An array formula is a formula used in spreadsheets for performing multiple calculations on a series of items in an array rather than on a single data value. The results returned from an array formula can be of two types: a single result or multiple results.

An array formula can be used to calculate a column or row of subtotals by placing it in a row of cells or it can be used to calculate a single value in a single cell. For example, an array formula =ROW(A1:A5) returns a series of numbers, which are the row numbers in the first cell from the range A1:A5; in other words, it returns {1, 2, 3, 4, 5}.

In an Excel spreadsheet, an array formula is enclosed between braces {}. A container function such as SUM or COUNT is always used while working with array formulas so as to aggregate a series of data to a single number result. The formula =ROW(A1:A5) in normal mode returns a single number 1, and the container function SUM just performs the sum of that single number. If the formula is used as an array formula then =ROW(A1:A5) returns an array of row numbers and SUM function adds up the elements of the array, returning a result of 15(=1+ 2+3+4+5).

Array formulas have the following advantages:

- Consistency: The array formula offers consistency, which helps to maintain data accuracy.
- Data Safety: The multicell array formula cannot be overwritten. This property helps to reduce errors due to manipulation.
- Single Array Formula: Single array formulas can be used rather than several intermediate formulas.

The problem with array formulas is that large formulas can slow down calculations. In addition, other users of the spreadsheet may not be able to understand the array formulas.