Prices are based on buying materials needed in bag format.
The length and width measurements should be the actual area you are laying pavers on. Material for the base will be calculated allowing for over excavation based on the depth of your gravel base.
If you require a 4" depth of gravel base the calculator will add material for 8" allowing for an over excavation 4" on each side.
Bags of sand appear to be sold by weight. A bag of 50 lbs of sand has a volume of 0.5 cubic foot. If you have a different weight bag divide the lbs by 100 to get the approximate cubic foot volume of the bag needed for the calculator entry.
Sand does not get used until you are ready to lay your pavers.
The Geotextile fabric is optional but well worth the investment.
The cost does not include rental of a compactor and if required, the cost of an excavator.