QR codes are all around us: café menus, checkout payments, links in ads. But what if you need to read a code without a smartphone, or check the contents of an unfamiliar code before following it?
What is a QR code and how does it work
QR (Quick Response) is a two-dimensional matrix barcode developed by Denso Wave in 1994. Unlike linear barcodes, it stores data in two dimensions.
A QR code's capacity depends on the data type:
| Data type | Maximum |
|---|---|
| Digits | 7 089 characters |
| Alphanumeric | 4 296 characters |
| Binary data | 2 953 bytes |
The three squares in the corners are positioning markers that let the scanner determine the code's orientation.
Data formats in QR codes
URL
The most common type. It contains a link such as https://example.com. When scanned, a smartphone automatically opens the browser.
Text
An arbitrary string without special formatting: promo codes, identifiers, short messages.
vCard (contact)
BEGIN:VCARD
VERSION:3.0
FN:Иван Петров
TEL:+79161234567
EMAIL:ivan@example.com
END:VCARDThe phone offers to save the contact as soon as the code is scanned.
Wi-Fi
WIFI:T:WPA;S:НазваниеСети;P:Пароль123;;Connect to a network without entering the password manually — convenient for guests.
Geolocation
geo:55.7558,37.6176The coordinates open in a maps app. This is used in real estate listings and tourist routes.
How online decoding works
Browser-based decoders (for example, those based on the jsQR library) process the image on the client side. The file is not sent to a server — this is safe for confidential codes.
Algorithm:
- The image is converted into an array of pixels
- jsQR finds the positioning markers and aligns the code
- The modules (black and white cells) are read and the mask is applied
- The data is decoded with error correction
Tips for better scanning results
If the decoder does not recognize the code, check the following:
- Clarity — the image should not be blurry
- Contrast — use a dark code on a light background; do not invert the colors
- Entire code in frame — the whole code must be visible, including the white “quiet zone” around the edges
- Resolution — at least 100×100 pixels for the code
- Format — PNG, JPG, and WEBP are supported
QR codes have built-in error correction (levels L/M/Q/H). A code can be recovered even if up to 30% of its surface is damaged — which is why logos can be placed over codes.
Security: why decode before following a link
QR codes have become a phishing tool: attackers stick fake codes over the original ones. An online decoder lets you see the URL before following it.
Be cautious if:
- The code contains a URL shortener (bit.ly, t.co) in an unexpected place
- The domain does not match the expected website
- The code physically looks like a sticker placed over the original
To create your own QR codes, use the QR code generator.
Tool on reChecker
The QR code decoder on reChecker works directly in your browser using jsQR — the file never leaves your computer. Upload an image or paste a URL to get the code's contents instantly.