I use a much more complex calculation for exact sizing, however, the following is accurate enough for what you require:
Halls/Kitchen/Utility/WC/Bedroom (18C) = Multiply Room Length x Width x Height x 33
Walk-in Wardrobe/Study/Office (21C) = Multiply Room Length x Width x Height x 40
Living Room/Family/Dining (21C) = Multiply Room Length x Width x Height x 40
Consevatory (21C) = Multiply Room Length x Width x Height x 60
Bathroom/En-suite (24C) = Multiply Room Length x Width x Height x 50