diff --git a/README.md b/README.md
index 40609c1..51e7ca1 100644
--- a/README.md
+++ b/README.md
@@ -6,7 +6,9 @@ one binary. It was called `narl-diary` until it grew everything but the diary.
 
 - **Rust + axum** backend, **SQLite** storage, **Svelte 5** frontend.
 - Spaces (`diary`, `work`, `uni`, …) with folders nested to any depth inside them.
-- Markdown documents with full-text search across a whole space (SQLite FTS5).
+- Markdown documents with full-text search across a whole space (SQLite FTS5),
+  and math in them set by KaTeX: `$x^2$` inline, `$$…$$` or a ```` ```math ````
+  block on its own.
 - Import a folder — or a zip of one — as a tree of documents, links and all.
 - A kanban board per space, so what is open at work is not mixed up with uni.
 - Drag, drop or paste images, video and audio straight into an entry.
diff --git a/web/bun.lock b/web/bun.lock
index 7eec19b..9990ec8 100644
--- a/web/bun.lock
+++ b/web/bun.lock
@@ -15,6 +15,7 @@
         "@lezer/highlight": "^1.2.1",
         "@replit/codemirror-vim": "^6.2.1",
         "dompurify": "^3.2.4",
+        "katex": "^0.19.0",
         "marked": "^15.0.6",
       },
       "devDependencies": {
@@ -212,6 +213,8 @@
 
     "clsx": ["clsx@2.1.1", "", {}, "sha512-eYm0QWBtUrBWZWG0d386OGAw16Z995PiOVo2B7bjWSbHedGl5e0ZWaq65kOGgUSNesEIDkB9ISbTg/JK9dhCZA=="],
 
+    "commander": ["commander@15.0.0", "", {}, "sha512-z67u4ZhzCL/Tydu1lJARtEZYWbWaN7oYLHbsuzocr6y4N6WZAagG3RQ4FW61V1/0+jImpj293XfrcYnd1qxtPg=="],
+
     "crelt": ["crelt@1.0.7", "", {}, "sha512-aK6BbWfhf4U/wCcLHKPJl/xa6VkVstRaPywWtMKGwuOLc/wZTyQYuoxgvZnNsBvv7Kg3YTBQYYBCggcviQczuA=="],
 
     "debug": ["debug@4.4.3", "", { "dependencies": { "ms": "^2.1.3" } }, "sha512-RGwwWnwQvkVfavKVt22FGLw+xYSdzARwm0ru6DhTVA3umU5hZc28V3kO4stgYryrTlLpuvgI9GiijltAjNbcqA=="],
@@ -234,6 +237,8 @@
 
     "is-reference": ["is-reference@3.0.3", "", { "dependencies": { "@types/estree": "^1.0.6" } }, "sha512-ixkJoqQvAP88E6wLydLGGqCJsrFUnqoH6HnaczB8XmDH1oaWU+xxdptvikTgaEhtZ53Ky6YXiBuUI2WXLMCwjw=="],
 
+    "katex": ["katex@0.19.0", "", { "dependencies": { "commander": "^15.0.0" }, "bin": { "katex": "cli.js" } }, "sha512-v6Tznz3zJ7u3niRCoDTsumM2+HA2XXcCu+WAacCeHD2z3p9A9Ks987o5FzfTGBN0e8A0vjEgIDvLbICrXpdw/Q=="],
+
     "kleur": ["kleur@4.1.5", "", {}, "sha512-o+NO+8WrRiQEE4/7nwRJhN1HWpVmJm511pBHUxPLtp0BUISzlBplORYSmTclCnJvQq2tKu/sgl3xVpkc7ZWuQQ=="],
 
