutions per mile.
Q
How do I calculate tire diameter?
Tire diameter = (Rim Diameter x 25.4) + (2 x Sidewall Height), all in mm. The sidewall height = Width x Aspect Ratio / 100. For a 225/45R17: sidewall = 225 x 0.45 = 101.25mm. Diameter = 17 x 25.4 + 2 x 101.25 = 431.8 + 202.5 = 634.3mm, or 24.97 inches.
Q
Why does tire size affect speedometer accuracy?
Your speedometer reads speed based on the number of wheel rotations per mile, which is calibrated for your stock tire's circumference. A taller tire (larger overall diameter) covers more ground per revolution, so the wheel spins fewer times per mile than your speedometer expects - causing it to read low. A shorter tire causes the opposite. The Compare mode in this calculator shows the exact speedometer error percentage between two tire sizes.
Q
What is the aspect ratio on a tire?
The aspect ratio is the sidewall height expressed as a percentage of the tire's section width. A tire with a 50 aspect ratio has sidewalls that are 50% as tall as the tire is wide. Lower aspect ratios (35, 40, 45) mean stiffer, sportier sidewalls that improve handling but transmit more road harshness. Higher aspect ratios (65, 70, 75) give a softer, more comfortable ride and are typical on trucks and SUVs.
Q
Can I put a different tire size on my car?
Yes, within limits. The new tire must physically fit (no rubbing fenders or suspension), and the overall diameter should ideally stay within ±3% of the stock size to keep your speedometer and ABS accurate. Width can typically vary by one size (e.g., 225 -> 235) as long as the rim width is compatible. Always check your vehicle's specifications and consult a tire professional before switching to a non-stock size for safety and legal compliance.
