Home › Engineering & Automotive › Tire Size Calculator

Tire Size Calculator

Parses a metric tire code such as 225/45R17 into section width, aspect ratio and rim size, computes sidewall height, overall diameter, circumference and revolutions per kilometre and mile, and optionally compares a second size (diameter difference and speedometer error).

When to use

You want the dimensions of a tire from its sidewall code, or want to check whether an alternative size keeps the overall diameter (and speedometer) within a few percent.

Do not use when: The tire uses a flotation size (31x10.50R15) or an old alphanumeric code, or you need road speed from rpm and gearing (use gear-ratio-speed).

Formula

sidewall_height_mm = width_mm × aspect_ratio_percent / 100; overall_diameter_mm = 2 × sidewall_height_mm + rim_diameter_in × 25.4; circumference_mm = π × overall_diameter_mm; revolutions_per_km = 1e6 / circumference_mm; actual_speed_at_indicated_100 = 100 × compare_diameter / original_diameter

Nominal dimensions from the code (ISO 4000 / ETRTO); actual tires vary by a few millimetres between brands and the loaded rolling circumference is 2–3 % smaller than the unloaded value.

Inputs

Parameter	Type	Unit	Required	Description
`tire_code`	string		yes	Metric tire size as printed on the sidewall: width mm / aspect ratio % R rim inches, e.g. 225/45R17 (prefixes like P or LT and suffixes like 91W are ignored).
`compare_tire_code`	string		no	Optional second (new) tire size to compare against tire_code (the original), e.g. 235/40R18.

Outputs

Output	Type	Unit	Description
`width_mm`	number	mm	Nominal section width.
`aspect_ratio_percent`	number	%	Sidewall height as a percentage of the width.
`rim_diameter_in`	number	in	Wheel diameter in inches.
`sidewall_height_mm`	number	mm	width × aspect ratio / 100.
`overall_diameter_mm`	number	mm	2 × sidewall + rim × 25.4.
`overall_diameter_in`	number	in	Overall diameter in inches.
`circumference_mm`	number	mm	π × overall diameter.
`revolutions_per_km`	number	rev/km	1,000,000 / circumference_mm.
`revolutions_per_mile`	number	rev/mile	1,609,344 / circumference_mm.
`compare_overall_diameter_mm`	number	mm	Overall diameter of compare_tire_code (only when given).
`diameter_difference_mm`	number	mm	compare − original (only when compare_tire_code is given).
`diameter_difference_percent`	number	%	(compare − original) / original × 100; keep within about ±3 %.
`actual_speed_at_indicated_100`	number	km/h or mph	With the compared tire fitted, true speed when the speedometer (calibrated for the original) reads 100 (only when compare_tire_code is given).
`size_change_assessment`	string		Whether the diameter change stays within the common ±3 % guideline (only when compare_tire_code is given).

Example

225/45R17: {"tire_code":"225/45R17"} → {"width_mm":225,"aspect_ratio_percent":45,"rim_diameter_in":17,"sidewall_height_mm":101.25,"overall_diameter_mm":634.3,"overall_diameter_in":24.97,"circumference_mm":1992.7,"revolutions_per_km":501.8,"revolutions_per_mile":807.6}

205/55R16 replaced by 225/45R17: {"tire_code":"205/55R16","compare_tire_code":"225/45R17"} → {"overall_diameter_mm":631.9,"compare_overall_diameter_mm":634.3,"diameter_difference_mm":2.4,"diameter_difference_percent":0.38,"actual_speed_at_indicated_100":100.38,"size_change_assessment":"Within the ±3 % diameter guideline"}

GET https://tttkmbb.com/api/v1/calculate/tire-size?tire_code=225%2F45R17

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/tire-size (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/tire-size · Markdown: https://tttkmbb.com/engineering/tire-size.md · JSON definition: https://tttkmbb.com/engineering/tire-size.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="tire-size"
OpenAPI operationId: calculate_tire_size
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

Wikipedia – Tire code (reference)
ETRTO – European Tyre and Rim Technical Organisation standards manual (standard)

FAQ

What do the numbers in 225/45R17 mean?

225 is the section width in mm, 45 the sidewall height as a percentage of the width, R radial construction and 17 the rim diameter in inches. A trailing 91W is the load index and speed rating.

How much can the diameter change?

Most manufacturers and fitters recommend staying within ±3 % of the original diameter so the speedometer, ABS and stability control remain accurate and the tire clears the wheel arch.

Related calculators

Gear Ratio and Speed Calculator — Road speed from rpm, gearing and this tire diameter.
Circle Calculator — Circumference and area of the tire circle.
Speed Converter — Convert the corrected speed between km/h and mph.