Seeing a cell full of ##### signs (sometimes called "pound signs" or "hash marks") looks alarming, but it's the most harmless "error" in Excel — and technically not an error at all. It simply means the column is too narrow to display the value. Here's the quick fix and why it happens.
What ##### means
When a cell contains a number or date that's wider than the column, Excel refuses to show a partial or misleading value. Instead of cutting the number off (which could look like a different, smaller number), it shows ##### to signal "the value is fine, but I can't fit it here." Your data is completely intact — only the display is affected.
This is important: ##### is not a calculation error like #REF! or #VALUE!. The value is correct; it's purely a column-width issue.
The instant fix: widen the column
Double-click the column border. Hover over the right edge of the column header (the boundary between column letters) until the cursor becomes a double-arrow, then double-click. Excel auto-fits the column to its widest value, and the ##### disappears.
Or drag the border. Drag the column's right edge to make it wider manually.
Or use the ribbon: Home > Format > AutoFit Column Width.
That's it for the vast majority of cases. Ninety percent of ##### situations are solved by widening the column.
Other causes and fixes
A negative date or time. If a date or time calculation produces a negative result (like subtracting a later time from an earlier one), Excel can't display it and shows ##### even in a wide column. The fix is to correct the calculation so it doesn't go negative, or if you need to show negative time differences, switch the workbook to the 1904 date system (File > Options > Advanced) — though that's rarely necessary. More often, the formula logic needs fixing.
Extremely wide numbers. A cell formatted to show many decimal places or a very large number may need a genuinely wide column, or you can reduce the displayed decimals (Home > Decrease Decimal) so it fits.
How to tell it apart from real errors
Real errors start with # and a word: #REF!, #VALUE!, #N/A, #DIV/0!, #NAME?, #NUM!, #NULL!. Those indicate something is wrong with the formula or data.