There is more to estimating the capacity of a loader bucket than simply multiplying the bucket's cross-section by its width, since the material you are loading can lie in a heap that rises above the bucket. The Society of Automotive Engineers has defined two different angles of repose for the purpose of comparatively rating buckets. If you do not know the actual angle of repose you will be using, estimate using 2:1 for wheel loader buckets and 1:1 for hydraulic excavator buckets.
Trace the outline of the bucket's interior cross-section on a large piece of cardboard.
Draw a straight base line to connect the open ends of the cross section. At the base line's midpoint, draw a perpendicular line to construct the heap area. For a 2:1 heap, make the perpendicular line a quarter of the length of the base line. For a 1:1 heap, make it a half of the base line's length.
Draw straight lines connecting the ends of the base line with the end of the perpendicular line to indicate how much additional area the cross section of the heap will have.
Draw a grid over your representation of the cross-section. For ease of estimation, make the grid squares 1 foot square for a larger bucket or, if extra precision is needed in a smaller bucket, 6 inches to a side such that each square represents a quarter of a square foot.
Count the number of squares that the cross section takes up to estimate the heap's cross sectional area. Combine parts of squares and count them as one square. Multiply the area you find with the width of the bucket to find the total capacity.