Home›Converters›Everyday›Tire Size Converter

Tire Size Converter

Everyday

Convert a metric tire size (e.g. 205/55R16) into overall diameter, sidewall height, and circumference in mm and inches — for tyre shopping and fitment checks.

Reviewed by the thecalcu.com team · Last updated July 26, 2026

Overall Diameter
631.9 mm (24.88 in)
Sidewall Height
112.8 mm
Circumference
1985 mm
Revolutions per km
504
Revolutions per mile
811

What is a Tire Size?

A Tire Size Converter calculates the actual physical dimensions of a tire from its standard sidewall size code, such as 205/55R16, a mixed metric-imperial notation that combines section width in millimetres, aspect ratio as a percentage, and wheel diameter in inches. Unlike most conversions on this platform, tire size isn't a single value with equivalent units; it's a compound calculation where three separate numbers combine to determine the tire's overall diameter, sidewall height, and circumference.

This tool performs that calculation directly, which is useful alongside the Length Converter or Area Converter for other automotive and DIY measurement conversions.

Why Use a Tire Size Converter?

Tire sidewall codes pack three different measurements into a compact notation that isn't immediately readable as a physical size, knowing that a tire is "205/55R16" doesn't tell you its actual diameter without doing the underlying calculation. This matters when comparing tire sizes for fitment, checking whether a replacement tire will affect ground clearance, or verifying speedometer accuracy after a tire size change.

Two concrete situations this solves: comparing your current tire's overall diameter against a proposed replacement size to check they're close enough to avoid speedometer and ABS calibration issues, and calculating a tire's circumference to estimate revolutions per kilometre for odometer or GPS-tracker calibration purposes.

Who Should Use This Converter?

  • Car and motorcycle owners checking whether a different tire size will fit their vehicle without affecting speedometer accuracy or ground clearance.
  • Mechanics and tyre shop staff quickly calculating overall diameter differences when advising customers on alternate tire sizes.
  • Off-road and performance enthusiasts comparing tire size options for larger wheel or lift kit fitments.
  • Fleet and logistics managers calculating tire circumference for odometer calibration across a mixed vehicle fleet.
  • Automotive students and hobbyists learning how tire sidewall codes translate into physical measurements.

What Insights Does the Tire Size Converter Give You?

The Overall Diameter result is the single most important figure for fitment, it determines whether a tire will physically clear the wheel well and whether it will affect your speedometer's accuracy compared to the vehicle's original tire size. The Sidewall Height shows just the rubber portion's height, useful for understanding ride comfort and clearance over bumps. The Circumference and Revolutions per km/mile figures are used for odometer, speedometer, and GPS-tracker calibration, since these instruments calculate distance based on how many times the tire rotates.

How to use this Tire Size calculator

  1. Find your tire's size code on the sidewall, formatted like 205/55R16.
  2. Enter the first number into the Section Width (mm) field, 205 in this example.
  3. Enter the second number into the Aspect Ratio (%) field, 55 in this example.
  4. Enter the number after the "R" into the Wheel Diameter (in) field, 16 in this example.
  5. Read the calculated Overall Diameter, Sidewall Height, and Circumference results, which update instantly.
  6. Repeat with a proposed alternate tire size to compare overall diameters before deciding on a replacement.

Formula & Methodology

Sidewall Height (mm) = Section Width × (Aspect Ratio ÷ 100)
Overall Diameter (mm) = (Wheel Diameter in inches × 25.4) + (2 × Sidewall Height)
Circumference (mm) = π × Overall Diameter

Worked example: For a 225/45R17 tire, Sidewall height = 225 × 0.45 = 101.25 mm. Overall diameter = (17 × 25.4) + (2 × 101.25) = 431.8 + 202.5 = 634.3 mm (about 24.97 inches). Circumference = π × 634.3 ≈ 1,993 mm, giving approximately 502 revolutions per kilometre.

Frequently Asked Questions

In 205/55R16, 205 is the section width in millimetres (the tire's width from sidewall to sidewall), 55 is the aspect ratio, the sidewall height as a percentage of the width, R indicates radial construction, and 16 is the wheel/rim diameter in inches. This converter calculates the tire's actual physical dimensions from these three numbers.
Sidewall height (mm) = Width × (Aspect Ratio ÷ 100), and Overall Diameter (mm) = (Wheel Diameter in inches × 25.4) + (2 × Sidewall Height). This converter applies both formulas together, since the wheel diameter contributes on both sides of the tire (top and bottom).
Sidewall height = 205 × (55 ÷ 100) = 112.75 mm. Overall diameter = (16 × 25.4) + (2 × 112.75) = 406.4 + 225.5 = 631.9 mm, or about 24.88 inches. Enter these three values into the converter to get this calculation automatically.
Aspect ratio is the sidewall height expressed as a percentage of the tire's width, so a lower number (like 40) means a shorter sidewall relative to the tire's width, giving the tire a flatter, more low-profile appearance. Higher-performance vehicles often use lower aspect ratio tires for sharper handling, at some cost to ride comfort.
Compare the overall diameter of your proposed new tire size against your current tire's diameter using this converter, most manufacturers recommend staying within about 3% of the original diameter to avoid affecting speedometer accuracy, ABS calibration, and ground clearance. Always confirm compatibility with your vehicle's specific fitment guide or a tyre professional before changing sizes.
Revolutions per kilometre indicates how many times the tire rotates over a fixed distance, which is directly used by your vehicle's speedometer and odometer calibration, a tire with a smaller diameter will have more revolutions per km than a larger one covering the same distance. This is why changing tire size without recalibration can throw off speedometer accuracy.
Enter your tire's section width in millimetres, aspect ratio as a percentage, and wheel diameter in inches, these three numbers come directly from the tire's sidewall markings (e.g. 205/55R16). The converter calculates the overall diameter, sidewall height, and circumference instantly as you type.
Sidewall height is the vertical measurement of just the tire's rubber portion between the wheel rim and the tread, while overall diameter is the full height of the mounted tire and wheel combination, including the wheel's own diameter. Overall diameter is the number that matters for fitment, ground clearance, and speedometer accuracy.
Tire sizing evolved from separate historical conventions, European manufacturers established the metric width and aspect ratio portion, while wheel diameter retained the inch-based sizing already standard for wheels in most markets. This mixed system, while unusual, has remained the global standard (ISO metric tire code) for decades.
Yes, a lower aspect ratio (shorter sidewall) generally results in a firmer, more responsive ride with less cushioning against road imperfections, while a higher aspect ratio (taller sidewall) provides more cushioning and a softer ride. This converter calculates the physical dimensions only; actual ride quality also depends on tire compound, inflation pressure, and suspension tuning.
Also known as
tire size calculator205/55r16 diametertyre size convertertire sidewall height calculatortire diameter chartmetric tire size to inches