Why your Excel SUM is wrong, and how to fix it
A total that is wrong by a little is worse than one that is wrong by a lot: nobody notices until it matters. Here are the reasons it happens, in the order they are most likely, with the check for each.
1. The range stops short
The most common cause. =SUM(B2:B40) was right when the sheet had 40 rows; now it has 52, and the last twelve are not counted. Click the cell with the total and look at the highlighted range: if it ends before your data does, that is it.
Fix: extend the range, =SUM(B2:B52), or better, sum the whole column, =SUM(B:B), so new rows are included automatically. If the header is text it is ignored. If you have a total row at the bottom, keep it out of the column you are summing.
2. Numbers stored as text
A value pasted from a website or a PDF often arrives as text. It looks like a number, sits in the cell like a number, and SUM skips it. The tell: text is left-aligned by default, numbers are right-aligned, and Excel usually shows a small green triangle in the corner.
Fix: select the column, open the warning triangle and choose Convert to Number, or multiply the range by 1 in a helper column. In Google Sheets, =SUM(VALUE(B2:B52)) as an array formula does the same.
3. Hidden characters
A non-breaking space or a stray apostrophe before the digits turns a number into text without the green triangle. =LEN(B7) on a suspect cell shows more characters than you can see.
Fix: =VALUE(TRIM(CLEAN(B7))) strips the usual culprits. Find and replace a non-breaking space by typing Alt + 0160 in the Find box on Windows.
4. Filtered or hidden rows
SUM adds hidden rows too. If you filtered a list and expected the total of what you see, SUM is telling the truth about the whole column, not the visible part.
Fix: =SUBTOTAL(109, B2:B52) sums only the visible rows and ignores other subtotals in the range.
5. The decimal is a comma
Data exported from a system set to another locale uses 1.234,50 where your Excel expects 1,234.50. The whole column is text, and the total is zero or oddly small.
Fix: use Data › Text to Columns and set the decimal separator in the advanced options, or change the file's locale on import.
Let the AI find it
If you would rather not hunt, attach the workbook to Chatular and ask "my total in column B is wrong, can you see why". It reads the sheet, names the cause and gives you the corrected formula. The example on the home page is exactly this: a SUM that stopped at row 40 when the data ran to row 52. You can also ask it to build the sheet from scratch with formulas that use whole columns, so this never happens again.