Tire Size Calculator

What is a tire size calculator?

A tire size calculator turns the cryptic code on your sidewall — something like 225/45R17 — into numbers you can actually picture: how tall the rubber sidewall is, the overall diameter of the mounted tire, its circumference, and how many revolutions per mile it makes. You feed it three values pulled straight off the tire (width, aspect ratio and rim diameter) and it reports the real-world dimensions instantly. Drivers use it to compare a new tire against the original, plus-size a wheel, check speedometer accuracy, or simply understand what the markings mean.

The headline output is overall diameter, because that single number drives almost everything else: ride height, speedometer reading, gearing feel and clearance. Get diameter close to stock and the rest falls into place.

How the tire size calculator works

The tool reads a tire size in the standard width / aspect / rim format and works through four short steps:

sidewall = width × aspect / 100 (then convert mm to inches by dividing by 25.4)

diameter = rim + 2 × sidewall (in)

circumference = π × diameter

revolutions per mile = 63360 / circumference

Here is what each input means and the unit it carries:

• width — the section width, the tire’s measurement sidewall to sidewall, in millimetres (mm). It is the first number, e.g. 225.
• aspect ratio — the sidewall height as a percentage of the width. An aspect of 45 means the sidewall is 45% of the width. It is the second number.
• rim diameter — the wheel diameter the tire mounts on, in inches (in). It is the number after the R, e.g. 17.

Why these steps

The aspect ratio is a percentage of the width, so multiplying width by aspect and dividing by 100 gives the sidewall height in millimetres. Dividing by 25.4 converts that to inches so it can be added to the rim, which is quoted in inches. There are two sidewalls stacked across the tire — one at the top and one at the bottom — which is why the diameter formula doubles the sidewall before adding the rim. Multiplying diameter by π gives the rolling circumference, and because there are 63360 inches in a mile (5280 feet × 12), dividing 63360 by the circumference gives revolutions per mile.

Examples

Every example uses only the formula above, so you can reproduce each answer by typing the same size into the calculator.

Example 1 — 225/45R17 (a common sport sedan size)

Width 225, aspect 45, rim 17.

sidewall = 225 × 0.45 = 101.25 mm = 101.25 ÷ 25.4 = 3.99 in

diameter = 17 + 2 × 3.99 = 24.97 in

circumference = π × 24.97 = 78.45 in

revolutions per mile = 63360 ÷ 78.45 = about 808

A 225/45R17 stands about 24.97 inches tall and turns roughly 808 times per mile.

Example 2 — 205/55R16 (a popular compact size)

Width 205, aspect 55, rim 16.

sidewall = 205 × 0.55 = 112.75 mm = 112.75 ÷ 25.4 = 4.44 in

diameter = 16 + 2 × 4.44 = 24.88 in

circumference = π × 24.88 = 78.16 in

revolutions per mile = 63360 ÷ 78.16 = about 811

The 205/55R16 comes out to about 24.88 inches and 811 revolutions per mile — very close to the 225/45R17 above, which is why the two are often cross-shopped.

Example 3 — 265/70R17 (a light-truck and SUV size)

Width 265, aspect 70, rim 17.

sidewall = 265 × 0.70 = 185.5 mm = 185.5 ÷ 25.4 = 7.30 in

diameter = 17 + 2 × 7.30 = 31.61 in

circumference = π × 31.61 = 99.30 in

revolutions per mile = 63360 ÷ 99.30 = about 638

A tall 265/70R17 measures about 31.61 inches and makes only about 638 revolutions per mile — fewer turns because each rotation covers more ground.

Tire size reference chart

This chart applies the same formula to common sizes. Sidewall and diameter are in inches; revolutions per mile use 63360 ÷ (π × diameter).

Tire size	Sidewall (in)	Overall diameter (in)	Circumference (in)	Revs per mile
205/55R16	4.44	24.88	78.16	811
225/45R17	3.99	24.97	78.45	808
235/45R18	4.16	26.33	82.70	766
245/40R19	3.86	26.72	83.92	755
265/70R17	7.30	31.61	99.30	638
275/65R18	7.04	32.07	100.76	629

Notice how a bigger rim with a thinner sidewall can keep overall diameter almost unchanged — that is the whole idea behind plus-sizing.

Common uses

The tire size calculator earns its keep wherever wheel and tire dimensions matter:

• Comparing a replacement — check that a new size has nearly the same diameter as the original so the speedometer stays honest.
• Plus-sizing wheels — pair a larger rim with a lower-profile tire to keep diameter constant while changing the look.
• Speedometer sanity checks — see whether a size change makes the gauge read fast or slow.
• Off-road and lift planning — confirm how much taller a 31- or 33-inch tire really is before checking clearance.
• Gearing and economy — taller tires lower engine rpm at a given speed; this tool shows the diameter change behind that effect.
• Decoding the sidewall — translate the marking into plain inches and millimetres.

Tips and common mistakes

• Width is in millimetres, rim is in inches. The format deliberately mixes units; the calculator handles the conversion, but do not assume the 17 in 225/45R17 is centimetres.
• Aspect ratio is a percentage, not a measurement. A 45 does not mean 45 mm — it means the sidewall is 45% of the 225 mm width.
• Lower aspect equals shorter sidewall. Dropping the aspect number gives a thinner sidewall and usually a firmer ride.
• There are two sidewalls in the diameter. Forgetting to double the sidewall is the most common error and roughly halves the height you add to the rim.
• Stay within about 3% of stock diameter. Larger jumps noticeably skew the speedometer and revolutions per mile and can cause rubbing.
• More revolutions per mile means a smaller tire. A taller tire covers more ground per turn, so it logs fewer revolutions per mile, not more.

Limitations and notes

This calculator returns the nominal calculated dimensions from the size code, using ideal geometry. Real mounted tires vary slightly: section width depends on the actual rim width, the loaded (rolling) radius is a touch smaller than the free diameter because the tire flattens under the car’s weight, and tread wear, inflation pressure and manufacturer tolerances all shift the numbers by a small amount. So treat the diameter, circumference and revolutions per mile as accurate reference figures, not a tape-measure guarantee — for a precise rolling check, measure the mounted, inflated tire on the vehicle. Everything runs privately in your browser: the width, aspect and rim values you enter stay on your device, the math is computed at full precision and only rounded for display, and nothing is uploaded.

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