diff --git a/index.js b/index.js
index 0c76f00..90058b0 100644
--- a/index.js
+++ b/index.js
@@ -1,102 +1,5 @@
 /**
- * @typedef {import('hast').Root} Root
- * @typedef {import('hast').Content} Content
+ * @typedef {import('./lib/index.js').Options} Options
  */
 
-/**
- * @typedef {Root | Content} Node
- *
- * @typedef Options
- *   Configuration
- * @property {number | null | undefined} [age=16]
- *   Target age group.
- *
- *   This is the age your target audience was still in school.
- *   Set it to 18 if you expect all readers to have finished high school,
- *   21 if you expect your readers to all be college graduates, etc.
- */
-
-import {toText} from 'hast-util-to-text'
-import readabilityScores from 'readability-scores'
-// @ts-expect-error: untyped.
-import median from 'compute-median'
-
-// See source [1].
-const firstGradeAge = 5
-const highschoolGraduationAge = 18
-const graduationAge = 22
-
-// See source [2].
-// Note that different other algorithms vary between 200, 230, 270, and 280.
-// 228 seems to at least be based in research.
-const reasonableWpm = 228
-const reasonableWpmMax = 340
-
-// See source [3].
-const addedWpmPerGrade = 14
-const baseWpm =
-  reasonableWpm - (highschoolGraduationAge - firstGradeAge) * addedWpmPerGrade
-
-const accuracy = 1e6
-
-/**
- * Estimate the reading time, taking readability of the document and a target
- * age group into account.
- *
- * *   [1] For more info on US education/grade levels, see:
- *     <https://en.wikipedia.org/wiki/Educational_stage#United_States>.
- * *   [2] For the wpm of people reading English, see:
- *     <https://en.wikipedia.org/wiki/Words_per_minute#Reading_and_comprehension>
- * *   [3] For information on reading rate, including how grade levels affect
- *     them, see: <https://en.wikipedia.org/wiki/Reading#Reading_rate>.
- *
- * And some more background info/history and a few insight on where this comes
- * from, see: <https://martech.org/estimated-reading-times-increase-engagement/>.
- *
- * @param {Node} tree
- *   Tree to inspect.
- * @param {Options | null | undefined} [options]
- *   Configuration.
- * @returns {number}
- *   Estimated reading time in minutes.
- */
-export function readingTime(tree, options) {
-  const settings = options || {}
-  // Cap an age to a reasonable and meaningful age in school.
-  const targetAge = Math.min(
-    graduationAge,
-    Math.max(firstGradeAge, Math.round(settings.age || 16))
-  )
-  const text = toText(tree)
-  const scores = readabilityScores(text) || {}
-  const score = median(
-    [
-      scores.daleChall,
-      scores.ari,
-      scores.colemanLiau,
-      scores.fleschKincaid,
-      scores.smog,
-      scores.gunningFog
-    ].filter((d) => d !== undefined)
-  )
-
-  if (score === null) {
-    return 0
-  }
-
-  /** @type {number} */
-  const readabilityAge = firstGradeAge + score
-
-  // WPM the target audience normally reads.
-  const targetWpm = baseWpm + (targetAge - firstGradeAge) * addedWpmPerGrade
-
-  // If the text requires higher comprehension than the target age group is,
-  // estimated to have, make it a bit slower (and vice versa).
-  const adjustedWpm =
-    targetWpm - (readabilityAge - targetAge) * (addedWpmPerGrade / 2)
-
-  // Cap it to a WPM that’s reasonable.
-  const wpm = Math.min(reasonableWpmMax, Math.max(baseWpm, adjustedWpm))
-
-  return Math.round((scores.wordCount / wpm) * accuracy) / accuracy
-}
+export {readingTime} from './lib/index.js'
diff --git a/lib/index.js b/lib/index.js
new file mode 100644
index 0000000..0c76f00
--- /dev/null
+++ b/lib/index.js
@@ -0,0 +1,102 @@
+/**
+ * @typedef {import('hast').Root} Root
+ * @typedef {import('hast').Content} Content
+ */
+
+/**
+ * @typedef {Root | Content} Node
+ *
+ * @typedef Options
+ *   Configuration
+ * @property {number | null | undefined} [age=16]
+ *   Target age group.
