Aspect ratio from a real measurement
Aspect ratio is sidewall height as a percentage of section width — normally read off a size designation, but this works it out from an actual measured sidewall instead, or goes the other way to find what sidewall a target ratio implies.
Getting a number you can act on
- 01Measure the section width
The tire's widest point, sidewall to sidewall, typically close to the middle of the tire's height rather than at the tread or bead.
- 02Measure the sidewall height
From the rim edge to the outer edge of the tread — this is the dimension aspect ratio expresses as a percentage of width.
- 03Read the calculated aspect ratio
Compare it against a nearby standard value (usually a multiple of 5) to sanity-check the measurement — real aspect ratios cluster around round numbers.
- 04Or solve the other direction
Given a target aspect ratio and a width, find the sidewall height it implies — useful when comparing tire options before a purchase.
What the calculator is actually doing
Nothing here is proprietary. If you would rather check it by hand, or explain it to someone at a counter, these are the same expressions the tool evaluates.
aspect ratio (%) = sidewall height ÷ section width × 100The definition itself — aspect ratio is simply sidewall height expressed as a percentage of section width.
sidewall height = section width × aspect ratio ÷ 100The same relationship rearranged to solve for sidewall instead.
Why aspect ratio is a percentage, not a fixed measurement
This is the detail that catches people out when comparing tire sizes: aspect ratio is not a sidewall height in millimetres, it is a percentage of whatever the section width happens to be. The same aspect ratio number means a completely different physical sidewall height depending on the width it is paired with.
A 55-series tire at 225mm width has a sidewall of about 123.75mm. The same 55-series aspect ratio at 275mm width produces a sidewall of about 151.25mm — nearly 28mm taller, despite an identical aspect ratio figure. This is exactly why widening a tire without also lowering the aspect ratio makes it taller, not just wider.
Tire Aspect Ratio Calculator FAQ
How do I calculate aspect ratio from a measurement?+
Divide the measured sidewall height by the section width and multiply by 100. A 146.25mm sidewall on a 225mm width gives an aspect ratio of 65.
Why do two tires with the same aspect ratio have different sidewall heights?+
Because aspect ratio is a percentage of section width, not a fixed dimension. A wider tire at the same aspect ratio has a taller sidewall in absolute terms.
What is a low aspect ratio tire?+
Generally anything at or below about 45 is considered low-profile — a shorter sidewall relative to width, which sharpens steering response at the cost of ride comfort and rim protection.
Standards and references behind these figures
The arithmetic on this page is fixed, but the boundaries and conventions around it come from published standards and manufacturer guidance. These are the documents they come from, so you can check them rather than take them on trust.
01The Tire and Rim Association — standards filing (NHTSA docket)TRA has been the US standardising body for tire and rim interchangeability since 1903; this filing sets out dimensional practice.↗02Yokohama Tire — how to read a sidewallManufacturer explanation of section width, aspect ratio and rim diameter in the standard size designation.↗Nominal sizes, not measured ones. Every figure here is calculated from the size marked on the sidewall. Real tires vary from nominal by a few millimetres between manufacturers, and more as tread wears or pressure changes. Use these numbers to plan and to sanity-check a quote — measure the actual tire before committing to a tight fitment.