A tyre size tells you the tyre's width, its sidewall height and the rim diameter it fits. In 540/65 R28, 540 is the width in millimetres, 65 is the sidewall height as a percentage of that width, R means radial, and 28 is the rim diameter in inches. Sealant doses depend on all three numbers, which is why the OKO dosage calculator asks for the full size rather than just the rim.
The three numbers that matter
Whatever the format, you are looking for the same three things:
- Width, in millimetres or inches.
- Aspect ratio, the sidewall height as a percentage of width. A low number means a low, stiff sidewall; a high number means a tall one.
- Rim diameter, always in inches.
Together they set how much tread there is to cover inside the tyre, and so how much sealant it needs. The table shows how the calculator reads each common format.
| Example size | Where you see it | Width | Aspect ratio | Rim |
|---|---|---|---|---|
| 540/65 R28 | Modern tractors | 540 mm | 65% (radial) | 28" |
| 18.4-38 | Older-style tractor sizes | 18.4" | 100% | 38" |
| 315/80 R22.5 | Trucks, buses, trailers | 315 mm | 80% (radial) | 22.5" |
| 13 R22.5 | Trucks, inch-width format | 13" | 80% | 22.5" |
| 26.5 R25 | Loaders and earthmovers (OTR) | 26.5" | 80% | 25" |
| 875/65 R29 | Large OTR, metric | 875 mm | 65% (radial) | 29" |
| 140/80-17 | Motorcycles, bias | 140 mm | 80% (bias) | 17" |
Metric sizes: 540/65 R28 and 315/80 R22.5
This is the format on most modern tractor, truck, van and motorcycle tyres. Read it left to right: width in millimetres, a slash, the aspect ratio, R for radial, then the rim diameter. A 315/80 R22.5 truck tyre is 315 mm wide with a sidewall 80% of that width, on a 22.5" rim. On the same 22.5" rim, width and profile are what change the dose: 2 litres for a 275/70 R22.5, 2.5 litres for a 315/80 R22.5 and 2.8 litres for a 385/65 R22.5.
Older-style sizes: 18.4-38 and 13 R22.5
Many tractor tyres still carry sizes like 18.4-38 or 16.9-30. These give width in inches and rim in inches, with no aspect ratio. The calculator treats them as 100%, a tall sidewall the same height as the tyre is wide. An 18.4-38 takes 8.75 litres of OKO Original Off-Road, compared with 6 litres for a 540/65 R28. Inch-width truck sizes such as 11 R22.5, 12 R22.5 and 13 R22.5 are read the same way, with the calculator assuming an 80% profile.
Earthmover and OTR sizes: 26.5R25
Loader, dozer and haul-truck tyres often use inch widths: 26.5 R25 is 26.5" wide on a 25" rim, and the calculator treats it as 80% profile. Sizes such as 18.00 R33 and 24.00 R35 follow the same pattern. Some large OTR tyres use mixed formats: 35/65 R33 is 35" wide at 65%, while 875/65 R29 gives width in millimetres. Getting the size right matters here because doses are large: OKO Original Mining needs 7.25 litres for a 26.5 R25 in normal service and 21.5 litres in severe service. For more, see sealant for mining and OTR tyres.
Motorcycle and small tyres: 120/70 ZR17, 4-9, 13x5.00-6
Motorcycle sizes use the metric pattern. In 120/70 ZR17, 120 is the width in millimetres, 70 the aspect ratio and 17 the rim; the Z marks a high-speed-rated radial. A dash instead of R, as in 140/80-17 or 110/70-17, means bias construction. A 110/70-17 takes 200 ml of OKO Original On-Road Motorcycle. Read more in tyre sealant for motorcycles.
Small tyres on trailers, mowers and barrows use simpler codes. A 4-9 is 4" wide on a 9" rim. A 13x5.00-6 gives the overall height (13"), the width (5.00") and the rim (6"). Small tyres take small doses, so the OKO Tyre Sealant Injector 150ml, marked in 5ml steps, is the easiest way to get them right.
Why the size decides more than the dose
The size also tells you whether a formula suits the tyre at all:
- Low profiles. OKO Original 4x4 & Van is not recommended for tyres of 40 profile or lower. On a 225/45 R17, the 45 is just above that line. See sealant for 4x4s and vans.
- Racing tyres. OKO Original On-Road Motorcycle is not for ultra-low-profile racing tyres.
- Rim diameter. OKO Original Truck & Bus is commonly used on wheels up to 22.5". OKO Original Mining is most effective above 25", and for the very largest tyres of 50" and over, OKO recommends OKO Premium Tyre Care Lifelong.
The calculator accepts every format in this guide, so type the size exactly as it appears on the sidewall.
Quick answers
What does R mean in a tyre size?
R means radial construction. In 540/65 R28 the R sits between the aspect ratio (65) and the rim diameter (28 inches). Sizes written with a dash instead, such as 140/80-17, are bias (cross-ply) tyres.
What does the 65 mean in 540/65 R28?
It is the aspect ratio: the sidewall height as a percentage of the tyre's width. Here the sidewall is 65% of 540 mm, or about 351 mm tall.
What does 18.4-38 mean on a tractor tyre?
The tyre is 18.4 inches wide and fits a 38 inch rim. No aspect ratio is printed, and the OKO dosage calculator treats it as 100%.
Is rim size enough to work out a sealant dose?
No. Width and sidewall height matter too. On the same 22.5 inch rim, a 275/70 R22.5 takes 2 litres of OKO Truck & Bus while a 385/65 R22.5 takes 2.8 litres.
Copy the size from your sidewall into the OKO dosage calculator to get the right formula and dose. Buy online if you are in the UK, or through a local OKO distributor anywhere else.
