Gzip: what it is and how to enable compression

What is Gzip

Gzip is a data compression algorithm widely used to compress HTTP responses. When a server sends HTML, CSS, JS, or JSON files, it can compress them with gzip before sending them to the browser. The browser automatically decompresses the data.

Gzip significantly reduces the amount of data transferred and speeds up page loading.

How effective is Gzip

The compression ratio depends on the file type:

Images (JPEG, PNG, WebP) already use their own compression, so additional gzip compression has no effect.

How the browser requests compression

Запрос браузера:
GET /page HTTP/1.1
Accept-Encoding: gzip, deflate, br

Ответ сервера:
HTTP/1.1 200 OK
Content-Encoding: gzip
Content-Type: text/html

br is Brotli, a more efficient compression algorithm (see below).

How to enable Gzip

Nginx

http {
    gzip on;
    gzip_comp_level 6;
    gzip_min_length 1000;
    gzip_types
        text/plain
        text/css
        text/javascript
        application/javascript
        application/json
        application/xml
        image/svg+xml;
    gzip_proxied any;
    gzip_vary on;
}

Apache (.htaccess)

<IfModule mod_deflate.c>
    AddOutputFilterByType DEFLATE text/html text/plain text/css
    AddOutputFilterByType DEFLATE application/javascript application/json
    AddOutputFilterByType DEFLATE image/svg+xml
</IfModule>

Node.js / Express

const compression = require('compression');
app.use(compression({
  level: 6,
  threshold: 1024 // не сжимать файлы < 1KB
}));

Next.js

Next.js enables gzip compression by default. For additional configuration:

// next.config.js
module.exports = {
  compress: true, // по умолчанию true
}

Gzip vs Brotli

Check compression on reChecker

Use HTTP/2 & Brotli to check whether gzip and Brotli are enabled on your website. The tool checks:

• Whether the Content-Encoding: gzip or br header is present
• Actual compression for common file types
• The HTTP/2 protocol and other performance parameters

FAQ

Does Gzip compress images? Technically, yes, but the effect is minimal—JPEG, PNG, and WebP already use their own compression. Apply gzip only to text formats: HTML, CSS, JS, JSON, XML, and SVG.

Should files smaller than 1 KB be compressed? No. The overhead from compression headers and processing time does not justify compressing small files. The minimum threshold is 1-2 KB.

Does the compression level affect server speed? Yes. Level 1 is fast but compresses less effectively. Level 9 provides maximum compression but puts more load on the CPU. Level 5-6 offers the best balance for most websites.