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
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310 litres- 100x100x31
Thank you !!!