Excel changed my SKU numbers and now they don't match in Shopify
You didn't edit the SKU column. You just opened the file. That's enough — Excel silently converts any all-digit column to a Number the moment the CSV opens, and depending on the SKU's length that can drop leading zeros or permanently round off digits you'll never get back. Drop your file into the free checker below to see whether your SKU column has already been altered before you upload it again.
What you'll see
You update an inventory or product CSV in Excel — maybe you just fixed a price in another column — save it, and re-upload. The upload "succeeds," no error appears, but rows that should have updated existing variants either get skipped or show up as brand-new variants with no stock history. Nothing in the SKU column looks obviously wrong to the eye; a SKU like 7421 instead of 007421 doesn't jump out unless you're comparing it directly against what Shopify has stored.
The root cause: Excel guesses your column type, silently
The moment you open a CSV in Excel — or paste digit-only data into a cell that hasn't been pre-formatted — Excel inspects the column and, if every value looks like a number, converts it to a Number type automatically. There's no dialog, no warning, no undo prompt. This has two distinct consequences for SKUs and barcodes:
Leading zeros disappear. A SKU stored and typed as "007421" becomes the number 7421, because leading zeros aren't meaningful in a numeric value. Visually in the cell it may even still show the full string depending on cached formatting, but the underlying value Excel writes back out on save has already lost them.
Long numeric SKUs lose precision permanently. Excel numbers are stored as floating-point values with roughly 15 significant digits of precision. A 16- or 17-digit SKU or barcode — not unusual for GS1/UPC-derived identifiers — can have its trailing digits silently zeroed out the instant the file is opened, before you've made a single edit. This isn't a display quirk; the actual stored value is altered, and saving the file writes the altered value back to disk.
Why this breaks the Shopify upload with no error
Shopify's inventory and product import matches rows to existing variants by comparing the SKU column in your upload against the SKU stored on the variant record. If Excel changed "007421" to 7421 (or rounded a 17-digit value), the string in your file no longer equals the string Shopify has on file. That row simply fails to match anything — Shopify either silently skips the update or, in some import types, creates a new variant rather than updating the existing one. There's no error because, from Shopify's side, nothing is malformed; it's just a SKU it's never seen before.
How to fix it
Never open a SKU-bearing CSV by double-clicking it, which lets Excel auto-detect types on the fly. Instead: pre-format the destination column as Text before pasting values in, or use Data → From Text/CSV and explicitly set the SKU column's type to Text during the import wizard, or edit the file in a plain text editor or in Google Sheets with the column pre-set to Plain text. The free checker above will flag SKU values in your spreadsheet that look like they've already lost leading zeros or precision, so you can catch it before the next upload.