Tire Size Calculator — Decode Any Tire Size
A tire size calculator takes the code moulded into a tire sidewall — something like 205/55R16 — and turns it into the measurements that actually matter: section width, sidewall height, overall diameter, circumference and revolutions per mile. American drivers usually meet the size code on the sidewall of the tires already on the car, on the door-jamb placard, or in an online listing for a used set — and in each case the code is the same three-number shorthand, read the same way. The search volume behind this tool is worth stating plainly: "tire size calculator" is a third-party estimate of 110,000 searches a month (Semrush, September 2026, via fitmentindustries.com), which is why so many drivers arrive here with a size code copied from a sidewall and no idea what it means. This page explains how the calculator works, shows the arithmetic step by step so you can check it by hand, and then shows what a change of size does to the speedometer, the gearing and the ride. Everything here uses computed dimensions from the size code, so the numbers are exact for a new tire of that size; real tires vary slightly by brand, model, wear and inflation pressure.
The calculator needs only the three numbers in the code. Type in a size such as 205/55R16 and it returns the width in millimetres, the aspect ratio as a percentage, the rim diameter in inches, and then the four derived figures: sidewall height, overall diameter, circumference and revolutions per mile. It also compares the size with alternatives, so you can see the diameter difference as a percentage and the speedometer error that follows. There is no guesswork in any of that — every figure is computed from the code by the formulas below, and the worked examples show each step.
This is a calculation, not a safety approval. Always check your vehicle's door-jamb placard, load index and speed rating, and confirm fitment with a tyre professional before fitting a different tyre size.
How to read a tire size code
A size code has three numbers and one letter: width, aspect ratio, the letter R, and rim diameter. In 205/55R16, 205 is the section width in millimetres — the width of the tire from sidewall to sidewall at its widest point. The 55 is the aspect ratio: the sidewall height as a percentage of the width, so the sidewall here is 55% of 205 mm. R stands for radial construction, the standard for modern road tires. And 16 is the rim diameter in inches, the wheel the tire mounts on. The most common misreading is to treat the aspect ratio as a measurement in millimetres — it is not; it is a percentage, so the same aspect ratio on a wider tire gives a taller sidewall. The second most common mistake is to read the width and ignore the rest, which is how two tires with the same width and different diameters end up confused. The size code also says nothing about load index or speed rating: those are separate markings on the sidewall, and the door-jamb placard specifies the minimum for both.
Worked example: 205/55R16
Start with 205/55R16, the size used through this page because it is familiar and its numbers are easy to follow. The width is 205 mm and the aspect ratio is 55%, so the sidewall height is 205 × 55 ÷ 100 = 112.8 mm. The rim is 16 inches, which is 16 × 25.4 = 406.4 mm. The overall diameter is the rim plus two sidewalls: 406.4 + 2 × 112.8 = 631.9 mm, or 24.88 inches. The circumference is pi times the diameter: 3.1416 × 631.9 = 1985.2 mm (78.16 inches). And the revolutions per mile are 63,360 inches in a mile divided by the circumference in inches: 63,360 ÷ 78.16 = 810.7. Those five numbers — 112.8 mm sidewall, 631.9 mm diameter, 1985.2 mm circumference, 810.7 revs per mile, 16-inch rim — are everything the calculator returns for this size, and every one of them is derived from the three numbers in the code.
| Measurement | 205/55R16 | 225/45R17 |
|---|---|---|
| Section width | 205 mm | 225 mm |
| Aspect ratio | 55% | 45% |
| Sidewall height | 112.8 mm | 101.2 mm |
| Overall diameter | 631.9 mm (24.88 in) | 634.3 mm (24.97 in) |
| Circumference | 1985.2 mm | 1992.7 mm |
| Revolutions per mile | 810.7 | 807.6 |
| Rim diameter | 16 in | 17 in |
A second worked example shows the same method on a wider, lower-profile size. 225/45R17 has a 225 mm width and a 45% aspect ratio, so its sidewall is 225 × 45 ÷ 100 = 101.2 mm, and its diameter is 431.8 + 2 × 101.2 = 634.3 mm. Against 205/55R16 that is a difference of 0.38%, so a car moving from 205/55R16 to 225/45R17 would see its speedometer reading shift by that percentage and its ride height move by half the diameter difference, 1.2 mm. The two sizes sit close together in diameter despite looking quite different on paper, which is the point of doing the arithmetic instead of guessing from the width alone.
tire size calculator: The formulas the calculator uses
Four formulas do all the work, and they are worth knowing because they let you check any result by hand. Sidewall height = width × aspect ratio ÷ 100. Overall diameter = rim diameter in millimetres + 2 × sidewall height, where the rim in millimetres is the rim in inches × 25.4. Circumference = pi × overall diameter. Revolutions per mile = 63,360 ÷ circumference in inches. The speedometer formulas build on the diameter: error % = (new diameter − old diameter) ÷ old diameter × 100, and actual speed = indicated speed × (new diameter ÷ old diameter). So if a replacement is 2% larger in diameter, the car travels 2% faster than the speedometer shows — at an indicated 70 mph that is 71.4 mph actual. If it is 2% smaller, the car travels 2% slower than the dial shows, and the odometer records 2% more miles than the car actually covers. All of these figures are nominal new-tire values, and the calculator shows its working so the result can be verified rather than taken on trust.
tire size calculator: What the numbers mean for your car
Diameter is the number that governs the speedometer, the gearing and the ground clearance. A larger diameter lengthens the gearing, so the engine turns more slowly at a given road speed, and raises the car by half the diameter increase. A smaller diameter shortens the gearing and lowers the car by half the decrease. Width governs the contact patch: more width generally means more dry grip and more rolling resistance, and it needs clearance at the wheel arch, the suspension and full steering lock. Sidewall height governs ride and steering feel: a taller sidewall absorbs bumps and protects the rim, while a shorter sidewall sharpens steering response at the cost of a firmer ride. The ±3% guideline ties these together — a replacement within ±3% of the original diameter keeps the speedometer error, the gearing change and the clearance change small, and it is the commonly used guideline for keeping speedometer error small, not a safety approval. Anything outside that band deserves the placard check and a professional's confirmation before fitting, because the effects grow with the percentage.
