Tire Size Calculator

Two calculators sit above this. The first turns a tire code into every dimension that follows from it, draws the tire to scale, and will list the alternative sizes that fit a different wheel while keeping the rolling diameter close. The second puts two tires side by side and shows what swapping one for the other does, including to your speedometer.

Reading the code on the sidewall

Every car tire carries an alphanumeric code that describes what it is. Being able to read it is the difference between buying the right tire and buying one that fits the rim but changes how the car behaves.

Intended use comes first, as one or two letters describing the class of vehicle.

Letter	Meaning	Fitted to
P	Passenger car	Cars, crossovers, minivans, smaller pickups
LT	Light truck	Heavier loads and towing
ST	Special trailer	Boat, utility and travel trailers
T	Temporary	Space-saver spares, not for normal use
(none)	Metric or Euro-metric	Common on European fitments

Nominal section width is the three-digit number, in millimetres, measured across the widest part of the tire from sidewall to sidewall. In the example above, 215 mm.

Aspect ratio follows the slash. It is the section height as a percentage of the width, so 65 means the sidewall is 65% of 215 mm, or 139.75 mm. This is the number people misread most often: it is a ratio, not a measurement, so the same 65 means a taller sidewall on a wider tire.

Internal construction is the single letter after the aspect ratio. R is radial, where the cords run across the tire perpendicular to the direction of travel, and it is what almost every passenger car uses. D is diagonal, with cords laid at 30 to 40 degrees from bead to bead. B is bias-belted, a diagonal carcass with extra stabiliser belts over the top to stiffen it. If the letter is missing, the tire is cross-ply.

Wheel diameter is next, and it is in inches even though everything before it was in millimetres. That inconsistency is a historical accident and it is why the calculators above ask for the parts separately.

Load index is the two or three digit number after the size. It is a code, not a weight: 95 means the tire can carry 690 kg, or about 1,521 lb, at its rated pressure. Multiply by four for the vehicle, and never fit a tire with a lower index than the placard in the door frame specifies.

Speed rating is the final letter, the maximum sustained speed at which the tire can carry that load.

Code	Q	S	T	H	V	W	Y
km/h	160	180	190	210	240	270	300
mph	99	112	118	130	149	168	186

The DOT code is separate and required in the United States. It identifies the plant, batch and mould, and its last four digits are the date of manufacture: two for the week, two for the year. A tire marked 1326 was built in the 13th week of 2026. This matters when buying, because rubber ages whether or not it is used, and a tire that has sat in a warehouse for years is older than it looks.

How tire size is calculated

Two lines do all of it:

section height = aspect ratio × section width
tire diameter = wheel diameter + 2 × section height

The doubling matters. Sidewall sits above and below the wheel, so a change in aspect ratio moves the overall diameter by twice as much as it moves the sidewall. Everything else on the results table comes from the diameter: circumference is π times it, and revolutions per mile is 63,360 divided by the circumference in inches.

Worked through on the default 265/70R17: the sidewall is 265 × 0.70 = 185.5 mm, which is 7.3 inches, so the diameter is 17 + 2 × 7.3 = 31.6 inches, or 803 mm. The circumference is 99.3 inches, and 63,360 divided by that gives 638.1 revolutions per mile.

What changes when you change size

The same rim takes many different tires, and a common modification is fitting larger wheels with lower-profile tires. The usual advice is to stay within 3% of the original rolling diameter, which is the tolerance the alternatives table above uses.

The reason is that a great deal of the car is calibrated against that diameter. Speedometer, odometer, ABS, traction control and stability control all infer speed from how fast the wheels turn. Change how far one revolution carries you and every one of those readings shifts.

A taller tire makes the speedometer read low. You are travelling faster than it says, and your odometer under-counts the miles. A shorter tire does the reverse: the needle reads high and the odometer over-counts. On a 3% larger tire, an indicated 70 mph is really about 72 mph, and a 12,000 mile year is really about 12,360.

This is what makes plus-sizing work. Going up an inch in rim while dropping the aspect ratio keeps the diameter nearly the same, so the calibration survives. Compare 265/70R17 with 265/40R23 in the second calculator and the wheel grows by 35% while the overall diameter changes by less than 1%, because the sidewall shrank by 43% to compensate.

What does change is ride and cost. Less sidewall means less rubber to absorb bumps, more road noise, more risk of pothole damage to an expensive rim, and usually a heavier wheel, which is unsprung mass that suspension has to control.

Why two tires of the same size measure differently

The section width in the code is nominal rather than a guarantee. It is the width the tire is designed to reach when mounted on a specified reference rim, and fitting the same tire to a narrower or wider rim pulls the sidewalls in or splays them out. The usual rule of thumb is that half an inch of rim width moves the section width by around 5 mm.

Manufacturing tolerance adds to that. Two 225-section tires from different makers can differ by several millimetres in measured width and by a little in diameter, which is why the same nominal size can rub on one brand and clear on another. Tread pattern and shoulder shape change the effective footprint again.

None of that makes the calculation wrong. It means the answer is the design intent, accurate to a millimetre or two in practice, and that clearance decisions at the margin want a physical check rather than arithmetic. If a size is only just going to fit on paper, it is not going to fit.

Practical rules when replacing tires

Replace all four at once where you can. If not, replace in pairs across an axle, never one corner alone, and keep type and brand the same across each axle. Mismatched grip left to right is a genuine handling problem, and on all-wheel drive a diameter mismatch loads the centre differential continuously, which is an expensive failure rather than a nuisance.

Check the age as well as the tread. Many manufacturers suggest replacing at six years regardless of mileage, and a tire more than ten years old should be taken off the car whatever its condition, because the rubber degrades from the inside where you cannot see it.

Tread depth is the other limit. The legal minimum in the United States and much of Europe is 1.6 mm, which is 2/32 of an inch, but wet braking falls off well before that, so 4/32 is a more sensible point to start shopping.

Reading your result

Compare the diameter first and the width second. Diameter is what the car's electronics care about; width is what your arches and your grip care about.

The alternatives table lists what matches on size alone. It cannot tell you whether a size is sold in your area, whether the load and speed ratings suit your car, or whether the tire will clear the suspension at full lock and full compression. The door placard is the authority on the first two, and only a physical check settles the third.

To convert any of the figures into other units, the Conversion Calculator handles length directly, and the Circle Calculator covers the circumference and diameter relationship the rolling numbers are built on.