Tire Size Calculator
Decode a P-metric tire size — overall diameter, sidewall height, circumference, revs per mile.
How to Use
- Enter section width (mm), aspect ratio (%), and wheel diameter (in).
- See the tire drawn to scale with its sidewall, overall diameter, and revs per mile.
- Comparing two sizes for an upgrade? Use the Tire & Wheel Calculator.
| 25 – 35 | Low profile · sharp, harsh ride |
| 40 – 50 | Sport · balanced |
| 55 – 65 | Touring · comfort |
| 70 – 85 | Truck / off-road · tall sidewall |
Show Work
Formulas
History of P-Metric Tire Sizing
The P-metric tire designation (e.g., P225/45R17) was standardized by the Tire and Rim Association in 1976, replacing the earlier alphanumeric codes (B78-14, F70-15) that had become unwieldy. The P-metric system compactly encodes three dimensions: section width in millimeters, aspect ratio (sidewall height as % of width), and wheel diameter in inches. The "R" denotes radial construction; "D" or "B" indicates diagonal/bias-ply (now obsolete for passenger cars).
Radial tires, invented by Michelin in 1946 and patented in 1949, revolutionized tire design by decoupling tread and sidewall construction. Radial cords run perpendicular to the wheel axis; a separate belt (steel, Kevlar, or nylon) stiffens the tread. Compared to bias-ply, radials last 2-3× longer, roll with 15-25% less energy loss, and grip better — which is why every car has worn radials since the 1970s.
The 45-series aspect ratio (sidewall = 45% of width) became mainstream with the 1990s-2000s performance-sedan trend. Lower aspect ratios (25, 30, 35) give sharper steering response but transmit every pavement imperfection to the driver and chew wheels on potholes. Today's EV-era tires are moving back toward taller sidewalls (50-55 series) to recover ride comfort and sidewall absorption of regenerative-braking torque.
About This Calculator
Enter section width (mm), aspect ratio (%), and wheel diameter (in). The tool computes sidewall height = width × aspect%, overall tire diameter = wheel + 2 × sidewall (in mixed metric/imperial per industry convention), circumference, and revolutions per mile (63,360 in/mi ÷ circumference).
When upsizing, keep overall diameter within ±3% of stock to preserve speedometer and ABS accuracy. A 225/45R17 (25.0" OD) is closely matched by 235/40R18 (25.4") or 245/35R19 (25.8"). Deviating more than 3% triggers speedometer error (which compounds across odometer readings, lease-return mileage, and toll systems) and may trip ABS/traction control calibration. Everything runs client-side; no values leave your browser.
About the Tire Size Calculator
The Tire Size Calculator is a simple, free helper for automotive and vehicle projects that runs entirely on your own device. Decode a P-metric tire size — overall diameter, sidewall height, circumference, revs per mile.
How it works
Enter your figures and the result appears instantly, updating the moment you change anything. There is no submit button and nothing to wait for, so it is easy to try a few what-if numbers and compare the results. Just check each box holds the kind of value it expects.
Want the deeper story? The Knowledge Base explains the ideas behind the tools in more detail.
Frequently Asked Questions
What does "225/45R17" mean?
225 mm section width, 45% aspect ratio (sidewall 45% of width), Radial, 17-inch wheel diameter.
How do I use the Tire Size Calculator?
Simply type your numbers and read the result, which refreshes the instant you change something. There is nothing to submit and nothing to wait for.
Does it cost anything or need an account?
No. The tool is completely free, there is no account to create, and it keeps working offline after the page first loads.
Is anything I type uploaded?
No. The tool works entirely on your device, so the values you enter never leave your browser.
Common Use Cases
Upsize / downsize
Keep overall diameter within 3% to preserve speedometer accuracy.
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