diff --git a/lib/fs.js b/lib/fs.js
index 1ea70ff192d6dd..1c1b29cede353e 100644
--- a/lib/fs.js
+++ b/lib/fs.js
@@ -59,6 +59,13 @@ const {
   O_SYMLINK,
 } = constants;
 
+function isUtf8Encoding(encoding) {
+  return encoding === 'utf8' ||
+         encoding === 'utf-8' ||
+         encoding === 'UTF8' ||
+         encoding === 'UTF-8';
+}
+
 const pathModule = require('path');
 const { isArrayBufferView } = require('internal/util/types');
 
@@ -531,8 +538,7 @@ function readFileSync(path, options) {
   validateReadFileBufferOptions(options);
   const hasUserBuffer = options.buffer !== undefined;
 
-  if ((options.encoding === 'utf8' || options.encoding === 'utf-8') &&
-      !hasUserBuffer) {
+  if (isUtf8Encoding(options.encoding) && !hasUserBuffer) {
     if (!isInt32(path)) {
       path = getValidatedPath(path);
     }
@@ -2906,7 +2912,7 @@ function writeFileSync(path, data, options) {
   const flag = options.flag || 'w';
 
   // C++ fast path for string data and UTF8 encoding
-  if (typeof data === 'string' && (options.encoding === 'utf8' || options.encoding === 'utf-8')) {
+  if (typeof data === 'string' && isUtf8Encoding(options.encoding)) {
     if (!isInt32(path)) {
       path = getValidatedPath(path);
     }
@@ -3197,7 +3203,7 @@ if (isWindows) {
 }
 
 function encodeRealpathResult(result, options) {
-  if (!options || !options.encoding || options.encoding === 'utf8')
+  if (!options || !options.encoding || isUtf8Encoding(options.encoding))
     return result;
   const asBuffer = Buffer.from(result);
   if (options.encoding === 'buffer') {