How values roll up to a parent

A parent item can show a figure for a field even when nobody typed one on it. The figure is calculated from the items below the parent. The field’s Rollup setting, in the field’s Hierarchy settings, decides how.

In a list, a calculated figure is shown in bold. Pointing at it shows a tooltip that explains how it was calculated.

The functions

A number field, and a formula whose result is a number, can use these:

Sum, the default: the values below added together.
Average: the mean of the values below.
Min or Max: the lowest or the highest value below.
None: nothing is calculated. A parent shows only its own value.

A date field can use Min, Max or None. Max is the default.

Tracked time always adds up

Tracked time has no Rollup setting. A parent shows its own logged time plus all the time logged below it.
A formula that only adds up tracked time works the same way and has no setting either. An example is {Tracked Time (h)} * 50.
If an edit makes such a formula stop adding up tracked time, the setting chosen before applies again.

A value typed on the parent wins

For a number or a formula, a value the parent holds itself replaces the figure from below. This is true for every function. A 0 counts as a value.

A date the parent holds itself also wins in its row. A parent with no date of its own shows the earliest or the latest date among the items below it, as Min or Max says.

How an average is counted

An average counts every value below the parent once. It never takes an average of averages.

A child that holds its own value counts as one value. A child with no value of its own passes on the values below it.

Group headers and the total row

A group header and the total row at the bottom of a list use the same function as the rows. An average there also counts every value once. Under None, both stay empty.

A view can choose its own function for a column under Total. The rows keep the field’s function. List view shows where to choose it.

For a number column, a view’s own Sum, Average, Min or Max reads the figures of the top-level rows. Each top-level row counts as one value.
When the field’s Rollup is None, each row shows only its own value. The view then reads the value of every item it shows.
When the view chooses the field’s own function, the totals do not change.
For a date column, Min or Max shows the earliest or the latest date in the view, whatever the field’s Rollup is.
Count non-empty counts every item in the view that shows a value, including a figure calculated from the items below it. A value a parent passes down to its children is not counted on them. For tracked time, an item that shows no time is not counted.
A total shows only the figure. Point at it to see which function made it, such as “Sum of all item values in this group.”

Sorting uses the field’s function too. A column sorts parents by the figure calculated with that function.

A formula that uses ROLLUP

A formula can read the figure this page describes with ROLLUP, for example {Budget} - ROLLUP({Spent}). Here Spent is a number field of your own. The Cost field cannot be used, because a formula cannot read it. Formula functions lists what ROLLUP accepts.

ROLLUP counts every item below, whatever the view shows. The view’s filter, its types and its Rollup choice of Filtered or All do not change it.
With Filtered, a list calculates a parent’s cell only from the rows that the view shows. The cell and ROLLUP can therefore show different figures for the same item.
Every screen shows the same ROLLUP figure for an item. The list, the board, the item page and a filter on the formula all agree.
ROLLUP counts the items below that your role can read. Two people with different roles can see different figures.
A formula that uses ROLLUP usually needs its own Rollup setting at None. Its figure already covers the items below, so a sum or an average of it in a group header means little.

Example

A project has two tasks. Task A has the value 10. Task B has no value of its own, and its two child tasks have 20 and 30.

Sum: Task B shows 50, and the project shows 60.
Average: Task B shows 25. The project shows 20, the mean of 10, 20 and 30. It does not show 17.5, the mean of Task A’s 10 and Task B’s 25.
Min: Task B shows 20, and the project shows 10.
None: Task B and the project show nothing.

Under Sum, a formula ROLLUP({Spent}) on the project shows 60, where Spent is the field above. A list view under Filtered that hides the child task with 30 shows 30 for the project. The formula on the project still shows 60.

Things that may surprise you

An average of a ratio is a mean of ratios. For a formula such as Spent divided by Budget, Average shows the mean of the children’s ratios. It does not divide the total of Spent by the total budget. To divide the totals, write ROLLUP({Spent}) / ROLLUP({Budget}).