diff --git a/src/lib/isISIN.js b/src/lib/isISIN.js
index 4576b684c..a6f6fa645 100644
--- a/src/lib/isISIN.js
+++ b/src/lib/isISIN.js
@@ -2,6 +2,12 @@ import assertString from './util/assertString';
 
 const isin = /^[A-Z]{2}[0-9A-Z]{9}[0-9]$/;
 
+// this link details how the check digit is calculated:
+// https://www.isin.org/isin-format/. it is a little bit
+// odd in that it works with digits, not numbers. in order
+// to make only one pass through the ISIN characters, the
+// each alpha character is handled as 2 characters within
+// the loop.
 
 export default function isISIN(str) {
   assertString(str);
@@ -9,27 +15,44 @@ export default function isISIN(str) {
     return false;
   }
 
-  const checksumStr = str.replace(/[A-Z]/g, character => (parseInt(character, 36)));
-
+  let double = true;
   let sum = 0;
-  let digit;
-  let tmpNum;
-  let shouldDouble = true;
-  for (let i = checksumStr.length - 2; i >= 0; i--) {
-    digit = checksumStr.substring(i, (i + 1));
-    tmpNum = parseInt(digit, 10);
-    if (shouldDouble) {
-      tmpNum *= 2;
-      if (tmpNum >= 10) {
-        sum += tmpNum + 1;
-      } else {
-        sum += tmpNum;
+  // convert values
+  for (let i = str.length - 2; i >= 0; i--) {
+    if (str[i] >= 'A' && str[i] <= 'Z') {
+      const value = str[i].charCodeAt(0) - 55;
+      const lo = value % 10;
+      const hi = Math.trunc(value / 10);
+      // letters have two digits, so handle the low order
+      // and high order digits separately.
+      for (const digit of [lo, hi]) {
+        if (double) {
+          if (digit >= 5) {
+            sum += 1 + ((digit - 5) * 2);
+          } else {
+            sum += digit * 2;
+          }
+        } else {
+          sum += digit;
+        }
+        double = !double;
       }
     } else {
-      sum += tmpNum;
+      const digit = str[i].charCodeAt(0) - '0'.charCodeAt(0);
+      if (double) {
+        if (digit >= 5) {
+          sum += 1 + ((digit - 5) * 2);
+        } else {
+          sum += digit * 2;
+        }
+      } else {
+        sum += digit;
+      }
+      double = !double;
     }
-    shouldDouble = !shouldDouble;
   }
 
-  return parseInt(str.substr(str.length - 1), 10) === (10000 - sum) % 10;
+  const check = (Math.trunc(((sum + 9) / 10)) * 10) - sum;
+
+  return +str[str.length - 1] === check;
 }
diff --git a/test/validators.js b/test/validators.js
index 2befa37d0..1f92e1480 100644
--- a/test/validators.js
+++ b/test/validators.js
@@ -4845,6 +4845,7 @@ describe('Validators', () => {
         'GB0001411924',
         'DE000WCH8881',
         'PLLWBGD00016',
+        'US0378331005',
       ],
       invalid: [
         'DE000BAY0018',