Transportation calculator

Free tire size calculator

Enter a metric tire code (e.g. 225/50R17) — this tire size calculator decodes it into sidewall height, overall diameter, circumference, and revolutions per mile, updated live, as you type.

On this page10 sections

InputsLive

Section width

Aspect ratio

Rim diameter

Result

Overall diameter

26.63"

Sidewall: 5.31" · Circumference: 83.7"

Overall diameter26.63"

Sidewall height5.31"

Circumference83.7"

Section width225 mm

Actual tire dimensions vary by manufacturer. Changing tire size affects speedometer, odometer, ABS, and clearance. Consult a tire professional before changing sizes.

Results are estimates. Consult a professional.

How it's calculated

How the tire size calculator works

Modern metric tire sizes encode three key dimensions in a standardized format such as 225/50R17. The calculator decodes this string and derives every geometric property of the tire — sidewall height, overall diameter, circumference, and revolutions per mile — from those three numbers alone.

Sidewall height (mm) = Section width (mm) × Aspect ratio ÷ 100

Overall diameter (mm) = Rim diameter (in) × 25.4 + 2 × Sidewall height

Circumference (in) = π × Overall diameter (mm) ÷ 25.4

Revolutions per mile = 63,360 ÷ Circumference (in)

For a 225/50R17: the section width is 225 mm, the sidewall height is 225 × 50 ÷ 100 = 112.5 mm, the overall diameter is 17 × 25.4 + 2 × 112.5 = 431.8 + 225 = 656.8 mm (25.86 in), and the circumference is π × 25.86 ≈ 81.24 in — giving 63,360 ÷ 81.24 ≈ 780 revolutions per mile. Changing overall diameter by even 1% alters speedometer accuracy and effective gear ratios.

Tire Rack: Tire Size Explained — Decoding Metric Tire Sizes

Example

Worked example: comparing OEM vs. plus-size fitment

Example: 205/55R16 (OEM) vs. 225/50R17 (plus-one)

A compact sedan ships with 205/55R16 tires. The owner wants to upgrade to 225/50R17 wheels and tires. Will the diameter change enough to affect the speedometer or cause rubbing?

OEM 205/55R16 → sidewall = 205 × 0.55 = 112.75 mm → diameter = 16 × 25.4 + 2 × 112.75 = 631.3 mm (24.86 in)

New 225/50R17 → sidewall = 225 × 0.50 = 112.5 mm → diameter = 17 × 25.4 + 2 × 112.5 = 656.8 mm (25.86 in)

Diameter difference: +25.5 mm (+1.0 in), or +4.0% — within the ±3% safe zone? No — borderline.

+4.0% diameter

The plus-one fitment increases overall diameter by 4%, which slightly exceeds the recommended ±3% threshold. Actual indicated speed of 60 mph equals a true speed of about 62.4 mph. Consider 225/45R17 (24.96 in) for a near-perfect diameter match.

Quick reference

Overall diameter and revolutions for common tire sizes

The table below lists calculated overall diameter, sidewall height, circumference, and revolutions per mile for ten of the most popular metric tire sizes in the US market. Use it to quickly compare fitments or check speedometer impact.

Tire Size	Sidewall (mm)	Overall Dia. (in)	Circumf. (in)	Rev/Mile
205/55R16	113	24.86	78.11	811
215/55R17	118	26.31	82.66	766
225/50R17	113	25.86	81.24	780
235/45R18	106	26.33	82.72	766
245/40R18	98	25.66	80.60	786
255/35R19	89	25.98	81.62	776
275/30R20	83	26.76	84.07	754
265/70R17	186	31.61	99.29	638
285/75R16	214	32.83	103.13	615
305/65R17	198	33.61	105.59	600

Calculated per ETRTO/Tire Rack methodology. Minor variation vs. mounted tire possible due to actual tread depth and load.

Practical tips

Tips for choosing the right tire size

Tire sizing involves trade-offs between ride comfort, handling, appearance, speedometer accuracy, and fitment clearance. These five tips cover the most important decisions.

Stay within ±3% of OEM diameter — Exceeding this range causes speedometer error, odometer drift, and may affect ABS and traction control calibration. Many states consider an inaccurate speedometer a vehicle defect.
Check wheel-well clearance before going wider — A wider section width (the first number in the code) risks rubbing on suspension components, inner fender liners, and sway bar end links — especially under full lock or maximum suspension compression.
Plus-sizing trade-offs — Moving to larger-diameter wheels (plus-one, plus-two) with lower-profile tires maintains diameter but reduces sidewall cushioning, making ride quality harsher and increasing susceptibility to pothole damage.
Load index and speed rating must meet or exceed OEM — Never substitute a tire with a lower load index or speed rating than specified in the owner's manual or door jamb placard. Under-rated tires can fail catastrophically at highway speed.
Recalibrate after any diameter change — Many modern vehicles (Ford, GM, BMW) allow speedometer and tire size calibration through the vehicle menu or a dealer scan tool. If your odometer is wrong, your maintenance intervals — oil changes, tire rotations — will be off too.

Accuracy & limits

Accuracy and limitations

Tire size calculations use nominal dimensions from the molded sidewall code — not the tire's actual inflated size under load. The real mounted diameter depends on rim width (a 225 mm tire mounts wider on a 9-inch rim than a 7.5-inch rim), inflation pressure, and load. Actual section width can vary ±6 mm from the nominal, and overall diameter can vary ±4 mm from the calculated figure.

Revolutions-per-mile figures assume the tire rolls at its full loaded radius without slippage. Heavy loads compress the sidewall and reduce effective radius, increasing actual revolutions per mile. For maximum accuracy — important for GPS speedometer calibration or gear ratio optimization — measure the actual tire circumference on your vehicle under normal load conditions.

Glossary

Tire size terms defined

The first number in a metric tire code (e.g., 225 in 225/50R17). It is the nominal width of the tire in millimeters from sidewall to sidewall when mounted on the specified rim width at the specified inflation pressure.

The second number (e.g., 50 in 225/50R17). It expresses the sidewall height as a percentage of the section width. A 50-series tire has a sidewall height equal to 50% of its width.

The third number (e.g., 17 in 225/50R17), expressed in inches. The diameter of the wheel at the bead seat where the tire seats.

The full outer diameter of the inflated tire from tread to tread, measured in millimeters or inches. Drives speedometer accuracy and gear ratio calculations.

How many full tire rotations are required to travel one mile. Higher for smaller tires; lower for larger tires. Used to calibrate speedometers and odometers.

Increasing wheel diameter by one or more inches while reducing the tire's aspect ratio to maintain the same overall tire diameter. Improves handling but stiffens the ride.

European Tyre and Rim Technical Organisation — the international standards body that defines metric tire dimensions and load/speed rating tables used worldwide.

About

About this tire size calculator

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Questions

Frequently asked questions about the free tire size calculator

A tire size calculator is a free online tool that helps you decode a metric tire spec (e.g. 225/50R17) into sidewall, diameter, circumference, and revs per mile. Tire size encodes section width, aspect ratio, and rim diameter. The math expands these into useful values. It runs entirely in your browser with instant results and no sign-up.

The base payment uses principal + APR. Sales tax can be added via the input. Doc fees, registration, and destination charges aren't included — add them to the principal.

Your new lender pays off the old loan and issues a new one in its place. The savings come from a lower rate or longer term. A longer term lowers monthly but raises total interest.

No — these are estimates for planning. Actual loan terms depend on credit score, lender, and current rates. Always read the disclosure (TILA box) before signing.