How to use

This page explains single-file check (validate one CSV) and compare two files (see differences between two CSVs).

Single-file check steps

1. Open the check page
   From the home page, click the single-file check card or go to the format check page.
2. Select a CSV
   Drag and drop a file onto the «Select CSV» area or click to choose. Row limits are shown in the footer under «Current plan».
3. Review the results
   Format, delimiter, encoding, header, empty rows, column mismatch, etc. are shown.
4. Choose columns (optional)
   Check the columns you want in the table, then click «Show table».
5. View and download
   Use the «Issues found» panel and «Download CSV», «To full-width» / «To half-width» as needed.

Compare two files steps

1. Open the compare page
   From the home page, click compare two files or go to compare page.
2. Select two CSVs
   Put the reference CSV in «Current data» (left) and the file to compare in «Reference data» (right).
3. View the comparison
   Added, deleted, and changed rows are colour-coded. Click the summary to list matching row numbers.
4. Columns and navigation
   Use «Choose visible columns» to show or hide columns. Click a row number to scroll to that row.

General

• All processing happens in your browser; nothing is sent to our servers.
• We recommend the latest Chrome or Edge.
• Row limits are shown in the footer. Log in or subscribe for higher limits.

Which tool should you use?

CSV Checker includes several specialised tools. Use this guide to pick the right one for your situation.

File shows garbled or broken characters

→ Encoding fix — detects the source encoding and converts to UTF-8 (BOM) so Excel opens it correctly.

Verify format, delimiter, column count before import

→ Format & basic check — see encoding, delimiter, BOM, row/column counts, empty lines at a glance.

Find duplicate IDs or invisible characters in data

→ Single-file check — detects duplicate keys, zero-width spaces, non-breaking spaces, and trailing whitespace.

See what changed between two versions of a file

→ Compare two files — colour-coded added/deleted/changed rows, WinMerge-style diff view.

File is too large or hits the row limit

→ CSV splitter — split into smaller chunks, then process each part.

Change encoding or delimiter and download

→ CSV converter — convert between encodings and delimiters in one step.

Common use cases

Before uploading to a database or system

Import failures usually come from encoding mismatches, duplicate primary keys, or hidden characters in the key column. The recommended workflow is:

1. Run Format & basic check — confirm the encoding and delimiter match what your system expects.
2. Run Single-file check — find duplicate IDs, invisible characters, and extra spaces in key columns.
3. Apply one-click fixes, download the cleaned file, then upload.

This takes about 2 minutes and catches the most common causes of import errors before they happen.

Reviewing a monthly-updated list or report

When a product catalog, member list, or price sheet is updated each month, it can be hard to know exactly what changed. Use Compare two files: load last month's file on the left and this month's on the right. Added, deleted, and changed rows are highlighted in different colours so you can spot unexpected changes at a glance — without scrolling through hundreds of rows manually.

Fixing garbled text from Excel

Excel sometimes saves CSV in a regional encoding (e.g. Windows-1252 or Shift-JIS) instead of UTF-8, which causes garbled characters when other systems open the file. Go to Encoding fix, drop in the file, and the tool automatically detects the source encoding. Preview the first few lines to confirm they look correct, then download as UTF-8 with BOM — a format that Excel reopens cleanly. The entire process happens in your browser; nothing is uploaded to a server.

Diagnosing an import that "looks fine" but still fails

Some errors only appear after import because the issue is invisible — a zero-width space in an ID, a trailing space that makes "user123" and "user123 " look identical on screen. Use Format & basic check to rule out encoding and column-count problems, then Single-file check to detect invisible characters. The CSV errors guide has a full checklist for tracing import failures step by step.

Tips for best results

• Fix encoding before anything else. Garbled text causes downstream errors in duplicate detection and compare. Always run Encoding fix first if characters look wrong.
• Pay attention to the key column. Invisible characters and trailing spaces most often hide in ID, email, or code columns. Trim those first to prevent false mismatches in compare.
• Use column selector in Compare. For wide files, hide irrelevant columns in Compare view to focus on the columns that matter and make the diff easier to read.
• Split large files first. If a file is slow or hits the row limit, use CSV Splitter to cut it into smaller chunks, then process each one.
• Your data never leaves your device. All processing happens in your browser. You can safely use CSV files that contain personal, confidential, or business-sensitive data.

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