Futon mattress is a phrase for a sitting and sleeping surface that folds. Another fundamental point is that the filling for the mattress is layered..
Very versatile, these mattresses are available in many sizes and thicknesses. Although the original concept is Japanese, the way we live in our homes in the Western world is very different.
Width sizes range from 2'6"/76 cm to 5'0"/150 cm and length 6'3"/190 cm to 6'6"/200 cm. There are 2 single sizes, metric measurement 76 cm and 90 cm. The Double middle sizes are called small double 120 cm and the most popular size is double 140 cm. The larger king size is another that is both imperial and metric. As a result it adds to the confusion. 5'0" x 6'6" or 150 x 200 cm is the imperial. The metric size is slightly bigger, hence 5'3" x 6'6" or 160 x 200 cm.
Also, there are lots of filling choices. Many people bought a futon mattress a long time ago when the filling option was narrower, cotton or cotton. These were not very good quality, heavy and much derided. Slowly more attention and a growing realisation that "futons" were versatile, useful and good value.
Cotton tends to compact and lose usefulness quickly. The filling of choice in the UK became wool felt. Reclaimed wool put through a centrifuge to clean then carded and combed. Specialist companies discovered Lamb's Wool was the best layer for the top and bottom of a futon mattress.
In between Wool felt and Lamb's Wool was a Coir core for a firm mattress. Latex for those with allergies and Pocket spring cores for the softer feel older folk wanted.
The point of choice is that you will find something to suit your needs.
The best mattress covers are 100% cotton drill. Whether its Natural or is dyed they qualities of cotton drill are durability and longevity.