Tyre Size Calculator & Decoder
Every number on your sidewall, decoded live - width, sidewall, diameter - so you replace or upsize with confidence.
Read it off your sidewall (e.g. 185/65 R15)
185/65 R15 decoded
185 mm
Tread width
120 mm
Sidewall height
622 mm
Overall diameter
💡 185/65 R15 means: 185 mm wide tread, a sidewall 65% as tall as the width (120 mm), on a 15-inch rim. The letter before the rim (R) means radial construction; the code after the size (like 87S) is the load index and speed rating.
Replacing tyres? Stay within ±3% of the original overall diameter or your speedometer misreads. Common speed ratings: S=180, T=190, H=210, V=240 km/h.
Frequently asked questions
Where do I find my tyre size?
On the sidewall of your current tyre (e.g. 185/65 R15 87S) and on the tyre placard inside the driver's door frame or fuel flap.
Can I fit a different size than stock?
Stay within ±3% of the original overall diameter and check rim width compatibility, or your speedometer and ABS calibration go off. Use the diameter output above to compare.
What do the last characters like 87S mean?
87 is the load index (545 kg per tyre) and S is the speed rating (180 km/h). Never go below your car's specified load index.