Solved: Creating a Notion formula to calculate progress percentage based on completed checkbox tasks within a related database

📌 The Problem Explained

A common challenge in Notion project management is visualizing progress. You might have a Projects database and a separate Tasks database linked via a Relation. While you can see the linked tasks, Notion doesn’t automatically show you what percentage of those tasks are finished.

Without a formula, you are forced to manually count checkboxes or use a “Rollup,” which is limited in how it can be styled or integrated into more complex calculations. To create a dynamic progress bar that updates in real-time as team members check off tasks, you need a formula that counts the items in the relation, filters for completion, and performs the division.

💡 The Quick Solution

Paste the following formula into your Formula property (assuming your relation is named “Tasks” and your checkbox is named “Done”):

let(
  totalTasks,
  prop("Tasks").length(),
  if(
    totalTasks > 0,
    prop("Tasks").filter(current.prop("Done")).length() / totalTasks,
    0
  )
)

Note: After pasting, click the formula icon (or the property name) > Edit Property > Number format and select Percent. You can then choose the Bar or Ring display.

🛠️ Step-by-Step Breakdown

Step	UI Path / Action	Description
1	Add Property > Relation	Connect your Project database to your Task database. Name this property Tasks.
2	Tasks Database > Checkbox	Ensure your Task database has a checkbox property. Name it Done.
3	Project Database > Formula	Create a new property in the Project database and select Formula.
4	Edit Formula	Use the let function to define totalTasks for cleaner code and to avoid “division by zero” errors.
5	Filter Logic	Use .filter(current.prop("Done")) to isolate only the rows where the checkbox is checked.
6	Display Setting	Set the Number format to Percent and select Bar to visualize the progress.

Argument Explanation

Function/Property	Purpose
prop("Tasks")	References the list of pages linked in the Relation property.
.length()	Counts the number of items in the list (total tasks).
.filter()	Iterates through the related tasks to find specific matches.
current	Refers to the specific task being evaluated during the filter process.
if(total > 0, ... , 0)	A safety check to ensure the formula doesn’t break if a project has zero tasks assigned.

⚠️ Common Mistakes to Avoid

• Property Name Mismatch: Notion formulas are case-sensitive. If your checkbox is named “Complete” but your formula says prop("Done"), it will return an error. Ensure the names in the code match your property titles exactly.
• Forgetting the “Percent” Format: By default, Notion formulas output a decimal (e.g., 0.5 instead of 50%). To see a progress bar, you must manually change the property’s format to Percent in the property configuration menu.
• Circular Dependencies: Ensure you aren’t trying to relate a database to itself in a way that creates an infinite loop, though this is rare for simple project/task setups.