Hello Rohnak! Welcome to the community. 
The best way to calculate sums of columns in Grist is with summary tables.
- Help Center Article: Summary tables - Grist Help Center
- Video Tutorial: Summary Tables Guide - Grist Help Center
Tl;dr:
Click on Add New > Select either New Page or Add Widget to Page > Select the table you wish to summarize > Click the sigma icon (ÎŁ) to summarize the table > (Optional) Select one or more columns by which to group data.
In the summary table, all numeric columns are automatically summed. You’ll also receive a “count” column by default which counts the total number of records in each group.
Sum of values in different tables
If you’re looking for a way to sum values in two different tables into a third table, this thread may be helpful: How do I Calculate Sums from multiple tables to a widget?