diff --git a/lib/output.js b/lib/output.js
index 6458c05c..6d309105 100644
--- a/lib/output.js
+++ b/lib/output.js
@@ -188,7 +188,7 @@ function withMetadata (options) {
  * @throws {Error} unsupported format or options
  */
 function toFormat (format, options) {
-  const actualFormat = formats.get(is.object(format) && is.string(format.id) ? format.id : format);
+  const actualFormat = formats.get((is.object(format) && is.string(format.id) ? format.id : format).toLowerCase());
   if (!actualFormat) {
     throw is.invalidParameterError('format', `one of: ${[...formats.keys()].join(', ')}`, format);
   }
diff --git a/test/unit/toFormat.js b/test/unit/toFormat.js
new file mode 100644
index 00000000..bc253d03
--- /dev/null
+++ b/test/unit/toFormat.js
@@ -0,0 +1,26 @@
+'use strict';
+
+const assert = require('assert');
+const sharp = require('../../');
+const fixtures = require('../fixtures');
+
+describe('toFormat', () => {
+  it('accepts upper case characters as format parameter (string)', function (done) {
+    sharp(fixtures.inputJpg)
+      .toFormat('PNG')
+      .toBuffer(function (err, data, info) {
+        if (err) throw err;
+        assert.strictEqual('png', info.format);
+        done();
+      });
+  });
+  it('accepts upper case characters as format parameter (object)', function (done) {
+    sharp(fixtures.inputJpg)
+      .toFormat({ id: 'PNG' })
+      .toBuffer(function (err, data, info) {
+        if (err) throw err;
+        assert.strictEqual('png', info.format);
+        done();
+      });
+  });
+});