Motorcycle tire size cross-reference

Reading the three notations

Metric	120/90-18 = 120 mm wide, 90% aspect ratio (height = width × 0.9), 18-inch rim. A hyphen as the separator means bias-ply; an R (as in 110/80R18) means radial.
Inch	4.60-18 = 4.6 inches wide, 18-inch rim. An older convention that survives on older tire designs, with no aspect ratio stated. 3.60, 4.10, 4.60 and 5.10 are the low-profile sizes — roughly equivalent to an 80-series.
Alpha	MT90-16 = M (motorcycle) plus a letter for width (T), 90% aspect ratio, 16-inch rim. Mostly seen on American machinery, Harley-Davidson above all.

Sizes off-road bikes actually run

Sizes on the same row are in roughly the same group — close dimensions that go on the same rim. When you are shopping and the old inch sizes and the current metric sizes are mixed together, this is how you read across. We have kept it to what off-road machines actually use.

Front, 21 inch (full-size off-road)

Metric	Inch	Notes
70/100-21	2.50-21	Narrow. Common on trials and lightweight machines
80/90-21	2.75-21
80/100-21, 90/90-21	3.00-21	The standard front group for full-size MX and trail bikes

Rear, 18 inch (trail and enduro)

Metric	Inch	Notes
90/90-18	2.75-18, 3.60-18
100/90-18, 90/100-18	3.25-18, 4.10-18	Common on 250 trail bikes
110/90-18 (120/80-18 is close too)	3.50-18, 4.60-18	The standard trail rear group
120/90-18	4.00-18, 4.25-18
130/90-18	4.50-18, 5.10-18	Rears for big adventure bikes and twins

Rear, 19 inch (motocross)

Metric	Inch	Notes
90/90-19	2.75-19 to 3.60-19
100/90-19, 90/100-19	3.25-19	Common on 250-class MX rears
110/90-19	3.50-19, 4.10-19	Common on 450-class MX rears

Rear, 17 inch (some trail bikes)

Metric	Inch	Notes
90/90-17	2.75-17, 3.00-17
110/90-17	3.50-17, 4.60-17

Small diameters, 14 and 12 inch (mini and kids)

Metric	Inch	Notes
60/100-14	2.25-14	kids' MX front slot
70/100-14	2.50-14	Cross Cub 50 and KLX110 front slot
80/100-14	2.75-14, 3.00-14	Cross Cub 50 rear slot (90/90-14 shares it)
80/100-12	3.00-12	KLX110 rear slot (90/90-12 shares it)

The 12-inch road minis (Grom, Monkey 125 and so on) are almost all metric-only in current production, so there is rarely anything to read across.

Alpha to metric

In the alpha sizes you find on American machines, the letter stands for the width, which maps onto a metric width. The width letters are an industry convention, so they read the same whoever made the tire.

Alpha	Metric equivalent
MH90	80/90
MJ90	90/90
MM90	100/90
MN90, MP85	110/90
MR90	120/90
MT90	130/90
MU85, MU90	140/90
MV85	150/80

How to read this

This cross-reference is an estimate, put together by us from several published sources.
"Corresponds to" is not "the same size." Sizes on one row go on the same rim at close dimensions, but actual width and overall diameter differ a little each way. That can show up in speedometer error and in clearances.
Conversions shift by around half a size depending on which source you read. There is no one-to-one conversion formula — that is the honest answer.
Before you change size, the manufacturer's specified size, the load index and speed rating, and clearance to the fender and elsewhere all have to be checked. The tire maker's own fitment table is the final word.
The markings after a size (51P, 4PR and the like) are load index, speed rating and ply rating — separate information from the size itself.