     "locate-character": ["locate-character@3.0.0", "", {}, "sha512-SW13ws7BjaeJ6p7Q6CO2nchbYEc3X3J6WrmTTDto7yMPqVSZTUyY5Tjbid+Ab8gLnATtygYtiDIJGQRRn2ZOiA=="],
diff --git a/web/package.json b/web/package.json
index 832e774..d35fb19 100644
--- a/web/package.json
+++ b/web/package.json
@@ -27,6 +27,7 @@
     "@lezer/highlight": "^1.2.1",
     "@replit/codemirror-vim": "^6.2.1",
     "dompurify": "^3.2.4",
+    "katex": "^0.19.0",
     "marked": "^15.0.6"
   }
 }
diff --git a/web/src/app.css b/web/src/app.css
index 845f77c..a30f6ac 100644
--- a/web/src/app.css
+++ b/web/src/app.css
@@ -517,6 +517,8 @@ input::placeholder { color: var(--muted); }
   margin: 0 0 1em;
 }
 .md pre code { background: none; border: 0; padding: 0; font-size: inherit; }
+/* A long equation scrolls on its own rather than widening the page. */
+.md .katex-display { overflow-x: auto; overflow-y: hidden; padding: 2px 0; }
 .md hr { border: 0; border-top: 1px solid var(--overlay); margin: 2.4em 0; }
 .md img, .md video {
   max-width: 100%;
diff --git a/web/src/lib/Help.svelte b/web/src/lib/Help.svelte
index 64d3e99..f315941 100644
--- a/web/src/lib/Help.svelte
+++ b/web/src/lib/Help.svelte
@@ -141,7 +141,7 @@
   <footer>
     inside the editor the full vim keymap is live — hjkl, w/b, dd, ciw, visual mode, macros, and
     <span class="key">:w</span> to write. drop or paste an image, video or audio file into the
-    editor to attach it. a file opens in place — pdf, text and code, markdown, csv, and
+    editor to attach it. <span class="key">$x^2$</span> and <span class="key">$$…$$</span> are set as math. a file opens in place — pdf, text and code, markdown, csv, and
     certificates and keystores, which ask for their password. <span class="key">:12</span> jumps straight to entry 12. yanks and
     deletes are copied to the system clipboard —
     <span class="key">:set noclipboard</span> keeps them in vim's registers.
diff --git a/web/src/lib/markdown.ts b/web/src/lib/markdown.ts
index 2fd0490..921b9e2 100644
--- a/web/src/lib/markdown.ts
+++ b/web/src/lib/markdown.ts
@@ -1,8 +1,66 @@
 import DOMPurify from 'dompurify'
-import { marked } from 'marked'
+import { marked, type TokenizerAndRendererExtension } from 'marked'
+import { mathEngine } from './math.svelte'
 
 marked.setOptions({ gfm: true, breaks: true })
 
+/*
+ * Math the way GitHub writes it: `$x^2$` inline, `$$…$$` on lines of its own
+ * or inline for display, and a ```math block. A `$` opening on a space or
+ * closing before a digit is a price, not math — "$5 and $10" stays text — and
+ * `\$` is a dollar sign as ever.
+ */
+const DISPLAY = /^\$\$((?:\\.|[^\\$])+?)\$\$/
+const INLINE = /^\$(?!\s)((?:\\.|[^\\$\n])+?)(?<!\s)\$(?!\d)/
+
+const mathBlock: TokenizerAndRendererExtension = {
+  name: 'mathBlock',
+  level: 'block',
+  start: (src) => src.match(/^\$\$/m)?.index,
+  tokenizer(src) {
+    const match = /^\$\$((?:\\.|[^\\$])+?)\$\$[^\S\n]*(?:\n+|$)/.exec(src)
+    if (match) return { type: 'mathBlock', raw: match[0], text: match[1].trim() }
+  },
+  renderer: (token) => tex(token.text, true),
+}
+
+const mathInline: TokenizerAndRendererExtension = {
+  name: 'mathInline',
+  level: 'inline',
+  start: (src) => src.indexOf('$'),
+  tokenizer(src) {
+    const display = DISPLAY.exec(src)
+    if (display) return { type: 'mathInline', raw: display[0], text: display[1].trim(), display: true }
+    const inline = INLINE.exec(src)
+    if (inline) return { type: 'mathInline', raw: inline[0], text: inline[1], display: false }
+  },
+  renderer: (token) => tex(token.text, token.display),
+}
+
+marked.use({
+  extensions: [mathBlock, mathInline],
+  renderer: {
+    code(token) {
+      return token.lang === 'math' ? tex(token.text, true) : false
+    },
+  },
+})
+
+/** TeX as KaTeX sets it, or as its source while KaTeX is still loading. */
+function tex(source: string, display: boolean): string {
+  const katex = mathEngine()
+  if (!katex) {
+    const text = escapeHtml(source)
+    return display ? `<pre class="math">${text}</pre>` : `<code class="math">${text}</code>`
+  }
+  // An error shows in place, in red, rather than throwing away the page.
+  return katex.renderToString(source, { displayMode: display, throwOnError: false })
+}
+
+function escapeHtml(text: string): string {
+  return text.replaceAll('&', '&amp;').replaceAll('<', '&lt;').replaceAll('>', '&gt;').replaceAll('"', '&quot;')
+}
+
 /**
  * Render entry markdown to sanitised HTML. When share credentials are given,
  * media URLs are rewritten to the public route — token and key both, since that
@@ -20,8 +78,9 @@ export function renderMarkdown(source: string, share?: { token: string; key: str
   const html = marked.parse(md, { async: false }) as string
 
