diff --git a/icu4c/source/common/ulocale.cpp b/icu4c/source/common/ulocale.cpp
index 28333143963..471ef0ce79a 100644
--- a/icu4c/source/common/ulocale.cpp
+++ b/icu4c/source/common/ulocale.cpp
@@ -10,6 +10,7 @@
 
 #include "charstr.h"
 #include "cmemory.h"
+#include "ustr_imp.h"
 
 U_NAMESPACE_USE
 #define EXTERNAL(i) (reinterpret_cast<ULocale*>(i))
@@ -56,14 +57,16 @@ int32_t ulocale_get ##N ( \
     CONST_INTERNAL(locale)->get ## N( \
         keywordLength < 0 ? StringPiece(keyword) : StringPiece(keyword, keywordLength), \
         sink, *err); \
-    if (U_FAILURE(*err)) return 0; \
-    if (sink.Overflowed()) { \
-       *err = U_BUFFER_OVERFLOW_ERROR; \
-       return sink.NumberOfBytesAppended()+1; \
+    int32_t reslen = sink.NumberOfBytesAppended(); \
+    if (U_FAILURE(*err)) { \
+        return reslen; \
     } \
-    int32_t len = sink.NumberOfBytesWritten(); \
-    if (len < bufferCapacity) valueBuffer[len] = '\0'; \
-    return len; \
+    if (sink.Overflowed()) { \
+        *err = U_BUFFER_OVERFLOW_ERROR; \
+    } else { \
+        u_terminateChars(valueBuffer, bufferCapacity, reslen, err); \
+    } \
+    return reslen; \
 }
 
 #define IMPL_ULOCALE_GET_KEYWORDS(N) \
diff --git a/icu4c/source/common/ulocbuilder.cpp b/icu4c/source/common/ulocbuilder.cpp
index 3adaabf9be3..40c4898df78 100644
--- a/icu4c/source/common/ulocbuilder.cpp
+++ b/icu4c/source/common/ulocbuilder.cpp
@@ -10,6 +10,7 @@
 #include "unicode/umachine.h"
 #include "unicode/ulocbuilder.h"
 #include "cstring.h"
+#include "ustr_imp.h"
 
 using icu::CheckedArrayByteSink;
 using icu::StringPiece;
@@ -95,18 +96,10 @@ int32_t ulocbld_buildLocaleID(ULocaleBuilder* builder,
     icu::Locale l = INTERNAL(builder)->build(*err);
     if (U_FAILURE(*err)) return 0;
     int32_t length = (int32_t)(uprv_strlen(l.getName()));
-    if (length <= bufferCapacity) {
-        uprv_strncpy(buffer, l.getName(), length);
-        if (length < bufferCapacity) {
-            buffer[length] = '\0';
-        } else {
-            *err = U_STRING_NOT_TERMINATED_WARNING;
-        }
-        return length;
+    if (0 < length && length <= bufferCapacity) {
+        uprv_memcpy(buffer, l.getName(), length);
     }
-    *err = U_BUFFER_OVERFLOW_ERROR;
-    uprv_memcpy(buffer, l.getName(), bufferCapacity);
-    return length;
+    return u_terminateChars(buffer, bufferCapacity, length, err);
 }
 
 int32_t ulocbld_buildLanguageTag(ULocaleBuilder* builder,
@@ -119,22 +112,16 @@ int32_t ulocbld_buildLanguageTag(ULocaleBuilder* builder,
     if (U_FAILURE(*err)) return 0;
     CheckedArrayByteSink sink(buffer, bufferCapacity);
     l.toLanguageTag(sink, *err);
+    int32_t reslen = sink.NumberOfBytesAppended();
     if (U_FAILURE(*err)) {
-        return 0;
+        return reslen;
     }
     if (sink.Overflowed()) {
         *err = U_BUFFER_OVERFLOW_ERROR;
-        return sink.NumberOfBytesAppended();
-    }
-    int32_t written = sink.NumberOfBytesWritten();
-
-    if (written <  bufferCapacity) {
-        // null terminate
-        buffer[written] = '\0';
     } else {
-        *err = U_STRING_NOT_TERMINATED_WARNING;
+        u_terminateChars(buffer, bufferCapacity, reslen, err);
     }
-    return written;
+    return reslen;
 }
 
 UBool ulocbld_copyErrorTo(const ULocaleBuilder* builder, UErrorCode *outErrorCode) {