Be careful as tyres are often not what they seem!
For instance there are 5 sizes of 20" tyres, and a 2 inch difference in diameter.
More accurate is the ERTRO numbers. These will be on the wheel in the format xxx-yy
xxx is the diameter of the bead, is accurate and fixed for that rim. yy is the width of the tyre.
Use these and you will guarrantee a fit
For instance there are 5 sizes of 20" tyres, and a 2 inch difference in diameter.
More accurate is the ERTRO numbers. These will be on the wheel in the format xxx-yy
xxx is the diameter of the bead, is accurate and fixed for that rim. yy is the width of the tyre.
Use these and you will guarrantee a fit