diff --git a/src/gaxios.ts b/src/gaxios.ts
index 51ef2f1..b60e8c2 100644
--- a/src/gaxios.ts
+++ b/src/gaxios.ts
@@ -115,10 +115,10 @@ export class Gaxios implements FetchCompliance {
 
     // prepare headers
     if (input && typeof input === 'object' && 'headers' in input) {
-      this.#mergeHeaders(headers, input.headers);
+      Gaxios.mergeHeaders(headers, input.headers);
     }
     if (init) {
-      this.#mergeHeaders(headers, new Headers(init.headers));
+      Gaxios.mergeHeaders(headers, new Headers(init.headers));
     }
 
     // prepare request
@@ -362,23 +362,6 @@ export class Gaxios implements FetchCompliance {
     return promiseChain;
   }
 
-  /**
-   * Merges headers.
-   *
-   * @param base headers to append/overwrite to
-   * @param append headers to append/overwrite with
-   * @returns the base headers instance with merged `Headers`
-   */
-  #mergeHeaders(base: Headers, append?: Headers) {
-    append?.forEach((value, key) => {
-      // set-cookie is the only header that would repeat.
-      // A bit of background: https://developer.mozilla.org/en-US/docs/Web/API/Headers/getSetCookie
-      key === 'set-cookie' ? base.append(key, value) : base.set(key, value);
-    });
-
-    return base;
-  }
-
   /**
    * Validates the options, merges them with defaults, and prepare request.
    *
@@ -390,7 +373,7 @@ export class Gaxios implements FetchCompliance {
   ): Promise<GaxiosOptionsPrepared> {
     // Prepare Headers - copy in order to not mutate the original objects
     const preparedHeaders = new Headers(this.defaults.headers);
-    this.#mergeHeaders(preparedHeaders, options.headers);
+    Gaxios.mergeHeaders(preparedHeaders, options.headers);
 
     // Merge options
     const opts = extend(true, {}, this.defaults, options);
@@ -664,4 +647,38 @@ export class Gaxios implements FetchCompliance {
 
     return this.#fetch;
   }
+
+  /**
+   * Merges headers.
+   * If the base headers do not exist a new `Headers` object will be returned.
+   *
+   * @remarks
+   *
+   * Using this utility can be helpful when the headers are not known to exist:
+   * - if they exist as `Headers`, that instance will be used
+   *   - it improves performance and allows users to use their existing references to their `Headers`
+   * - if they exist in another form (`HeadersInit`), they will be used to create a new `Headers` object
+   * - if the base headers do not exist a new `Headers` object will be created
+   *
+   * @param base headers to append/overwrite to
+   * @param append headers to append/overwrite with
+   * @returns the base headers instance with merged `Headers`
+   */
+  static mergeHeaders(base?: HeadersInit, ...append: HeadersInit[]): Headers {
+    base = base instanceof Headers ? base : new Headers(base);
+
+    for (const headers of append) {
+      const add = headers instanceof Headers ? headers : new Headers(headers);
+
+      add.forEach((value, key) => {
+        // set-cookie is the only header that would repeat.
+        // A bit of background: https://developer.mozilla.org/en-US/docs/Web/API/Headers/getSetCookie
+        key === 'set-cookie' ? base.append(key, value) : base.set(key, value);
+      });
+    }
+
+    return base;
+  }
 }
+
+type HeadersInit = ConstructorParameters<typeof Headers>[0];
diff --git a/test/test.getch.ts b/test/test.getch.ts
index debef58..d02102b 100644
--- a/test/test.getch.ts
+++ b/test/test.getch.ts
@@ -1511,3 +1511,50 @@ describe('fetch-compatible API', () => {
     assert.deepStrictEqual(res.data, {});
   });
 });
+
+describe('merge headers', () => {
+  it('should merge Headers', () => {
+    const base = {a: 'a'};
+    const append = {b: 'b'};
+    const expected = new Headers({...base, ...append});
+
+    const matrixBase = [{...base}, Object.entries(base), new Headers(base)];
+    const matrixAppend = [
+      {...append},
+      Object.entries(append),
+      new Headers(append),
+    ];
+
+    for (const base of matrixBase) {
+      for (const append of matrixAppend) {
+        const headers = Gaxios.mergeHeaders(base, append);
+
+        assert.deepStrictEqual(headers, expected);
+      }
+    }
+  });
+
+  it('should merge multiple Headers', () => {
+    const base = {a: 'a'};
+    const append = {b: 'b'};
+    const appendMore = {c: 'c'};
+    const expected = new Headers({...base, ...append, ...appendMore});
+
+    const headers = Gaxios.mergeHeaders(base, append, appendMore);
+
+    assert.deepStrictEqual(headers, expected);
+  });
+
+  it('should merge Set-Cookie Headers', () => {
+    const base = {'set-cookie': 'a=a'};
+    const append = {'set-cookie': 'b=b'};
+    const expected = new Headers([
+      ['set-cookie', 'a=a'],
+      ['set-cookie', 'b=b'],
+    ]);
+
+    const headers = Gaxios.mergeHeaders(base, append);
+
+    assert.deepStrictEqual(headers.getSetCookie(), expected.getSetCookie());
+  });
+});