Inch tire sizes: what 33x12.50R15 means

Some truck tires show the size in inches, for example 33x12.50R15LT. The official table lists this size at 32.52 in tall, not 33 in.¹

Last reviewed 2026-11-01 · 2 source documents · Written in ASD-STE100 Simplified Technical English

33Nominal overall diameter, in

x12.50Nominal section width, in

RConstruction

15Rim diameter, in

LTLight truck

The parts of an inch tire size.

Read an inch size

UN R54 calls these tires high flotation tires. It lists them in a table of United States tire sizes.¹ The first number is close to the overall diameter in inches, and the second number is close to the section width in inches.¹ The letter R shows radial construction.² The last number is the rim diameter in inches.

Nominal 33 is not 33.00 inches

The table gives the design diameter and width of each size. For 33x12.50R15LT, the outer diameter is 826 mm (32.52 in) and the section width is 318 mm (12.52 in).¹ A new tire must be between 31.99 in and 33.75 in.³

US high flotation sizes ¹. Radial tires use R in place of the dash.
Size (R54)	Diameter (mm)	Diameter (in)	Width (mm)	Width (in)	Allowed diameter (in)
24x7.50-13LT	597	23.50	191	7.52	23.19–24.24
27x8.50-14LT	674	26.54	218	8.58	26.16–27.41
28x8.50-15LT	699	27.52	218	8.58	27.14–28.40
29x9.50-15LT	724	28.50	240	9.45	28.10–29.45
30x9.50-15LT	750	29.53	240	9.45	29.09–30.54
31x10.50-15LT	775	30.51	268	10.55	30.05–31.60
31x11.50-15LT	775	30.51	290	11.42	30.05–31.60
31x13.50-15LT	775	30.51	345	13.58	30.05–31.60
31x15.50-15LT	775	30.51	390	15.35	30.05–31.60
32x11.50-15LT	801	31.54	290	11.42	31.04–32.69
33x9.50R15LT	826	32.52	240	9.45	31.99–33.75
33x12.50-15LT	826	32.52	318	12.52	31.99–33.75
35x12.50-15LT	877	34.53	318	12.52	33.94–35.89
37x12.50-15LT	928	36.54	318	12.52	35.89–38.04
37x14.50-15LT	928	36.54	372	14.65	35.89–38.04
30x9.50-16.5LT	750	29.53	240	9.45	29.14–30.44
31x10.50-16.5LT	775	30.51	266	10.47	30.09–31.49
33x12.50-16.5LT	826	32.52	315	12.40	32.04–33.64
35x12.50R16.5LT	877	34.53	318	12.52	33.99–35.79
37x12.50-16.5LT	928	36.54	315	12.40	35.93–37.94
37x14.50-16.5LT	928	36.54	365	14.37	35.93–37.94
37x12.50R17LT	928	36.54	318	12.52	35.95–37.90

The allowed range uses H = 0.5 × (D − d), a lower limit of 0.97 and an upper limit of 1.07.⁴ The overall width can be up to 7 percent more than the table width.⁵

Sizes that are not in the table

Many newer inch sizes, for example 35x12.50R20LT, are not in the 2004 revision of the table that we use. For these sizes, TireSizeCalc shows the numbers in the size and marks them as numbers from the size. Read the tire manufacturer's specification for the real diameter.

Metric sizes near 33 inches

These metric sizes have a nominal diameter near 33 in.⁶ Compare each with 33x12.50R15LT at 32.52 in.

Metric sizes with a nominal diameter of 32.5 to 33.5 in.
Size	Diameter (in)	Width (in)	Rim (in)	vs 33x12.50R15LT (in)
265/60R20	32.52	10.43	20	0.00
285/65R18	32.59	11.22	18	+0.07
265/70R18	32.61	10.43	18	+0.09
305/65R17	32.61	12.01	17	+0.09
265/75R17	32.65	10.43	17	+0.13
285/70R17	32.71	11.22	17	+0.19
295/55R20	32.78	11.61	20	+0.26
305/45R22	32.81	12.01	22	+0.29
305/70R16	32.81	12.01	16	+0.29
275/50R22	32.83	10.83	22	+0.31
285/75R16	32.83	11.22	16	+0.31
275/60R20	32.99	10.83	20	+0.47
255/80R17	33.06	10.04	17	+0.54
255/85R16	33.07	10.04	16	+0.55
295/65R18	33.10	11.61	18	+0.58
275/70R18	33.16	10.83	18	+0.64
305/55R20	33.21	12.01	20	+0.69
285/50R22	33.22	11.22	22	+0.70
275/75R17	33.24	10.83	17	+0.72
295/70R17	33.26	11.61	17	+0.74
315/70R16	33.36	12.40	16	+0.84
295/75R16	33.42	11.61	16	+0.90
285/60R20	33.46	11.22	20	+0.94

Convert an inch size to metric numbers

Multiply the width by 25.4: 12.50 × 25.4 = 317.5 mm.
Subtract the rim diameter from the overall diameter, then divide by 2: (33 − 15) ÷ 2 = 9 in of sidewall.
Divide the sidewall by the width, then multiply by 100: 9 ÷ 12.50 × 100 = 72.

No metric size has a 317.5 mm width and a 72 aspect ratio. For this reason, no metric size is an exact match. Compare diameters in the calculator instead.