Tire Size Calculator
This free tire size calculator instantly converts any tire size code (like 225/45R17) into real-world measurements: overall diameter, sidewall height, circumference, and revolutions per mile. It's both a tire size estimator and a tire size dimension calculator. Use the Compare Sizes tab to check whether a different tire size will affect your speedometer accuracy - a must-check before fitting non-stock tires.
All calculations use standard published formulas. Results are for informational use only.
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How to read a tire size code - the tire profile calculator guide
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 in millimeters. This is the widest point of the tire from sidewall to sidewall. A wider tire provides more contact patch area, which can improve grip but may require wider rims.
45 = Aspect ratio (%). The sidewall height is 45% of 225mm = 101.25mm. Lower aspect ratios (35, 40) mean sportier, stiffer sidewalls; higher ones (65, 70) mean softer, more comfortable rides.
R17 = Radial construction, 17-inch rim diameter. The R is almost universal on modern tires. The rim number tells you which wheel size the tire fits.
How the tire diameter calculator formula works
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.
Revolutions per mile: why it matters for your car
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 - you're going faster than you think. A smaller tire has the opposite effect.
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.
How to use the tire size comparison tool
Switch to Compare Sizes mode and enter two tire codes (or manual dimensions). The table shows every measurement side by side - diameter, sidewall, circumference, and rev/mile - plus the exact speedometer error percentage if Tire 2 replaces a vehicle calibrated for Tire 1.
This is especially useful when: upsizing wheels (going from 17" to 18" rims with lower profile tires to keep the same diameter), choosing winter tires (slightly different sizing is common), or comparing a substitute size when your stock size is unavailable.
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 (0.6% difference) and are a common sport upgrade that keeps speedometer accuracy.
Winter tires: Many drivers run a slightly narrower winter tire on the same rims (e.g., 205/55R17 instead of 225/45R17). 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 via a tuner or programmer is typically required.
Quick tips for figuring tire size correctly
- Always check your door jamb sticker first. 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.
- Rim width matters as much as rim diameter. Each tire has a recommended rim width range. Too narrow causes sidewall bulge; too wide spreads the tread insufficiently for safe cornering grip.
- 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 before you buy.
- Load and speed ratings must meet or exceed 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 whether the overall diameter is acceptable. Two tires can share the same section width but differ by 2 full inches in overall diameter, which affects everything from ground clearance to fuel economy.
Not recalibrating after a plus-size upgrade
Plus-sizing (larger rim, lower profile) keeps overall diameter similar but rarely identical. Even a 1.5% increase in overall diameter adds nearly 1 mph of speedometer error at 60 mph. The car's ABS, traction control, and odometer all drift with an uncorrected size change over time. Use Compare mode to check the speedometer error percentage before purchasing.
Forgetting that winter tires use different sizing
Many drivers run a narrower winter tire (e.g., 205/55R17 instead of 225/45R17) to improve snow traction. The narrower width cuts through snow better, and the higher aspect ratio compensates to maintain a similar overall diameter. Always calculate the diameter for both sizes to verify the fit is correct before purchasing winter tires.
Frequently Asked Questions
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.
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.
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.
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.
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.