The calculation itself is pretty straightforward:
Remaining Time = Original Estimate – Time Spent
So in your example:
15 (original estimate) – 6 (time spent) = 9 hours remaining.
If you’re doing this often, a simple spreadsheet (Excel/Google Sheets) works well — just put the two values in cells and use a formula like =A1-B1.
There are also a few online calculators and project management tools that do this automatically if you want something quick without opening a sheet. Tools like Jira, Trello (with plugins), or even basic time-tracking apps will show you remaining time as you log work.
