What is the difference between an Australian and American fat quarter?

An Australian fat quarter will generally measure 50cm x 55cm (19.7” x 21.7”). This is because fabrics in Australia are usually cut to metric measurements (ie by the meter instead of by the yard).

An American fat quarter will generally measure 18” x 21” (45.7cm x 53.3cm).

A fat quarter is usually cut as a half metre/half yard off a bolt of fabric first. This strip of fabric is then cut in half width-wise, creating the measurements above.