Quick answer
Overall tire diameter = 2 × (section width × aspect ratio ÷ 100) + rim diameter in mm (rim inches × 25.4). A 225/45R17 tire has a 101.3 mm sidewall and a 634.3 mm (24.97 in) diameter, turning about 807.6 revolutions per mile.
Enter the section width, aspect ratio and rim diameter from the tire's sidewall code. The calculator returns diameter, sidewall height, circumference and revolutions per mile or kilometre.
Related tools: Tire Size Comparison Calculator: Compare two tire sizes and the resulting speedometer error. Carburetor Size Calculator: Carburetor airflow needed from displacement, peak rpm and volumetric efficiency. Sample Size Calculator: Sample size for estimating a mean or a proportion.
How to use the Tire Size Calculator
- Enter section width (mm) (above 0).
- Enter aspect ratio (%) (above 0).
- Enter rim diameter (in) (above 0).
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Section width | A number, in mm | > 0 |
| Aspect ratio | A number, in % | > 0 |
| Rim diameter | A number, in in | > 0 |
Formula and method
Formula
sidewall = width × aspect ÷ 100; diameter = 2 × sidewall + rim diameter × 25.4 mm
IEEE-754 double precision; shown to the decimals indicated.
ISO metric tire code: width (mm) / aspect ratio (%) R rim (in). Real tires vary a little with load and wear.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Section width = 225; Aspect ratio = 45; Rim diameter = 17.
| Result | Value |
|---|---|
| Overall diameter | 634.3 mm |
| Sidewall height | 101.3 mm |
| Overall diameter | 24.97 in |
| Circumference | 1,992.7 mm |
| Revolutions per mile | 807.6 |
| Revolutions per km | 501.8 |
Example 2
Inputs: Section width = 205; Aspect ratio = 55; Rim diameter = 16.
| Result | Value |
|---|---|
| Overall diameter | 631.9 mm |
| Sidewall height | 112.8 mm |
| Overall diameter | 24.88 in |
| Circumference | 1,985.2 mm |
| Revolutions per mile | 810.7 |
| Revolutions per km | 503.7 |
Example 3
Inputs: Section width = 265; Aspect ratio = 70; Rim diameter = 17.
| Result | Value |
|---|---|
| Overall diameter | 802.8 mm |
| Sidewall height | 185.5 mm |
| Overall diameter | 31.61 in |
| Circumference | 2,522.1 mm |
| Revolutions per mile | 638.1 |
| Revolutions per km | 396.5 |
The first line is the headline result (overall diameter). Below it the tire Size Calculator lists sidewall height, overall diameter, circumference, revolutions per mile and revolutions per km, which describe the same case from other angles. IEEE-754 double precision; shown to the decimals indicated.
Reading the tire code
In 225/45R17, 225 is the width in millimetres, 45 is the sidewall height as a percentage of that width, R means radial construction and 17 is the rim diameter in inches. Sidewall = 225 × 0.45 = 101.25 mm; diameter = 2 × 101.25 + 17 × 25.4 = 634.3 mm.
Comparing sizes
Changing tire size alters the speedometer, gearing and fit. A 245/40R17 tire is 627.8 mm tall, about 1.0% smaller, so the speedometer would read about 1% high. Many sources advise staying within about 3% of the original overall diameter, and checking clearance in the wheel arch and with the vehicle handbook.
How the result changes with section width
Holding every other input at the first worked example, this table changes only section width. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Section width | Overall diameter | Sidewall height | Overall diameter |
|---|---|---|---|
| 112.5 | 533.1 mm | 50.6 mm | 20.99 in |
| 225 | 634.3 mm | 101.3 mm | 24.97 in |
| 450 | 836.8 mm | 202.5 mm | 32.94 in |
| 900 | 1,241.8 mm | 405 mm | 48.89 in |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Section width: entering "0" shows "Section width must be greater than 0." beside that field and no result.
- Rim diameter: entering "0" shows "Rim diameter must be greater than 0." beside that field and no result.
Choosing between the Tire Size Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Tire Size Comparison Calculator](/automotive/tire-comparison-calculator/) | old tire: Section width, old tire: Aspect ratio and old tire: Rim diameter | Diameter difference |
| [Carburetor Size Calculator](/automotive/carburetor-size-calculator/) | displacement, peak engine speed and volumetric efficiency | Required airflow |
| [Sample Size Calculator](/statistics/sample-size-calculator/) | estimate a, confidence level and margin of error | Required sample size |
| [Wire Size Calculator](/engineering/wire-size/) | current, one-way length and supply voltage | Smallest AWG for the voltage drop |
| [Screen Size Calculator](/technology/screen-size-calculator/) | diagonal, aspect ratio width and aspect ratio height | Screen size |
Background: vehicle sizing and speed
A taller tyre travels farther per revolution, so the speedometer reads low and the effective gear ratio becomes numerically lower; a shorter tyre does the opposite. True speed is roughly the speedometer reading times the new tyre diameter divided by the old one. Verify with GPS over a known distance before changing parts.
Assumptions and limitations
- Loaded radius is slightly smaller
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for section width and rim diameter to confirm this.
Frequently asked questions
What does the aspect ratio mean?
It is the sidewall height divided by the section width, shown as a percentage.
Does tire pressure change the diameter?
Slightly; loaded radius is a little smaller than the unloaded size the calculator uses.
Can I use it for speedometer correction?
Use the revolutions per mile figures of two sizes to compare them.
What does the Tire Size Calculator calculate?
Overall diameter, sidewall height and revolutions per mile from a tire size such as 225/45R17. It returns Overall diameter, Sidewall height, Overall diameter, Circumference and Revolutions per mile.
What formula does the Tire Size Calculator use?
sidewall = width × aspect ÷ 100; diameter = 2 × sidewall + rim diameter × 25.4 mm. ISO metric tire code: width (mm) / aspect ratio (%) R rim (in).
What do I need to enter in the tire Size Calculator?
You enter Section width (mm), Aspect ratio (%) and Rim diameter (in).
How is the result rounded or limited?
IEEE-754 double precision; shown to the decimals indicated. Loaded radius is slightly smaller.
What is an example of a tire Size result?
With section width 225, aspect ratio 45 and rim diameter 17, the overall diameter is 634.3 mm.
Where does the method come from?
The method follows: ISO 4000-1 / ETRTO tire designation; Standard arithmetic and unit definitions (international inch 25.4 mm, avoirdupois pound 0.45359237 kg).
Related calculators
- Tire Size Comparison Calculator — Compare two tire sizes and the resulting speedometer error.
- Carburetor Size Calculator — Carburetor airflow needed from displacement, peak rpm and volumetric efficiency.
- Sample Size Calculator — Sample size for estimating a mean or a proportion.
- Screen Size Calculator — Width, height and area of a screen from its diagonal and aspect ratio.
Sources
- ISO 4000-1 / ETRTO tire designation; Standard arithmetic and unit definitions (international inch 25.4 mm, avoirdupois pound 0.45359237 kg)
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.