TireSizeCalc

How the numbers are calculated

Every dimension on TireSizeCalc is computed from the official tire sizing formula, and every chart value is copied from a named official document.

The formula

For a metric size such as 285/70R17, the sidewall height is the section width times the aspect ratio: 285 mm × 70% = 199.5 mm. The overall diameter is the rim diameter plus two sidewalls: 17 in + 2 × 7.85 in = 32.71 in. This is the definition in UN Regulation No. 30 §2.15 and §6.1.2.1, repeated in UN Regulation No. 54 for light-truck tires.

Circumference is π × diameter. Revolutions per mile is 63,360 inches divided by the circumference. Speedometer change is the ratio of the two diameters.

Nominal versus real tires

These are nominal dimensions: the size the designation describes. A real new tire may differ. The regulations allow a radial tire's diameter to fall between 97% and 104% of the sidewall portion (UN R30 §6.1.5), and for passenger tires US rule FMVSS 139 allows the width to exceed the specified width by 7% (or 10 mm, if larger, for some pressure classes). Each size page shows that allowed range.

Manufacturers publish measured diameters on their own specification pages. Check the maker's page for the exact tire you plan to buy.

Inch (flotation) sizes

For inch sizes such as 33x12.50R15, the numbers in the name are rounded. Where UN Regulation No. 54 lists the size in its table of US high flotation tires (Annex 5, Part II, Table B), we use the table's diameter and width. Where it does not, we show the numbers in the size and say so on the page. See inch tire sizes.

How sizes are chosen and verified

A size gets a page only after it has been confirmed on an official manufacturer page. We record the page and the date it was checked. Sizes are prioritised by how often people search for them.

What this site does not do

TireSizeCalc does not tell you whether a tire fits your vehicle. Wheel width, offset, suspension and body clearance decide that. Start with the tire size on your door-jamb placard and ask the tire or vehicle maker.

How the guides are written

The guides follow the writing rules of ASD-STE100 Simplified Technical English: short sentences, one instruction per step, active voice and a controlled vocabulary. A build check tests every guide against the rules that a program can test. Quotations from regulations keep their original words.

TireSizeCalc is developed and maintained by Sri Murugan Systems, Madurai, India. Questions or corrections: contactus@tiresizecalc.net.

Printed from https://tiresizecalc.net/about/. Figures as published on 2026-11-01. The sources are listed below.

Sources

Every source used anywhere on TireSizeCalc, with the date we retrieved it.

