2 Answers

Depends on the size of the gallons.

HI      THE FORMULEA   FOR VOLUME IN A CUBE SHAPE IS  LENGTH X BREADTH X HEIGHT

SO AS AN EXAMPLE IF YOU MEASURE THE DIMENSIONS   AND YOU HAVE   1.5MTRS X 2.2MTRS X 0.6MTRS THE SUM WOULD BE

1.5 X 2.2 X 0.6  = 1.98 CU MTRS 

THERE ARE 219.9692 GALLONS UK   IN 1 CU MTR

THEREFORE  1.98 X 219.9692 = 435.539 GALLONS UK

  THERE ARE  264.172 GALLONS US   IN 1 CU MTR

THEREFORE   1.98 X 264.172 = 523.06 GALLONS US

IF YOU ARE MEASURING IMPERIAL FT AND INCHES

EXAMPLE          3'-4'' X 2'-6'' X 2'-0''  CONVERT TO DECIMALS  3.33 X 2.5 X 2 = 16.65 CUFT

THERE ARE   6.229 GALLONS UK  PER CU FT

THEREFORE      16.65 X 6.229 = 103.71   GALLONS UK

THERE ARE  7.481 GALLONS  US  PER CU FT

THEREFORE    16.65 X 7.481 = 124.56 GALLONS US

I HOPE THESE EXAMPLES WILL HELP YOU IF NOT ASK ANOTHER QUESTION