diff --git a/src/utils.js b/src/utils.js
index 9a11350..f57a59d 100644
--- a/src/utils.js
+++ b/src/utils.js
@@ -59,9 +59,11 @@ export function equals(a, b) {
  *
  * @param {string} url
  * @param {RequestInit} [requestInit] fetch options
+ * @param {typeof globalThis.fetch} [customFetch] fetch function to use
  * @returns {Promise<number>}
  */
-export async function byteLengthFromUrl(url, requestInit) {
+export async function byteLengthFromUrl(url, requestInit, customFetch) {
+  const fetch = customFetch ?? globalThis.fetch
   return await fetch(url, { ...requestInit, method: 'HEAD' })
     .then(res => {
       if (!res.ok) throw new Error(`fetch head failed ${res.status}`)
@@ -74,18 +76,21 @@ export async function byteLengthFromUrl(url, requestInit) {
 /**
  * Construct an AsyncBuffer for a URL.
  * If byteLength is not provided, will make a HEAD request to get the file size.
+ * If fetch is provided, it will be used instead of the global fetch.
  * If requestInit is provided, it will be passed to fetch.
  *
  * @param {object} options
  * @param {string} options.url
  * @param {number} [options.byteLength]
+ * @param {typeof globalThis.fetch} [options.fetch] fetch function to use
  * @param {RequestInit} [options.requestInit]
  * @returns {Promise<AsyncBuffer>}
  */
-export async function asyncBufferFromUrl({ url, byteLength, requestInit }) {
+export async function asyncBufferFromUrl({ url, byteLength, requestInit, fetch: customFetch }) {
   if (!url) throw new Error('missing url')
+  const fetch = customFetch ?? globalThis.fetch
   // byte length from HEAD request
-  byteLength ||= await byteLengthFromUrl(url, requestInit)
+  byteLength ||= await byteLengthFromUrl(url, requestInit, fetch)
 
   /**
    * A promise for the whole buffer, if range requests are not supported.
diff --git a/test/utils.test.js b/test/utils.test.js
index 13577ef..3a52c83 100644
--- a/test/utils.test.js
+++ b/test/utils.test.js
@@ -83,6 +83,18 @@ describe('byteLengthFromUrl', () => {
 
     await expect(byteLengthFromUrl('https://example.com')).rejects.toThrow('fetch head failed 401')
   })
+
+  it ('uses the provided fetch function, along with requestInit if passed', async () => {
+    const customFetch = vi.fn().mockResolvedValueOnce({
+      ok: true,
+      headers: new Map([['Content-Length', '2048']]),
+    })
+
+    const requestInit = { headers: { authorization: 'Bearer token' } }
+    const result = await byteLengthFromUrl('https://example.com', requestInit, customFetch)
+    expect(result).toBe(2048)
+    expect(customFetch).toHaveBeenCalledWith('https://example.com', { ...requestInit, method: 'HEAD' })
+  })
 })
 
 describe('asyncBufferFromUrl', () => {
@@ -226,4 +238,43 @@ describe('asyncBufferFromUrl', () => {
       expect(fetch).toBeCalledTimes(1)
     })
   })
+
+  describe('when a custom fetch function is provided', () => {
+    it ('is used to get the byte length', async () => {
+      const customFetch = vi.fn().mockResolvedValueOnce({
+        ok: true,
+        headers: new Map([['Content-Length', '2048']]),
+      })
+
+      const requestInit = { headers: { authorization: 'Bearer token' } }
+      const buffer = await asyncBufferFromUrl({ url: 'https://example.com', requestInit, fetch: customFetch })
+      expect(buffer.byteLength).toBe(2048)
+      expect(customFetch).toHaveBeenCalledWith('https://example.com', { ...requestInit, method: 'HEAD' })
+    })
+    it ('is used to fetch the slice', async () => {
+      const mockArrayBuffer = new ArrayBuffer(35)
+      let counter = 0
+      function rateLimitedFetch() {
+        counter++
+        if (counter === 2) {
+          return Promise.resolve({ ok: true, status: 206, body: {}, arrayBuffer: () => Promise.resolve(mockArrayBuffer) })
+        }
+        return Promise.resolve({ ok: false, status: 429 })
+      }
+      const customFetch = vi.fn().mockImplementation(async () => {
+        while (true) {
+          const result = await rateLimitedFetch()
+          if (result.ok) {
+            return result
+          }
+          await new Promise(resolve => setTimeout(resolve, 100)) // wait for 100ms before retrying
+        }
+      })
+      const requestInit = { headers: { authorization: 'Bearer token' } }
+      const buffer = await asyncBufferFromUrl({ url: 'https://example.com', byteLength: 1024, requestInit, fetch: customFetch })
+      const result = await buffer.slice(50, 85)
+      expect(result).toBe(mockArrayBuffer)
+      expect(customFetch).toHaveBeenCalledWith('https://example.com', { headers: new Headers({ ...requestInit.headers, Range: 'bytes=50-84' }) })
+    })
+  })
 })