Subtitle Comparison Tool — WER, Timing Drift & Line Diff
Upload two SRT or VTT files and DiffALL calculates Word Error Rate, measures timestamp drift, and shows a colour-coded line-by-line diff of every change.
Free, no sign-up — your files never leave the comparison.
Drop file here or click to browse
SRT · VTT
Drop file here or click to browse
SRT · VTT
Comparing two different recordings of people talking to see if it's the same person? Use the Voice Comparison tool →
Free to use — no sign-up needed
Processing...
API Keys
Use the comparison API from scripts and CI. Each call costs one credit.
Copy this key now — for security it won't be shown again.
No keys yet — create one to start using the API.
What DiffALL measures
Word Error Rate (WER)
Industry-standard metric counting substitutions, deletions, and insertions at the word level — the same metric used to evaluate speech recognition systems.
Timing Drift
Measures the millisecond offset between corresponding subtitle entries. Average, maximum, and standard deviation help you spot sync drift accumulating over time.
Line-by-Line Diff
Colour-coded diff showing every changed, added, and removed cue. Equal lines are shown in context; differences are highlighted inline.
Drift Chart
Scatter plot of timing drift over cue index — instantly spot sync errors that grow toward the end of the file.
Summary Counts
Total lines, matched, changed, removed, and added — all in a single metrics row at the top of the report.
Unicode & BOM Support
Handles UTF-8, UTF-8 BOM, and common subtitle encodings. HTML tags and ASS override codes are stripped before comparison.
Supported formats
Use cases
Localisation QC
Verify that translated subtitles have the correct timing and that no lines were accidentally dropped.
Transcription Accuracy
Compare auto-generated captions to a human-corrected transcript to measure WER and find errors.
Subtitle Sync Check
Detect timing drift between a subtitle file and a re-timed version after video editing.
Version Control
Track changes between two subtitle revisions — see exactly which lines were reworded or re-timed.
Frequently asked questions
How do I compare two SRT subtitle files online?
Upload your two SRT or VTT files to DiffALL. It parses timestamps and text, calculates WER, measures timing drift, and shows a colour-coded line-by-line diff.
What is Word Error Rate (WER)?
WER counts how many words were substituted, deleted, or inserted relative to the reference file. 0% means perfect match; 100% means completely different. Below 5% is excellent for professional subtitles.
What does timing drift mean?
Timing drift is the millisecond difference between the start timestamp of matching subtitle entries. A drift of 0 ms is perfect sync. Drifts above ±500 ms are usually noticeable to viewers.
Does DiffALL support VTT files?
Yes. Both SRT and WebVTT (.vtt) are supported. The WEBVTT header and NOTE/STYLE blocks are stripped before parsing.
Ready to compare subtitles?
Free, no install, no sign-up required for your first comparison.
Start comparing now ↑