ERR:DIM MISMATCH means you asked the TI-84 to combine two lists or matrices that aren't the same size. The calculator pairs elements up one-by-one, and when one side runs out before the other, it stops and throws this error. The fix is always the same: find the size difference and make the two objects match.
What triggers it
Three situations cause almost every ERR:DIM MISMATCH:
- List arithmetic with unequal lengths. Typing
L1+L2(orL1*L2, etc.) when L1 has 6 entries and L2 has 5. The calculator can't add a 6th element to nothing. - A stat plot with mismatched Xlist and Ylist. Your scatter plot points at L1 for x-values and L2 for y-values, but the lists hold different numbers of entries. Every x needs a partner y.
- Matrix addition with different-size matrices. Adding a 2×3 matrix to a 3×2 matrix (or any two shapes that don't match) fails — matrix addition requires identical dimensions.
Step 1: Compare the lengths
The visual way:
- Press
STATand choose1: Edit. - Look at L1 and L2 side by side.
- Scroll to the bottom of each column and compare — one will be longer than the other.
The quick way, using the dim( command:
- Press
2ndSTATto open the LIST menu. - Arrow over to OPS and select
dim(. - Type the list name and close the parenthesis, e.g.
dim(L1), then pressENTER. - Repeat for the other list:
dim(L2).
If one says 12 and the other says 11, you've found your problem.
Step 2: Fix the mismatch
You have two options, depending on which list is wrong:
- Delete the extra entry. In
STAT1: Edit, move the cursor onto the entry that shouldn't be there and pressDEL. The list closes the gap automatically. - Add the missing entry. Scroll to the bottom of the shorter list, type the value that got skipped, and press
ENTER.
Before deleting, figure out which data point is the extra one — don't just chop off the last value. Read down both columns together: each row should be one (x, y) pair from your data. The row where the pairing goes off the rails is where the typo lives. Usually someone entered a value twice, or skipped one, and everything below it is shifted by one row.
Why this hits during linear regression
This error shows up constantly during regression homework, because that's when you're hand-typing two parallel columns of data. One duplicated y-value in L2 — or one skipped x-value in L1 — and your LinReg command or scatter plot dies with ERR:DIM MISMATCH.
The habit that prevents it: after entering your data, check dim(L1) and dim(L2) match before running the regression. It takes ten seconds and saves the frustration. For the full walkthrough of entering data and running the regression itself, see /guides/linear-regression/.
Quick reference
| Where it happened | Likely cause | Fix |
|---|---|---|
L1+L2 on home screen | Lists have different lengths | STAT 1: Edit, delete or add an entry |
Pressing GRAPH with a scatter plot on | Xlist and Ylist lengths differ | Compare lists, fix the shorter/longer one |
[A]+[B] matrix addition | Matrices are different sizes | Re-check dimensions; both must match exactly |
On the error screen itself, 2: Goto will jump you to the command that failed (choose 1: Quit if you just want out). For other errors that look similar — ERR:INVALID DIM is the empty-list cousin of this one — see the overview at /error-fixes/error-messages/.
Try the fix right now
If your calculator isn't in front of you, open the free online TI-84 calculator — a full TI-84 Plus CE that runs in your browser with nothing to download. Press STAT 1: Edit, put 3 numbers in L1 and 2 in L2, then try L1+L2 to see the error — and fix it by evening out the lists. Practicing once now means it won't rattle you on test day.