Tire Size Calculator: How to Compare Sizes and Work Out Tire Diameter

A tire size calculator turns the code on a tire's sidewall into usable dimensions: overall diameter, sidewall height, section width, circumference and revolutions per mile. When you compare two sizes, the most important result is the percentage change in overall diameter. That figure drives speedometer error, effective gearing and whether the new tire is likely to clear the suspension and bodywork. You can do the same math by hand with three numbers from the tire code. The results are nominal, though. Real tires differ by brand, rim width, inflation and wear, so treat a calculator as a screening tool. It does not replace fitment approval.

Reading the tire size code

Take a common metric format, 245/45R18, as an example. The first number, 245, is the section width in millimeters, measured from sidewall to sidewall. The second number, 45, is the aspect ratio: the sidewall height as a percentage of the section width. R means radial construction. The last number, 18, is the wheel (rim) diameter in inches. Some codes start with a prefix such as P (passenger) or LT (light truck). Most are followed by a service description such as 98W, which combines a load index and a speed rating. Those last two items do not affect diameter, but they matter just as much when you choose a replacement.

The tire diameter formula

Sidewall height (in)   = section width (mm) × aspect ratio ÷ 100 ÷ 25.4
Overall diameter (in)  = rim diameter (in) + 2 × sidewall height (in)
Section width (in)     = section width (mm) ÷ 25.4
Circumference (in)     = overall diameter (in) × 3.1416
Revolutions per mile   = 63,360 ÷ circumference (in)

Worked example for 245/45R18: the sidewall is 245 × 0.45 = 110.25 mm, which is about 4.34 in. Both sidewalls together add about 8.68 in to the 18 in wheel, giving an overall diameter of roughly 26.68 in. The circumference is about 83.8 in, which works out to around 756 revolutions per mile. The section width is about 9.65 in. A tire height calculator and a wheel diameter calculator both run this same arithmetic. Here, wheel diameter means the rim size in the code, while tire height means the overall diameter.

Comparing two tire sizes side by side

To compare sizes, calculate the overall diameter of each and express the difference as a percentage of the original. Your speedometer and odometer are calibrated to the original rolling diameter. A larger tire makes the vehicle travel farther per revolution, so actual speed is higher than indicated and the odometer under-records distance. A smaller tire does the opposite.

Diameter change (%) = (new diameter − original diameter) ÷ original diameter × 100
Actual speed        = indicated speed × new diameter ÷ original diameter

Can I fit 255 instead of 245?

Often you can, but it depends on how you change the rest of the size and on the space available on your specific vehicle. Going from 245 to 255 adds 10 mm of nominal width, about 0.39 in. If you keep the same aspect ratio, the sidewall also grows, because aspect ratio is a percentage of width. The comparison below uses 245/45R18 as the original and shows two ways to go wider on the same 18 in wheel.

Measurement | 245/45R18 (original) | 255/45R18 | 255/40R18

Section width | 9.65 in | 10.04 in | 10.04 in

Sidewall height | 4.34 in | 4.52 in | 4.02 in

Overall diameter | 26.68 in | 27.04 in | 26.03 in

Diameter change | — | +1.3% | −2.4%

Revolutions per mile | about 756 | about 746 | about 775

Actual speed at indicated 60 mph | 60 mph | about 60.8 mph | about 58.5 mph

In this example, 255/45R18 stays closer to the original diameter. It is also 0.18 in taller in each sidewall, which reduces clearance at the top of the wheel arch as well as at the sides. 255/40R18 keeps the overall tire lower, but it moves the diameter further from the original in the other direction and gives a shorter sidewall. A shorter sidewall generally means less cushioning over impacts. Whichever you consider, confirm that your wheel's width falls within the rim-width range the tire maker approves for that size. Also confirm that the load index and speed rating meet or exceed what your vehicle's tire placard specifies.

How to choose between candidate sizes

• Diameter: stay as close to the original overall diameter as practical. This keeps speedometer, odometer and gearing changes small.
• Clearance: a wider or taller tire can contact suspension parts, brake components, fender liners or bodywork, especially at full steering lock or under suspension compression. A calculator cannot check this for your vehicle.
• Rim compatibility: the wheel width must be within the approved range for the new tire size.
• Load and speed capability: the replacement's load index and speed rating should meet the vehicle maker's requirement, not just match the old tire's dimensions.
• Drivetrain sensitivity: some all-wheel-drive and four-wheel-drive systems are sensitive to diameter differences between tires. Check the vehicle maker's guidance for your model before mixing sizes or replacing only some tires.
• Ride and handling trade-offs: shorter sidewalls usually sharpen steering response but transmit more impact harshness. Taller sidewalls do the opposite.

