Tire Size Calculator

About the Tire Size Calculator

A tyre size such as 205/55 R16 91V describes the tyre’s shape: 205 mm section width, a sidewall 55% as tall as that (112.75 mm), radial construction, a 16-inch rim, load index 91 (615 kg per tyre) and speed symbol V (240 km/h). Type any size from the sidewall to get its width, sidewall height, overall diameter, circumference and revolutions per kilometre and per mile, with every part of the code explained.

The calculator reads metric car, van and motorcycle sizes (P-metric, LT and C sizes, ZR, run-flat and XL too), older sizes without an aspect ratio (165 R13, from the official UN dimensions), flotation sizes (31x10.50R15LT) and bicycle sizes in all four systems: ETRTO 37-622, French 700x35C, decimal 29x2.25 and fractional 28 × 1⅝ × 1⅜. Compare two sizes to see the difference in diameter and what your speedometer shows. This is geometry only — an estimate, not a fitment check: fit only sizes approved for your vehicle, as listed on its tyre placard or by its maker.

Common uses

• Checking how a new wheel and tyre combination compares with the original size before buying.
• Working out the speedometer error after fitting bigger or smaller tyres.
• Reading the load index and speed symbol on a tyre to check them against the vehicle’s requirements.
• Finding the ETRTO size and wheel circumference of a bicycle tyre for a bike computer.

How a tyre size is read

• 205 — section width in millimetres, sidewall to sidewall.
• 55 — aspect ratio: the sidewall (section height) is 55% of the width.
• R — radial construction (ZR: radial for speeds above 240 km/h; D or a hyphen: diagonal; B: bias-belted; F before the rim size: run-flat).
• 16 — rim diameter in inches (a number over 100 is the rim diameter in mm).
• 91 — load index: the maximum load per tyre (91 = 615 kg).
• V — speed symbol: the maximum speed (V = 240 km/h).

A leading P (P-metric), LT (light truck), T (temporary spare) or ST (special trailer), or a C after the rim size (commercial, with two load indices for single and dual fitment, such as 104/102), says what the tyre is for. XL or Reinforced marks an extra-load tyre, and M+S a snow tyre. Makers’ run-flat markings (RFT, SSR, ZP, ROF and others) and car makers’ approval marks (MO, N0, AO, ★) do not change the size: type them with it and the calculator says what they are.

The formulas

• sidewall height H = width × aspect ratio ÷ 100 (205 × 0.55 = 112.75 mm)
• rim diameter d = rim size × 25.4 mm (16 × 25.4 = 406.4 mm)
• overall diameter D = d + 2 × H (406.4 + 225.5 = 631.9 mm)
• circumference = π × D; revolutions per km = 1,000,000 ÷ circumference in mm
• true speed with a new size = speed shown × new diameter ÷ old diameter

These are the design dimensions of UN Regulation No. 30 (paragraph 6.1). A new tyre may measure a little less or more: for a radial car tyre the sidewall part of the diameter may be 3% smaller to 4% larger (D = d + 2H × 0.97 to d + 2H × 1.04), and the calculator shows that range. Sizes without an aspect ratio, such as 165 R13 and 185 R14 C, take their dimensions from the tables in Annex V of UN Regulations No. 30 and 54.

Load index and speed symbol

The load index is a code for the maximum load per tyre under the conditions the tyre maker sets: 91 = 615 kg, 95 = 690 kg, 100 = 800 kg, 110 = 1,060 kg, 120 = 1,400 kg (Annex IV of UN Regulations No. 30 and No. 54). Van and light-truck tyres often carry two, such as 120/116: the load per tyre fitted single and fitted in pairs.

The speed symbol is the maximum speed: Q 160 km/h, R 170, S 180, T 190, U 200, H 210, V 240, W 270, Y 300. On a ZR tyre a service description in brackets, such as (94Y), means more than 300 km/h, with the limit set by the tyre maker.

Changing tyre size and your speedometer

A speedometer counts wheel turns, so a taller tyre makes the vehicle go faster than it shows: with a diameter 3% larger, a shown 100 km/h is a true 103 km/h. UN Regulation No. 39 requires the speed shown never to be less than the true speed, and at most 10% plus 4 km/h more (0 ≤ V1 − V2 ≤ 0.1 V2 + 4 km/h); a taller tyre can make a speedometer read low. A different diameter also changes the odometer, the gearing and the clearance in the wheel arch, and it can affect ABS and stability control. Fit only sizes the vehicle maker approves for your vehicle — they are on the tyre placard and in the owner’s manual.

Bicycle sizes

Bicycle tyres carry an ETRTO size (ISO 5775) such as 37-622: 37 mm wide, for a rim with a 622 mm bead seat diameter. The bead seat diameter decides whether a tyre fits a rim, and the older names map onto it: 700C and 29-inch are 622 mm (in Europe also 28-inch), 650B and 27.5-inch are 584 mm, decimal 26-inch mountain-bike tyres are 559 mm and 27-inch is 630 mm. Fractional inch names are ambiguous — 26 × 1⅜ is usually 590 mm but sometimes 597 mm — so the calculator says when it had to choose. A bicycle tyre’s diameter is estimated as the bead seat diameter plus twice the width, as it is about as tall as it is wide. Bead seat diameters for the traditional names are as tabulated by Sheldon Brown.

Limitations

• Geometry only: whether a size fits depends on the rim width and offset, the load and speed ratings and the space in the wheel arch. Fit only sizes approved for your vehicle, and have a tyre professional check the fitment.
• Real tyres differ from the design dimensions: UN R30 lets a new radial car tyre’s sidewall part of the diameter be 3% smaller to 4% larger, the diameter shrinks as the tread wears, and load and pressure change the rolling radius.
• Bicycle diameters are estimates (tyre height ≈ width); for a bike computer, measure the roll-out of your own wheel.
• Inch sizes without an aspect ratio (such as 7.50R16) and agricultural or industrial tyre codes are not decoded.

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