Travel, Auto & Lifestyle
Tire Size Calculator
Tire Size — tire Size Calculator derives overall tyre diameter from the size code and the speedometer effect of a size change. This page runs that same calculation on your numbers, in your browser: no signup, no uploads, nothing stored.
Tire Size Calculator turns your tire code, section width in mm, aspect ratio in percent and rim diameter in inches into the tyre diameter, circumference and speedometer effect using the tyre size code (width, aspect ratio and rim). The maths runs locally, so nothing you enter leaves the page.
Tire size math does not confirm vehicle fitment, load rating, speed rating, clearance, legality, or safety. Verify with the vehicle manufacturer or a tire professional. Everything runs locally in your browser and nothing you enter is uploaded.
About this tool
Get tire size right and the rest of the task gets easier. Tire Size Calculator derives overall tyre diameter from the size code and the speedometer effect of a size change. It matters for trips, vehicles and daily decisions where a quick reliable number beats a rough guess. A 205/55 R16 tyre has a sidewall of 205 × 0.55 = 112.75 mm and an overall diameter of 16″ (406.4 mm) + 2 × 112.75 = 631.9 mm . The tool above does the same computation in your browser — free, private, nothing stored.
How to use
- Enter a tire code such as 205/55R16 in the first field.
- Or fill the width, aspect ratio, and rim fields instead.
- Leave the fields you are not using blank.
- Press Calculate to see diameter, sidewall, and circumference.
Why use the Tire Size Calculator
Instant
Each result is worked out on your device the moment you press Calculate.
Private
The numbers you enter stay in your browser and are never uploaded.
Free
No signup and no paywall to use any tool in this set.
Works offline
Once the page has loaded it keeps working on a weak or dropped connection.
Clear units
Inputs and outputs name their units so you know exactly what you are reading.
Honest by design
Uses simplified browser-side formulas for estimates; real fitment depends on the tyre code, the wheel, clearance and the load and speed ratings.
Common uses
Use it when matching replacement tyres
Use it when fitting plus-size tyres
Use it for revs-per-mile reference
Use it for checking the speedometer effect of a change
Use it when comparing tyre sizes
Use it for working out overall diameter
Technical notes
Tire Size Calculator derives overall tyre diameter from the size code and the speedometer effect of a size change.
Results assume the tyre code you enter; real fitment also depends on the wheel, clearance and the load and speed ratings.
All arithmetic runs in your browser using double-precision floating point, so extreme inputs may show tiny rounding differences.
Worked example
A 205/55 R16 tyre has a sidewall of 205 × 0.55 = 112.75 mm and an overall diameter of 16″ (406.4 mm) + 2 × 112.75 = 631.9 mm.
FAQ
Does this confirm a tire fits my vehicle?
No. It only computes tire geometry. It does not check fitment, load rating, speed rating, clearance, legality, or safety; confirm those with the manufacturer or a tire professional.
Can I type the size as 205/55R16?
Yes. Enter the code in the first field and the tool parses the width, aspect ratio, and rim. You can also fill the separate fields instead.
What does the aspect ratio mean?
The aspect ratio is the sidewall height as a percentage of the section width. An aspect of 55 means the sidewall is 55 percent of the width.
How is overall diameter calculated?
Sidewall equals width times aspect divided by 100. Diameter equals the rim in millimetres plus two sidewalls, since there is a sidewall above and below the rim.