   return DOMPurify.sanitize(html, {
-    ADD_TAGS: ['video', 'audio', 'source'],
-    ADD_ATTR: ['controls', 'target', 'loading', 'playsinline', 'preload'],
+    // `semantics` and `annotation` carry KaTeX's MathML, for screen readers.
+    ADD_TAGS: ['video', 'audio', 'source', 'semantics', 'annotation'],
+    ADD_ATTR: ['controls', 'target', 'loading', 'playsinline', 'preload', 'encoding'],
   })
 }
 
diff --git a/web/src/lib/math.svelte.ts b/web/src/lib/math.svelte.ts
new file mode 100644
index 0000000..bf9e11b
--- /dev/null
+++ b/web/src/lib/math.svelte.ts
@@ -0,0 +1,20 @@
+/*
+ * KaTeX, fetched the first time something on the page has math in it rather
+ * than with the app: it is larger than the rest of the app put together.
+ * Until it arrives the math shows as its source. The engine is state, so
+ * markdown rendered without it renders again once it is here.
+ */
+import type Katex from 'katex'
+
+let engine = $state.raw<typeof Katex | null>(null)
+let loading = false
+
+export function mathEngine(): typeof Katex | null {
+  if (!engine && !loading) {
+    loading = true
+    Promise.all([import('katex'), import('katex/dist/katex.min.css')])
+      .then(([katex]) => (engine = katex.default))
+      .catch((e) => console.error('math: could not load KaTeX', e))
+  }
+  return engine
+}
diff --git a/web/src/vite-env.d.ts b/web/src/vite-env.d.ts
new file mode 100644
index 0000000..4078e74
--- /dev/null
+++ b/web/src/vite-env.d.ts
@@ -0,0 +1,2 @@
+/// <reference types="svelte" />
+/// <reference types="vite/client" />
diff --git a/web/vite.config.ts b/web/vite.config.ts
index 8e2cbf1..4e6630c 100644
--- a/web/vite.config.ts
+++ b/web/vite.config.ts
@@ -1,8 +1,22 @@
 import { defineConfig } from 'vite'
 import { svelte } from '@sveltejs/vite-plugin-svelte'
+import type { Plugin } from 'vite'
+
+/*
+ * KaTeX lists each font as woff2, woff and ttf. Every browser the app runs in
+ * takes woff2, and the other two would be another megabyte in the binary.
+ */
+const katexWoff2Only: Plugin = {
+  name: 'katex-woff2-only',
+  enforce: 'pre',
+  transform(code, id) {
+    if (!/katex(\.min)?\.css$/.test(id)) return
+    return code.replace(/,\s*url\([^)]*\.(?:woff|ttf)\)\s*format\("(?:woff|truetype)"\)/g, '')
+  },
+}
 
 export default defineConfig({
-  plugins: [svelte()],
+  plugins: [svelte(), katexWoff2Only],
   build: { target: 'es2022', chunkSizeWarningLimit: 900 },
   server: {
     port: 4243,