diff --git a/src/internal/__tests__/format-ext.spec.ts b/src/internal/__tests__/format-ext.spec.ts
new file mode 100644
index 00000000..5aaa8049
--- /dev/null
+++ b/src/internal/__tests__/format-ext.spec.ts
@@ -0,0 +1,22 @@
+/**
+ * @file Unit Tests - formatExt
+ * @module pathe/internal/tests/unit/formatExt
+ */
+
+import testSubject from '../format-ext'
+
+describe('unit:internal/formatExt', () => {
+  let ext: string
+
+  beforeEach(() => {
+    ext = '.mjs'
+  })
+
+  it('should do nothing if value begins with dot character', () => {
+    expect(testSubject(ext)).to.equal(ext)
+  })
+
+  it('should return formatted file extension', () => {
+    expect(testSubject(ext.slice(1))).to.equal(ext)
+  })
+})
diff --git a/src/internal/format-ext.ts b/src/internal/format-ext.ts
new file mode 100644
index 00000000..c3131c7d
--- /dev/null
+++ b/src/internal/format-ext.ts
@@ -0,0 +1,20 @@
+/**
+ * @file Internal - formatExt
+ * @module pathe/internal/formatExt
+ */
+
+import type { Ext } from '#src/types'
+import validateString from './validate-string'
+
+/**
+ * File extension formatter.
+ *
+ * @param {string} ext - File extension to format
+ * @return {Ext} `ext` formatted
+ */
+const formatExt = (ext: string): Ext => {
+  validateString(ext, 'ext')
+  return ext.replace(/^([^.])/, '.$1') as Ext
+}
+
+export default formatExt