Many tire guides suggest keeping the overall diameter within about 3% of the original. Treat that as a common rule of thumb, not a regulation or a vehicle-specific approval. The safest references for your car are the sizes listed by its manufacturer for your model year and trim, and the fitment guidance from the tire maker.

Converting metric tire sizes to standard (inch) sizes

Inch-based sizes, often called flotation sizes and common on light trucks, look like 33x12.50R15. They state overall diameter, section width and rim diameter, all in inches. To convert a metric size to this format, calculate the overall diameter with the formula above and divide the section width in millimeters by 25.4. For example, 285/70R17 works out to about 32.7 in tall and 11.2 in wide on a 17 in wheel, so it behaves roughly like a 33x11R17. To go the other way, subtract the rim diameter from the overall diameter and halve the result to get the sidewall height. Then multiply the inch width by 25.4 and divide the sidewall height by the width. For 33x12.50R15, that gives a width of about 318 mm and an aspect ratio of about 72%. Real metric sizes come only in standard width and aspect-ratio steps, so the nearest available metric size will not match exactly. Compare the overall diameters before treating any two sizes as equivalent.

Metric size | Overall diameter | Section width | Approximate inch equivalent

205/55R16 | 24.9 in | 8.1 in | 24.9x8.1R16

225/65R17 | 28.5 in | 8.9 in | 28.5x8.9R17

265/70R17 | 31.6 in | 10.4 in | 31.6x10.4R17

275/60R20 | 33.0 in | 10.8 in | 33.0x10.8R20

285/70R17 | 32.7 in | 11.2 in | 32.7x11.2R17

Every figure in this chart is calculated from the nominal size code. None are measured values for any particular tire. The sizes were chosen to illustrate the math, not to represent market popularity.

How wheel and rim width affect tire width

The section width in a tire code is a nominal figure, specified when the tire is mounted on a particular measuring rim width. Each tire size also has a range of approved rim widths. Mounting the tire on a wider or narrower wheel within that range changes its actual section width somewhat, and can slightly change its shape and height. That is why two tires with the same nominal size can measure differently on different wheels. Check the approved rim-width range in the tire maker's published specifications for the exact tire you are considering. The information supplied for this guide does not include a general table of rim widths by tire size.

Motorcycle tire sizes and what 4.00-18 means

Metric motorcycle sizes such as 120/70ZR17 follow the same logic as car sizes: width in millimeters, aspect ratio as a percentage, and rim diameter in inches. The same diameter formula applies. Older inch-designated motorcycle sizes such as 4.00-18 (sometimes written 4.00 x 18) give the nominal section width in inches followed by the rim diameter. A 4.00 in section width is about 102 mm, and the tire fits an 18 in rim. There is no aspect ratio in this format, so you cannot calculate the overall diameter from the code alone. No single metric size is an exact equivalent. The evidence supplied for this guide does not include a cross-reference chart, so confirm any metric replacement with the motorcycle or tire manufacturer's fitment data. Then check the actual overall diameter, rim-width range and load rating rather than relying on width alone.

Why calculated and real-world diameters differ

• Brand and model variation: tires with the same size code can differ slightly in actual dimensions.
• Rim width: a wider or narrower wheel changes the mounted width and profile.
• Inflation and load: the loaded radius under the vehicle's weight is smaller than the unloaded diameter, and the tire's effective rolling circumference is smaller than the calculated circumference.
• Tread wear: diameter falls as tread wears, so a new tire is taller than a worn one of the same size.
• Rounding: calculators round their outputs, which matters when you compare sizes that differ only slightly.

What a calculator cannot tell you

A size calculator or side-by-side visualizer shows geometry only. It cannot confirm physical clearance on your vehicle. It also cannot tell you whether a size is approved for your model or whether a change affects electronic systems calibrated to the original rolling diameter. The supplied evidence also contains no data on which tire sizes are most common in 2026. Any ranking of popular sizes would need current sales or fitment data from a tire maker or retailer. For a definitive answer, start with the tire placard (typically on the driver's door jamb) and the owner's manual for your model year. Then use the calculator to compare those approved sizes with any alternative you are considering.