Raised bed contents

After going around the houses for a while I am hoping somebody knows ..... I think I am missing something obvious!

I have a raised bed that measures 1mtr x 1mtr x 31cms

I am trying to calculate the volume in litres so as I can work out how many bags of top soil  that I need as they all seem to quote litres on the bags.

No maths lessons please, just an answer.

Many thanks