OMR Scanner Online — Bubble Sheet Grader & CSV Export

Step 1 — Upload Bubble Sheet Photo

Remove & upload different image

Step 2 — Configure Sheet Layout

Step 3 — Scan & Grade

The scanner will detect the bubble grid using OpenCV.js, read filled marks, and score against your key — entirely in your browser.

How it works

• 1Image decode: Your photo is drawn onto an HTML Canvas element — no pixels are sent anywhere. The browser reads the raw pixel data locally.
• 2Preprocessing (OpenCV.js): The image is converted to grayscale, Gaussian-blurred, and Otsu-thresholded to produce a clean black-and-white binary image, separating filled bubbles from paper background.
• 3Contour detection: OpenCV finds the largest rectangular contour in the image. A four-point perspective transform ("deskew") corrects for camera angle, producing a front-on, flat view of the sheet.
• 4Grid subdivision: The deskewed sheet region is divided into a uniform grid of (questions × choices) cells based on your configuration.
• 5Bubble reading: For each cell, the tool counts dark pixels as a proportion of the cell area. If that proportion exceeds the fill threshold (default 40%), the bubble is marked filled. The choice with the highest fill ratio per row wins.
• 6Grading & CSV: Detected answers are compared to the optional answer key to produce a score. All data is formatted as CSV and can be downloaded in one click.

Tips for best results: Use a flat, well-lit photo taken straight-on (not at an angle). The sheet should take up most of the frame. Standard Scantron 882-E, NCS, and generic 4/5-choice sheets work well.

Frequently Asked Questions

Is my test data private? Does the image get uploaded anywhere?

Completely private. The entire OMR scanning process runs in your web browser using WebAssembly (OpenCV.js). Your image never leaves your device — no server receives it. This makes it safe for grading real student answer sheets without any FERPA or privacy concerns.

What types of bubble sheets does this support?

Any standard grid-style bubble sheet where answers are arranged in rows (one row per question) with bubbles for each choice across the row. This includes Scantron 882-E, NCS/Pearson form F-288, generic teacher-made sheets, and custom sheets with 2–8 choices. Sheets with staggered, diagonal, or circular layouts are not supported by the automatic grid detector — use manual grid mode instead.

What if the scanner marks the wrong answer for a question?

Accuracy depends heavily on photo quality and sheet design. For best results: (1) photograph the sheet flat on a desk, not hand-held; (2) use even lighting without harsh shadows or glare; (3) make sure the sheet fills most of the camera frame; (4) if one side is darker, increase the threshold setting. You can also run the scan again on a better photo — multiple scans are free and instant.

Can I grade multiple sheets at once?

Currently one sheet per scan. Upload the next student's sheet after downloading the CSV for the first. For batch grading of a whole class, scan each sheet and append the resulting rows — the CSV download includes a Student column so you can concatenate results in a spreadsheet.

How does the fill threshold work?

After binarising the image to black-and-white, the tool counts what percentage of pixels inside each bubble cell are dark. The default threshold is 40% — a cell darker than 40% is read as "filled". If lightly penciled bubbles are missed, lower the threshold to 25–30%. If smudges or printed borders cause false positives, raise it to 50–55%.

What does the CSV contain?

One row per question. Columns: Question number, Detected answer (A/B/C…), Answer key (if provided), Result (Correct/Wrong/Blank), and Fill% for each choice (so you can audit the confidence of each read). This is ready to open in Excel, Google Sheets, or any gradebook software.