  1. UN Regulation No. 30 - Uniform provisions concerning the approval of pneumatic tyres for motor vehicles and their trailers (Revision 3). United Nations Economic Commission for Europe (UNECE). E/ECE/324 E/ECE/TRANS/505 Rev.1/Add.29/Rev.3. 2007-03-29. Retrieved 2026-11-01. SHA-256 of the copy used: 1d13d312ed644bb8ecaad4cacb3ce6e3102dc4b8d125bd004cb79b43d2d4263b. Passenger-car tyres. Revision 3 (2007); later supplements to the regulation exist.
  2. UN Regulation No. 54 - Uniform provisions concerning the approval of pneumatic tyres for commercial vehicles and their trailers (Revision 2). United Nations Economic Commission for Europe (UNECE). E/ECE/324 E/ECE/TRANS/505 Rev.1/Add.53/Rev.2. 2004-04-16. Retrieved 2026-11-01. SHA-256 of the copy used: fbd7b6544b767178473c9af871d5d85897f7d736b29ebb88c37f6ab4952366c8. Commercial-vehicle tyres, including LT tyres and the Annex 5 Part II table of United States tyre sizes. Revision 2 (2004); later supplements exist.
  3. Toyo Tires 2024 Commercial Tires - Dimensions and Load Limits. Toyo Tire U.S.A. Corp. 2024-04-04. Retrieved 2026-11-01. SHA-256 of the copy used: 08cf369e249635f9cba5619c1762cc531eb50f5244c794c0398db9d40c861f12. Document date taken from the PDF's modification date. Toyo site terms restrict copying; only the short load-range/ply-rating correspondence is used, with attribution.
  4. 49 CFR 571.139 - Standard No. 139; New pneumatic radial tires for light vehicles. National Highway Traffic Safety Administration (NHTSA), Code of Federal Regulations (eCFR). 49 CFR 571.139. eCFR up to date as of 2026-10-15. Retrieved 2026-11-01. S3 definitions, S5.5 tire markings, S6.1.2 dimension tolerance. The eCFR is an authoritative but unofficial editorial compilation of the CFR.
  5. 49 CFR 571.110 - Standard No. 110; Tire selection and rims for motor vehicles with a GVWR of 4,536 kilograms (10,000 pounds) or less. National Highway Traffic Safety Administration (NHTSA), Code of Federal Regulations, annual edition (GovInfo). 2006-10-01. Retrieved 2026-11-01. S4.2.1 load limits and S4.3 placard, cited from the 2006 annual edition of the CFR. NHTSA's current Tires page confirms the placard location.
  6. 49 CFR 575.104 - Uniform tire quality grading standards. National Highway Traffic Safety Administration (NHTSA), Code of Federal Regulations (eCFR). 49 CFR 575.104. eCFR up to date as of 2026-10-21. Retrieved 2026-11-01. (c)(1) scope and exclusions, (d)(2) grades, Figure 2 explanation text.
  7. Tire Identification and Recordkeeping - Final rule (49 CFR Part 574). National Highway Traffic Safety Administration (NHTSA), Federal Register. FR Doc. 2015-08418. 2015-04-13. Retrieved 2026-11-01. Regulatory text of 49 CFR 574.5(a) and (b): 13-symbol TIN, plant code, manufacturer's code, date code; two-symbol plant codes allowed until 2025-04-13.
  8. Tires (Tire Safety). National Highway Traffic Safety Administration (NHTSA). Undated web page. Retrieved 2026-11-01. Placard location, recommended pressure, monthly cold check, TIN date code example, tire age.
  9. Interpretation letter: ply rating and load range (to Federazione Italiana Fuoristrada). National Highway Traffic Safety Administration (NHTSA), Office of Chief Counsel. Undated on the web page. Retrieved 2026-11-01. States that the ply rating is an older system that the load range replaced.
  10. UN Regulation No. 39 - Uniform provisions concerning the approval of vehicles with regard to the speedometer equipment including its installation (Revision 1). UNECE; Official Journal of the European Union. OJ L 120, 13.5.2010. 2010-05-13. Retrieved 2026-11-01. Para 5.3 speedometer accuracy. Applies in countries that apply UN R39; it is not a US federal rule.
  11. 2025 MX-5 Owner's Manual - Tire Information (U.S.A.). Mazda North American Operations. mazdausa.com manual 70180200. 2025 model year. Retrieved 2026-11-01. Explains P (T&RA) versus no prefix (ETRTO or JATMA) and each part of the size designation.
  12. How to Read Your Tire Sidewall. Continental Tire the Americas. Undated web page. Retrieved 2026-11-01. Size parts; date code before and after 2000.
  13. Reading a Sidewall. Toyo Tire U.S.A. Corp. Undated web page. Retrieved 2026-11-01. P and LT prefixes; tires without P are hard-metric.
  14. Tire Upgrade (plus sizing). BFGoodrich Tires (Michelin North America (Canada) Inc.). Undated web page. Retrieved 2026-11-01. States that BFGoodrich does not recommend up-sizing, and that a large change can affect ABS, speedometer and stability systems.
  15. Commercial Tire Safety Tips. Federal Motor Carrier Safety Administration (FMCSA), US DOT. FMCSA-ADO-16-003. 2016-06. Retrieved 2026-11-01. Labels the load index (single/dual) and load range on a sidewall marking.
  16. How Google uses information from sites or apps that use our services. Google LLC. Undated web page. Retrieved 2026-11-01. Linked from the privacy policy, as Google asks of sites that use AdSense and Analytics.
  17. UN Regulation No. 124 - Uniform provisions concerning the approval of wheels for passenger cars and their trailers. UNECE; Official Journal of the European Union. 42006X1227(08), OJ L 375, 27.12.2006. 2006-12-27. Retrieved 2026-11-01. Para 2.6 defines inset (US: offset) as the distance from the attachment face of the disc to the centre line of the rim; it can be positive, zero or negative. English wording checked in the adopted draft TRANS/WP.29/2005/46 (7 April 2005).
  18. 49 CFR 571.138 - Standard No. 138; Tire pressure monitoring systems. National Highway Traffic Safety Administration (NHTSA), Code of Federal Regulations, annual edition (GovInfo). 2022-10-01. Retrieved 2026-11-01. S2 application, S4.2(a) warning threshold and Table 1 minimum activation pressures, S4.4 malfunction indication, S4.5 owner's manual statement. Amendment history in this edition ends at 72 FR 38025 (2007).
  19. Computed by tiresizecalc.net from the cited formula. Sri Murugan Systems. Retrieved 2026-11-01. Unit conversions use exact definitions: 1 in = 25.4 mm, 1 lb = 0.45359237 kg, 1 mile = 1.609344 km.
  20. OpenRush US keyword index - keyword research pull. OpenRush (third-party SEO data). Retrieved 2026-11-01. Search volume estimates. Used only to order the most-searched list on the home page; the volume values are not published on the site.

Last reviewed 2026-11-01. The full source list, with dates and file checksums, is in How the numbers are calculated. To report an error, email contactus@tiresizecalc.net.