+ *
+ *   This is the age your target audience was still in school.
+ *   Set it to 18 if you expect all readers to have finished high school,
+ *   21 if you expect your readers to all be college graduates, etc.
+ */
+
+import {toText} from 'hast-util-to-text'
+import readabilityScores from 'readability-scores'
+// @ts-expect-error: untyped.
+import median from 'compute-median'
+
+// See source [1].
+const firstGradeAge = 5
+const highschoolGraduationAge = 18
+const graduationAge = 22
+
+// See source [2].
+// Note that different other algorithms vary between 200, 230, 270, and 280.
+// 228 seems to at least be based in research.
+const reasonableWpm = 228
+const reasonableWpmMax = 340
+
+// See source [3].
+const addedWpmPerGrade = 14
+const baseWpm =
+  reasonableWpm - (highschoolGraduationAge - firstGradeAge) * addedWpmPerGrade
+
+const accuracy = 1e6
+
+/**
+ * Estimate the reading time, taking readability of the document and a target
+ * age group into account.
+ *
+ * *   [1] For more info on US education/grade levels, see:
+ *     <https://en.wikipedia.org/wiki/Educational_stage#United_States>.
+ * *   [2] For the wpm of people reading English, see:
+ *     <https://en.wikipedia.org/wiki/Words_per_minute#Reading_and_comprehension>
+ * *   [3] For information on reading rate, including how grade levels affect
+ *     them, see: <https://en.wikipedia.org/wiki/Reading#Reading_rate>.
+ *
+ * And some more background info/history and a few insight on where this comes
+ * from, see: <https://martech.org/estimated-reading-times-increase-engagement/>.
+ *
+ * @param {Node} tree
+ *   Tree to inspect.
+ * @param {Options | null | undefined} [options]
+ *   Configuration.
+ * @returns {number}
+ *   Estimated reading time in minutes.
+ */
+export function readingTime(tree, options) {
+  const settings = options || {}
+  // Cap an age to a reasonable and meaningful age in school.
+  const targetAge = Math.min(
+    graduationAge,
+    Math.max(firstGradeAge, Math.round(settings.age || 16))
+  )
+  const text = toText(tree)
+  const scores = readabilityScores(text) || {}
+  const score = median(
+    [
+      scores.daleChall,
+      scores.ari,
+      scores.colemanLiau,
+      scores.fleschKincaid,
+      scores.smog,
+      scores.gunningFog
+    ].filter((d) => d !== undefined)
+  )
+
+  if (score === null) {
+    return 0
+  }
+
+  /** @type {number} */
+  const readabilityAge = firstGradeAge + score
+
+  // WPM the target audience normally reads.
+  const targetWpm = baseWpm + (targetAge - firstGradeAge) * addedWpmPerGrade
+
+  // If the text requires higher comprehension than the target age group is,
+  // estimated to have, make it a bit slower (and vice versa).
+  const adjustedWpm =
+    targetWpm - (readabilityAge - targetAge) * (addedWpmPerGrade / 2)
+
+  // Cap it to a WPM that’s reasonable.
+  const wpm = Math.min(reasonableWpmMax, Math.max(baseWpm, adjustedWpm))
+
+  return Math.round((scores.wordCount / wpm) * accuracy) / accuracy
+}
diff --git a/package.json b/package.json
index 8bb3f7f..593b182 100644
--- a/package.json
+++ b/package.json
@@ -30,6 +30,7 @@
   "main": "index.js",
   "types": "index.d.ts",
   "files": [
+    "lib/",
     "index.d.ts",
     "index.js"
   ],