Frequently asked questions

15 questions about Excel Diff · Two-Sheet Compare.

Q1. How big a file can I compare?

Depends on your browser memory. On a 16 GB laptop we comfortably handle ~200k rows × 30 columns. A warning banner appears past 100k rows. Over 500k rows we recommend splitting the workbook into 2–3 passes.

Q2. Are files really not uploaded?

Yes. The site is static S3 hosting — there is no upload endpoint to send anything to. Open DevTools → Network, run a compare, and the upload tab stays at zero bytes. After the page first loads you can go offline and it still works.

Q3. How are merged cells handled?

The top-left cell of a merged range is treated as the value for the whole range — the same simplification Excel uses when reading merged cells. So a merged header or summary row is compared by that single top-left value.

Q4. Can you diff Excel formulas?

By default formulas are compared by their computed value. If you need to diff the raw formula text (=SUM(A1:A10) vs =AVERAGE(A1:A10)), convert to values first (Paste Special → Values) and re-upload.

Q5. Does CSV work?

Yes — we auto-detect commas, tabs, semicolons and pipes. UTF-8, UTF-8 with BOM, and CP949 (Korean) encodings are attempted. If Korean characters look garbled, re-save as UTF-8 in Excel first.

Q6. Can I use multiple key columns?

The MVP supports a single key column. For composite keys, add a helper column like =A2&"-"&B2 in Excel and pick that. Native composite-key support is on the roadmap.

Q7. How do I compare multiple sheets?

Pick the sheet on each side separately — the names can differ (e.g. A "2024" vs B "2025"). A workbook-wide diff across all sheets at once is on the roadmap.

Q8. Does row order matter?

No — matching is by key-column value, so rows can be in any order on each side. Duplicate keys fall back to positional pairing within that key group.

Q9. Are numbers and dates matched across formats?

Numbers compare as their underlying double, so 1,000 / 1000 / 1.0e3 are equal. Dates stored as Excel serials compare across different display formats. Dates stored as plain strings (e.g. "2025-03-01") fall back to string comparison.

Q10. What if columns were added or removed?

We union the headers from both files. A-only columns show empty on the B side (and vice versa) and surface as modifications. Identical headers that differ only in whitespace, case, or fullwidth/halfwidth forms are auto-normalised.

Q11. Can I export only the changes?

Yes — the XLSX export has 4 tabs (Summary · Modified · Added · Removed). The Modified tab is a Key / Column / Before / After table that review meetings can use directly. CSV export bundles all three sections into one file.

Q12. Can the exported XLSX highlight changed cells with color?

On-screen the table uses yellow highlights. The current XLSX Modified tab is a plain Key / Column / Before / After table. A full-sheet export with cell-level yellow/green/red fills is planned.

Q13. Does it work on mobile?

Works on iOS Safari 16+ and Android Chrome. Mobile memory/screen constraints make it best suited for small sheets (up to a few thousand rows). Use a desktop for bigger files.

Q14. My company blocks file uploads. Is this tool safe to use?

The site is pure static hosting (S3 + CloudFront) — there is no backend to receive files. Parsing happens in the SheetJS runtime inside the browser. Some enterprises restrict even third-party JS, so check with your security team. An on-prem build is available on request.

Q15. Do you log or store anything?

We use GA4 and Naver Analytics for page-view counts only. No file contents or cell values ever leave your browser — nothing is transmitted to begin with. Only your language preference is stored in localStorage.