About SidewallMath
SidewallMath is an independent project run by one person, not a company or news organization. It is a free, no-signup tool site: Read the sidewall. Know if it fits.
Purpose
I built SidewallMath to help people work out what a tire size means in inches, whether a new tire size is a close match, and how it changes your speedometer, gearing and wheel fit with real numbers instead of guesses. The calculators run in your browser, show their formulas, and let you change every assumption. The guides explain the numbers behind the tools in plain language.
Data sources
Tire dimensions follow standard geometry: sidewall height = section width × aspect ratio, overall diameter = wheel diameter + 2 × sidewall, circumference = π × diameter, revolutions per mile = 63,360 ÷ circumference. True speed, gearing and RPM formulas are the usual ratios of tire diameters. The ±3% diameter rule of thumb is quoted by Discount Tire's tire size calculator (discounttire.com/learn/tire-size-calculator) and 1010tires (1010tires.com/Tools/Tire-Size-Calculator). Speed rating symbols follow Discount Tire's speed rating page and the load index table follows the standard published index table; the date code layout follows Continental's DOT code guide. Sizes in the finder and converter are calculated, not a list of tires on sale, and real tires vary by brand and model. SidewallMath is currently served at matt5516.github.io/tire-size-calc; sidewallmath.com is the planned domain, pending the owner's approval.
How we keep it accurate
The math is covered by automated tests with hand-computed answers, figures show their source and update date where possible, and the site was last updated October 19, 2026. If you spot an error, please tell me and I will fix it.
Disclaimers
Estimates from the tire size. Actual tire dimensions vary by brand and model; confirm fitment, load index and speed rating for your vehicle. See the terms of use and privacy policy.
Contact and feedback
Email mathtools@mail.grokbot.com, use the Feedback button on any page, or open an issue on GitHub. More on the contact page.