diff --git a/lib/mathlib.cpp b/lib/mathlib.cpp
index 60d569b7a..5dfb360fe 100644
--- a/lib/mathlib.cpp
+++ b/lib/mathlib.cpp
@@ -251,6 +251,15 @@ std::string MathLib::calculate(const std::string &first, const std::string &seco
     case '%':
         return MathLib::mod(first, second);
 
+    case '&':
+        return MathLib::toString(MathLib::toLongNumber(first) & MathLib::toLongNumber(second));
+
+    case '|':
+        return MathLib::toString(MathLib::toLongNumber(first) | MathLib::toLongNumber(second));
+
+    case '^':
+        return MathLib::toString(MathLib::toLongNumber(first) ^ MathLib::toLongNumber(second));
+
     default:
         throw InternalError(0, std::string("Unexpected action '") + action + "' in MathLib::calculate(). Please report this to Cppcheck developers.");
     }
diff --git a/lib/templatesimplifier.cpp b/lib/templatesimplifier.cpp
index a6a1759e4..db7808927 100644
--- a/lib/templatesimplifier.cpp
+++ b/lib/templatesimplifier.cpp
@@ -897,12 +897,8 @@ bool TemplateSimplifier::simplifyCalculations(Token *_tokens)
                     const MathLib::bigint rightInt(MathLib::toLongNumber(tok->strAt(2)));
                     std::string result;
 
-                    if (cop == '&')
-                        result = MathLib::toString<MathLib::bigint>(leftInt & rightInt);
-                    else if (cop == '|')
-                        result = MathLib::toString<MathLib::bigint>(leftInt | rightInt);
-                    else if (cop == '^')
-                        result = MathLib::toString<MathLib::bigint>(leftInt ^ rightInt);
+                    if (cop == '&' || cop == '|' || cop == '^')
+                        result = MathLib::calculate(tok->str(), tok->strAt(2), cop);
                     else if (cop == '<') {
                         if (tok->previous()->str() != "<<") // Ensure that its not a shift operator as used for streams
                             result = MathLib::toString<MathLib::bigint>(leftInt << rightInt);
diff --git a/lib/tokenize.cpp b/lib/tokenize.cpp
index 9137a1f78..0226cfd1a 100644
--- a/lib/tokenize.cpp
+++ b/lib/tokenize.cpp
@@ -7008,6 +7008,32 @@ void Tokenizer::simplifyEnum()
                         }
 
                         if (simplify) {
+                            // Simplify calculations..
+                            while (Token::Match(enumValueStart, "%num% %op% %num% %op%") &&
+                                   enumValueStart->strAt(1) == enumValueStart->strAt(3)) {
+                                const std::string &op = enumValueStart->strAt(1);
+                                if (op.size() != 1U)
+                                    break;
+                                const std::string &val1 = enumValueStart->str();
+                                const std::string &val2 = enumValueStart->strAt(2);
+                                const std::string result = MathLib::calculate(val1, val2, op[0]);
+                                enumValueStart->str(result);
+                                enumValueStart->deleteNext(2);
+                            }
+                            if (Token::Match(enumValueStart, "%num% %op% %num% [,}]")) {
+                                const std::string &op   = enumValueStart->strAt(1);
+                                if (op.size() == 1U) {
+                                    const std::string &val1 = enumValueStart->str();
+                                    const std::string &val2 = enumValueStart->strAt(2);
+                                    const std::string result = MathLib::calculate(val1, val2, op[0]);
+                                    enumValueStart->str(result);
+                                    enumValueStart->deleteNext(2);
+                                    enumValue = enumValueStart;
+                                    enumValueStart = enumValueEnd = 0;
+                                }
+                            }
+
+
                             if (enumValue)
                                 tok2->str(enumValue->str());
                             else {
diff --git a/test/testsimplifytokens.cpp b/test/testsimplifytokens.cpp
index a4c81d72c..345d25282 100644
--- a/test/testsimplifytokens.cpp
+++ b/test/testsimplifytokens.cpp
@@ -344,6 +344,7 @@ private:
         TEST_CASE(enum26); // ticket #2975 (segmentation fault)
         TEST_CASE(enum27); // ticket #3005 (segmentation fault)
         TEST_CASE(enum28);
+        TEST_CASE(enum29); // ticket #3747 (bitwise or value)
 
         // remove "std::" on some standard functions
         TEST_CASE(removestd);
@@ -6985,6 +6986,11 @@ private:
         ASSERT_EQUALS("void f ( ) { char x [ 4 ] ; memset ( x , 0 , 4 ) ; { x } } ; void g ( ) { 0 ; }", checkSimplifyEnum(code));
     }
 
+    void enum29() {  // #3747 - bitwise or value
+        const char code[] = "enum { x=1, y=x|2 }; i = (3==y);";
+        ASSERT_EQUALS("i = 3 == 3 ;", checkSimplifyEnum(code));
+    }
+
     void removestd() {
         ASSERT_EQUALS("; strcpy ( a , b ) ;", tok("; std::strcpy(a,b);"));
         ASSERT_EQUALS("; strcat ( a , b ) ;", tok("; std::strcat(a,b);"));