---
title: "What is Brotli? The compression upgrade over gzip, explained — warmup.rocks Glossary"
description: "Brotli compresses text content 15–25% smaller than gzip. Quality levels for static vs. dynamic content, how it interacts with Vary and CDN caching, and how to check what your site really delivers."
canonical: https://warmup.rocks/glossary/brotli
---

# Brotli

**Brotli** is a compression algorithm developed at Google and standardized in RFC 7932, negotiated over HTTP as `Content-Encoding: br`. For text-based content — HTML, CSS, JavaScript, JSON, SVG — it typically produces **15–25% smaller output than gzip**, which translates directly into faster downloads on every request. All modern browsers support it (and advertise it via `Accept-Encoding: gzip, deflate, br`).

## Compression levels: static vs. on-the-fly

Brotli has quality levels 0–11. The trade-off:

-   **Level 10–11** — maximum savings but slow to compress. Ideal for _static_ assets compressed once at build time and served pre-compressed.
-   **Level 4–5** — what servers and CDNs typically use when compressing _dynamically_ per response. Still beats gzip's default output while staying cheap enough for on-the-fly use.

Newer CDN stacks also offer **Zstandard** (`zstd`), which compresses faster at similar ratios — Brotli remains the most widely supported upgrade over gzip.

## How Brotli interacts with caching

Compressed responses and caching meet at the [Vary header](https://warmup.rocks/glossary/vary-header): a response served with `Vary: Accept-Encoding` gets a separate cache entry per encoding, so the cache can hand Brotli to browsers that accept it and gzip to clients that don't. Most CDNs handle this transparently — Cloudflare, for example, stores one representation and compresses at the edge on the way out. What matters for you: whether the _edge_ actually delivers Brotli to end users, regardless of what your origin sends.

## Check what your site really delivers

Plenty of sites serve gzip (or nothing) despite Brotli being one config line away. The [compression test](https://warmup.rocks/compression-test) shows which encoding your site delivers, what Brotli would save per URL, and whether your CDN re-compresses for you — from a real browser's `Accept-Encoding`, measured at the edge.