Tyre Size Calculator — Decode Any Tyre Size
A tyre size calculator takes the code moulded into a tyre sidewall — something like 195/65R15 — and turns it into the measurements that actually matter: section width, sidewall height, overall diameter, circumference and revolutions per mile. UK drivers usually meet the size code on the sidewall of the tyres 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. 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 tyre of that size; real tyres vary slightly by brand, model, wear and inflation pressure.
Give the calculator any size code and it works out the rest: 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 lines the size up against alternatives, showing the diameter difference as a percentage and the speedometer error that follows from it. Nothing in that is estimated: every figure is computed from the code by the formulas further down this page, and the worked examples show each step so the result can be checked by hand.
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 tyre size code
A size code has three numbers and one letter: width, aspect ratio, the letter R, and rim diameter. In 195/65R15, 195 is the section width in millimetres — the width of the tyre from sidewall to sidewall at its widest point. The 65 is the aspect ratio: the sidewall height as a percentage of the width, so the sidewall here is 65% of 195 mm. R stands for radial construction, the standard for modern road tyres. And 15 is the rim diameter in inches, the wheel the tyre 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 tyre gives a taller sidewall. The second most common mistake is to read the width and ignore the rest, which is how two tyres 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: 195/65R15
Start with 195/65R15, the size used through this page because it is familiar and its numbers are easy to follow. The width is 195 mm and the aspect ratio is 65%, so the sidewall height is 195 × 65 ÷ 100 = 126.8 mm. The rim is 15 inches, which is 15 × 25.4 = 381.0 mm. The overall diameter is the rim plus two sidewalls: 381.0 + 2 × 126.8 = 634.5 mm, or 24.98 inches. The circumference is pi times the diameter: 3.1416 × 634.5 = 1993.3 mm (78.48 inches). And the revolutions per mile are 63,360 inches in a mile divided by the circumference in inches: 63,360 ÷ 78.48 = 807.4. Those five numbers — 126.8 mm sidewall, 634.5 mm diameter, 1993.3 mm circumference, 807.4 revs per mile, 15-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 | 195/65R15 | 205/55R16 |
|---|---|---|
| Section width | 195 mm | 205 mm |
| Aspect ratio | 65% | 55% |
| Sidewall height | 126.8 mm | 112.8 mm |
| Overall diameter | 634.5 mm (24.98 in) | 631.9 mm (24.88 in) |
| Circumference | 1993.3 mm | 1985.2 mm |
| Revolutions per mile | 807.4 | 810.7 |
| Rim diameter | 15 in | 16 in |
A second worked example shows the same method on a taller, narrower size that is common on family cars. 205/55R16 has a 205 mm width and a 55% aspect ratio, so its sidewall is 205 × 55 ÷ 100 = 112.8 mm, and its diameter is 406.4 + 2 × 112.8 = 631.9 mm. Against 195/65R15 that is a difference of -0.41%, so a car moving from 195/65R15 to 205/55R16 would see its speedometer reading shift by that percentage and its ride height move by half the diameter difference, 1.3 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.
tyre 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-tyre values, and the calculator shows its working so the result can be verified rather than taken on trust.
tyre size calculator: What the numbers mean for your car
Start with diameter, because it is the figure that ties everything together. A larger diameter lengthens the gearing, so the engine turns more slowly at a given road speed, and it raises the car by half the diameter increase; a smaller one shortens the gearing and lowers the car by half the decrease. On a UK speedometer, which reads in mph, a 2% larger diameter means 71.4 mph of real speed at an indicated 70 mph — and in kilometres, 102 km/h of real speed at an indicated 100 km/h. Width sets the size of the contact patch: more of it usually means more dry grip and more rolling resistance, and it needs room at the wheel arch, the suspension and full steering lock. Sidewall height sets the ride and the steering feel — a taller sidewall soaks up bumps and shields the rim, a shorter one sharpens the steering and firms the ride. The ±3% guideline is the thread that ties the three together: a replacement inside ±3% of the original diameter keeps the speedometer error, the gearing change and the clearance change small. It is a commonly used guideline for keeping speedometer error small, not a safety approval, and anything outside it deserves the placard check and a professional's confirmation before fitting.
Finding your size and checking a replacement for tyre size calculator
The size to match lives on the door-jamb placard, usually on the driver's door frame. It gives the size the manufacturer specified, the recommended pressures and, in most cases, the load index and speed rating the car needs. The owner's manual carries the same information and adds the spare-tyre size, which is often different. Check both against the size moulded into the sidewall of the tyres fitted now: a mismatch is a question to answer before ordering, because the tyres on the car may be a previous owner's choice rather than the manufacturer's. Then look at the load index and speed rating on any replacement — both have to meet or exceed what the placard asks for, since a tyre that is the right size but the wrong load index is still the wrong tyre. Rim width needs the same treatment: the section width only mounts correctly on a rim inside its approved width range. Cars with tyre pressure monitoring may need a reset or relearn after a circumference change, and winter tyres go through every one of these checks too.
Common mistakes when using a size calculator for tyre size calculator
Six mistakes cover most of the trouble with a size calculator. First, typing in the size already on the car instead of the placard size, which repeats whatever the last owner fitted. Second, matching the width alone and ignoring the aspect ratio, even though two tyres with the same width and different aspect ratios have different diameters. Third, forgetting that the aspect ratio is a percentage of the width rather than a fixed height: 65% of 195 mm is 126.8 mm, but 65% of 215 mm is 139.8 mm, so the same aspect ratio on a wider tyre is a taller tyre. Fourth, treating the calculator's answer as a fitment approval — it is a calculation of dimensions, and clearance, load index, speed rating and rim width still have to be checked. Fifth, changing size on one axle only, which leaves different rolling circumferences front and rear and can confuse traction control, ABS and all-wheel-drive systems. Sixth, fitting the new tyres at the old pressures instead of the placard figure for the size fitted, checked cold.
Try the calculator and related sizes for tyre size calculator
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 195/65R15 tyre size, 205/55R16 tyre 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 tyre size like 195/65R15?
195 is the section width in millimetres, 65 is the aspect ratio (the sidewall is 65% of the width, so 126.8 mm), R means radial construction, and 15 is the rim diameter in inches. The overall diameter is the rim (381.0 mm) plus two sidewalls (126.8 mm each), giving 634.5 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 tyre 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 195/65R15 that is a 126.8 mm sidewall, a 634.5 mm diameter, a 1993.3 mm circumference and 807.4 revolutions per mile. The calculator shows its working so every figure can be checked by hand.
How do I find the right tyre 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 tyres 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 tyre 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 tyre professional before fitting.
Does a bigger tyre make the speedometer read slow?
Yes, if the overall diameter is bigger. Actual speed = indicated speed × (new diameter ÷ old diameter), so a tyre 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.
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-tyre figures. Real tyres vary slightly by brand, model, wear and inflation pressure, so two tyres with the same size code can differ by a millimetre or two in practice.