A 20x1 3/8 bike tyre is a narrow 20 inch size used on some folding, compact, junior, mobility and utility wheels. The safe compatibility check is the ETRTO marking, commonly 37-451: about 37 mm wide on a 451 mm rim.
Why 37-451 matters
The phrase 20 inch is not precise enough by itself. A 20 inch tyre can use several rim diameters, including 406 and 451. If your current sidewall says 37-451, the replacement must match the 451 mm bead seat diameter. A nearby 20 inch label is not automatically interchangeable.
Pneumatic tyre or solid tyre?
For a normal ride feel and easier servicing, start with a pneumatic 37-451 tyre and a compatible tube. For utility wheels, workshop fleets or riders who want to avoid tube punctures, a solid 20x1 3/8 / 37-451 tyre can be useful, but it may feel firmer and needs careful rim-width and fitting checks.
Check rim width and frame clearance
Before ordering, compare the full sidewall marking, measure available clearance around the fork, frame or mudguards, and check any product-specific rim-width guidance. Solid tyres in particular can have stricter internal-rim-width requirements than standard pneumatic tyres.
Choosing the right tube
If you keep an air-filled tyre, choose a tube whose printed range includes 37-451 or 20x1 3/8, then match the valve type to your rim. Schrader and Presta tubes both exist in this size range, so check the old valve before buying.
Shop 20x1 3/8 tyres from 1Bike
1Bike lists 20 inch 37-451 tyres, solid-tyre options and compatible tubes with product-page ETRTO details. Use the exact sidewall number first, then compare tyre type, rim-width notes and valve compatibility.
