Tire Size Diameter Calculator
Calculate a tire's overall rolling diameter, sidewall height, circumference, and revolutions per mile from its P-metric size (width / aspect / wheel). Runs in your browser.
From a P-metric tire size (width/aspect Rwheel): sidewall height = width ร aspect%, and overall diameter = wheel diameter + 2 ร sidewall. Revolutions per mile = 63,360 in รท (ฯ ร diameter). Changing tire size alters your speedometer reading and effective gearing โ a larger diameter makes the speedo read low. Compare a new size to your original to see the % difference. Everything runs in your browser.
About this tool
A tire's size, written in the familiar P-metric format like 225/45R17, encodes everything needed to compute its real-world dimensions, and this calculator does that math. The three numbers are the section width in millimetres (225), the aspect ratio as a percentage (45), and the wheel diameter in inches (17). From them, the sidewall height is the width times the aspect ratio โ here 225 ร 0.45 = 101.25 mm โ and the overall (rolling) diameter is the wheel diameter plus two sidewalls: 17 inches (431.8 mm) plus 2 ร 101.25 mm, giving about 634 mm or 24.97 inches. The tool also reports the circumference (ฯ ร diameter) and the revolutions per mile, found by dividing 63,360 inches (one mile) by the circumference โ about 808 revolutions per mile for that example. These figures matter for more than curiosity. The overall diameter determines your effective gearing and, critically, your speedometer accuracy: a speedometer is calibrated for the factory tire diameter, so fitting taller tires makes it read slower than you're actually going (and shorter tires make it read fast), by the same percentage as the diameter change. Revolutions per mile feeds into that calibration and into odometer accuracy. When choosing replacement or plus-size tires, the standard advice is to keep the overall diameter within about 3% of the original so the speedometer, ABS, and transmission shift points stay accurate โ so the most useful workflow is to compute your original size, compute the candidate size, and compare the diameters. This calculator gives you the precise numbers to do that. Everything runs in your browser; nothing is uploaded.
How to use it
- Read your tire's size from the sidewall, e.g. 225/45R17.
- Enter the width (mm), aspect ratio (%), and wheel diameter (in).
- Read the overall diameter, sidewall height, circumference, and revolutions per mile.
- To check a tire swap, compute both sizes and keep the diameter within ~3%.
Frequently asked questions
- How do you calculate tire diameter from the size?
- Sidewall height = width ร aspect%. Overall diameter = wheel diameter + 2 ร sidewall. For 225/45R17: sidewall = 101.25 mm, diameter = 17โณ + 2 ร 101.25 mm โ 24.97โณ (634 mm).
- What does the tire size 225/45R17 mean?
- 225 = section width in mm; 45 = aspect ratio (sidewall height is 45% of the width); R = radial construction; 17 = wheel diameter in inches.
- How does tire size affect my speedometer?
- The speedometer is calibrated for the factory tire diameter. A larger-diameter tire travels farther per revolution, so the speedometer reads lower than your true speed (and the odometer undercounts); a smaller tire does the opposite, by the same percentage as the diameter change.
- What are revolutions per mile?
- How many times the tire rotates to cover one mile: 63,360 inches รท (ฯ ร diameter in inches). It is used for speedometer/odometer calibration. A 24.97โณ tire turns about 808 times per mile.
- How much can I change tire size safely?
- A common guideline is to keep the overall diameter within about 3% of the original so the speedometer, ABS, and transmission behavior stay accurate. Compare the two diameters with this tool before changing.
- Is anything uploaded?
- No. All calculations run entirely in your browser.