the sum of part of a group of numbers
"The subtotal on the invoice listed the cost of just the items she had selected before adding tax and shipping fees."
The total for a part of a list of numbers being summed.
"The subtotal shows the cost before tax is added to the final sum."
In plain English: A subtotal is an intermediate sum calculated before adding everything together to get the final total.
"The subtotal on my grocery receipt was lower than I expected before sales tax added more to the total."
Usage: Use the noun form to refer specifically to an intermediate sum calculated before adding remaining items in a longer list, such as line totals on a receipt that precede the grand final amount. Avoid using it for the ultimate aggregate of all figures unless you are explicitly distinguishing this stage from subsequent additions.
To calculate a subtotal.
"The cashier asked me to subtotal my purchases before applying any discounts."
Less than total; partial.
"The subtotal on the receipt showed less than the final amount after taxes were added."
The word subtotal comes from combining the prefix sub-, meaning under or below, with total to describe a sum that is part of a larger whole. It entered English as a straightforward formation to denote an intermediate calculation in accounting before the final grand total.