diff --git a/icu4c/source/i18n/number_modifiers.h b/icu4c/source/i18n/number_modifiers.h
index 6fd0bad8d8e..e3820b6b384 100644
--- a/icu4c/source/i18n/number_modifiers.h
+++ b/icu4c/source/i18n/number_modifiers.h
@@ -322,9 +322,9 @@ class U_I18N_API AdoptingModifierStore : public ModifierStore, public UMemory {
     const Modifier *mods[4 * StandardPlural::COUNT] = {};
 
     inline static int32_t getModIndex(Signum signum, StandardPlural::Form plural) {
-        U_ASSERT(signum >= 0 && signum <= 3);
+        U_ASSERT(signum >= 0 && signum < SIGNUM_COUNT);
         U_ASSERT(plural >= 0 && plural < StandardPlural::COUNT);
-        return static_cast<int32_t>(plural) * 4 + signum;
+        return static_cast<int32_t>(plural) * SIGNUM_COUNT + signum;
     }
 };
 
diff --git a/icu4c/source/i18n/number_patternstring.cpp b/icu4c/source/i18n/number_patternstring.cpp
index b2e6edc5bfe..55b83f1ac63 100644
--- a/icu4c/source/i18n/number_patternstring.cpp
+++ b/icu4c/source/i18n/number_patternstring.cpp
@@ -1072,6 +1072,8 @@ PatternSignType PatternStringUtils::resolveSignDisplay(UNumberSignDisplay signDi
                 case SIGNUM_POS_ZERO:
                 case SIGNUM_POS:
                     return PATTERN_SIGN_TYPE_POS;
+                default:
+                    break;
             }
             break;
 
@@ -1084,6 +1086,8 @@ PatternSignType PatternStringUtils::resolveSignDisplay(UNumberSignDisplay signDi
                 case SIGNUM_POS_ZERO:
                 case SIGNUM_POS:
                     return PATTERN_SIGN_TYPE_POS_SIGN;
+                default:
+                    break;
             }
             break;
 
@@ -1097,6 +1101,8 @@ PatternSignType PatternStringUtils::resolveSignDisplay(UNumberSignDisplay signDi
                     return PATTERN_SIGN_TYPE_POS;
                 case SIGNUM_POS:
                     return PATTERN_SIGN_TYPE_POS_SIGN;
+                default:
+                    break;
             }
             break;
 
diff --git a/icu4c/source/i18n/number_types.h b/icu4c/source/i18n/number_types.h
index 49b77eeb4d6..641e082d361 100644
--- a/icu4c/source/i18n/number_types.h
+++ b/icu4c/source/i18n/number_types.h
@@ -95,7 +95,8 @@ enum Signum {
     SIGNUM_NEG = 0,
     SIGNUM_NEG_ZERO = 1,
     SIGNUM_POS_ZERO = 2,
-    SIGNUM_POS = 3
+    SIGNUM_POS = 3,
+    SIGNUM_COUNT = 4,
 };
 
 
diff --git a/icu4j/main/classes/core/src/com/ibm/icu/impl/number/AdoptingModifierStore.java b/icu4j/main/classes/core/src/com/ibm/icu/impl/number/AdoptingModifierStore.java
index 83652480173..93da08b48bb 100644
--- a/icu4j/main/classes/core/src/com/ibm/icu/impl/number/AdoptingModifierStore.java
+++ b/icu4j/main/classes/core/src/com/ibm/icu/impl/number/AdoptingModifierStore.java
@@ -86,6 +86,6 @@ public class AdoptingModifierStore implements ModifierStore {
     private static int getModIndex(Signum signum, StandardPlural plural) {
         assert signum != null;
         assert plural != null;
-        return plural.ordinal() * 4 + signum.ordinal();
+        return plural.ordinal() * Signum.COUNT + signum.ordinal();
     }
 }
diff --git a/icu4j/main/classes/core/src/com/ibm/icu/impl/number/Modifier.java b/icu4j/main/classes/core/src/com/ibm/icu/impl/number/Modifier.java
index 43542e1c048..699ba3bc95a 100644
--- a/icu4j/main/classes/core/src/com/ibm/icu/impl/number/Modifier.java
+++ b/icu4j/main/classes/core/src/com/ibm/icu/impl/number/Modifier.java
@@ -21,7 +21,9 @@ public interface Modifier {
         NEG,
         NEG_ZERO,
         POS_ZERO,
-        POS
+        POS;
+
+        static final int COUNT = Signum.values().length;
     };
 
     /**