Finding your size and checking a replacement for tire size calculator
The size to match is on the door-jamb placard, usually on the driver's door frame, which lists the manufacturer-specified size together with the recommended pressures and often the load index and speed rating. The owner's manual repeats that information and adds the spare-tire size, which is often different. Compare both with the size moulded on the sidewall of the tires fitted now: if they do not match, find out why before ordering replacements, because the fitted size may have been a previous owner's choice rather than the manufacturer's. Then check the load index and the speed rating on the replacement — both must meet or exceed what the placard specifies, because a tire that is the right size but the wrong load index is still the wrong tire. Check the rim width as well: the section width needs a rim within its approved width range. If the car has tire pressure monitoring, remember that a change in rolling circumference can affect it, so budget for a reset or relearn after fitting. Winter tires go through the same checks, not fewer.
Common mistakes when using a size calculator for tire size calculator
The first mistake is entering the size from the tire on the car instead of the placard, which repeats whatever the last owner fitted. The second is matching only the width and ignoring the aspect ratio — two tires with the same width and different aspect ratios have different diameters, so the speedometer and gearing change. The third is forgetting that the aspect ratio is a percentage of the width, not a fixed height: 55% of 205 mm is 112.8 mm, but 55% of 225 mm is 123.8 mm, so the same aspect ratio on a wider tire is a taller tire. The fourth is treating the calculator's output as a fitment approval — it is a calculation of dimensions, and clearance, load index, speed rating and rim width still need checking. The fifth is changing the size on one axle only, which leaves different rolling circumferences front and rear and can confuse traction control, ABS and all-wheel-drive systems. And the sixth is fitting the new tires at the old pressures: the correct pressure is the placard figure for the size fitted, checked cold.
tire size calculator: Try the calculator and related sizes
The tyre size calculator tool runs these formulas for any size in the chart, and the tyre sizes hub lists every size covered on this site with its full dimensions. To see the method applied to a single size, start with 205/55R16 tire size, 225/45R17 tire size and 205/55R16 tyre size — each page gives the diameter, sidewall, circumference and revs per mile for that size, plus its three closest alternatives. The tyre size chart shows the full computed table side by side for comparison.
Frequently asked questions
How do I read a tire size like 205/55R16?
205 is the section width in millimetres, 55 is the aspect ratio (the sidewall is 55% of the width, so 112.8 mm), R means radial construction, and 16 is the rim diameter in inches. The overall diameter is the rim (406.4 mm) plus two sidewalls (112.8 mm each), giving 631.9 mm. The size code does not include the load index or speed rating — those are separate sidewall markings, and the placard specifies the minimum for both.
What does the tire size calculator tell me?
It turns the three numbers in the size code into the measurements that matter: sidewall height, overall diameter, circumference and revolutions per mile, plus a comparison with alternative sizes showing the diameter difference as a percentage and the speedometer effect. For 205/55R16 that is a 112.8 mm sidewall, a 631.9 mm diameter, a 1985.2 mm circumference and 810.7 revolutions per mile. The calculator shows its working so every figure can be checked by hand.
How do I find the right tire size for my car?
Check the door-jamb placard first — it lists the size the manufacturer specified, with the recommended pressures and usually the load index and speed rating. The owner's manual repeats it. The size moulded on the tires fitted now is a useful cross-check, but if it disagrees with the placard, the placard is the one to investigate, because a previous owner may have fitted a different size.
How much can I change my tire size?
The commonly used guideline is ±3% of the original overall diameter: a replacement inside that band keeps the speedometer error, the gearing change and the clearance change small. It is a guideline, not a safety approval. Outside that band the speedometer error at motorway speed becomes noticeable, the odometer drifts by the same percentage, and the load index, speed rating, rim width and arch clearance all need checking with a tire professional before fitting.
Does a bigger tire make the speedometer read slow?
Yes, if the overall diameter is bigger. Actual speed = indicated speed × (new diameter ÷ old diameter), so a tire 2% larger in diameter makes the car travel 2% faster than the dial shows — 71.4 mph actual at an indicated 70 mph. A smaller diameter does the opposite: the dial reads fast and the odometer records more miles than the car covers. The effect is proportional to the diameter change, which is why the ±3% guideline exists.
How popular is the tire size calculator?
"tire size calculator" is a third-party estimate of 110,000 searches a month (Semrush, September 2026, via fitmentindustries.com). That figure is an estimate from a third-party tool, not a count from this site, and it is the reason this page exists: a lot of drivers need the same decoding, done properly, with the working shown.
Where does the calculator get its dimensions?
From the size code itself, using the formulas on this page: sidewall = width × aspect ratio ÷ 100, diameter = rim in millimetres + 2 × sidewall, circumference = pi × diameter, revs per mile = 63,360 ÷ circumference in inches. The values are nominal new-tire figures. Real tires vary slightly by brand, model, wear and inflation pressure, so two tires with the same size code can differ by a millimetre or two in practice.