diff --git a/icu4c/source/common/unicode/unistr.h b/icu4c/source/common/unicode/unistr.h
index 48fe2b3b1d6..f841594abbe 100644
--- a/icu4c/source/common/unicode/unistr.h
+++ b/icu4c/source/common/unicode/unistr.h
@@ -1,6 +1,6 @@
 /*
 **********************************************************************
-*   Copyright (C) 1998-2014, International Business Machines
+*   Copyright (C) 1998-2015, International Business Machines
 *   Corporation and others.  All Rights Reserved.
 **********************************************************************
 *
@@ -173,15 +173,52 @@ class UnicodeStringAppendable;  // unicode/appendable.h
 # endif
 #endif
 
+/**
+ * \def UNISTR_OBJECT_SIZE
+ * Desired sizeof(UnicodeString) in bytes.
+ * It should be a multiple of sizeof(pointer) to avoid unusable space for padding.
+ * The object size may want to be a multiple of 16 bytes,
+ * which is a common granularity for heap allocation.
+ *
+ * Any space inside the object beyond sizeof(vtable pointer) + 2
+ * is available for storing short strings inside the object.
+ * The bigger the object, the longer a string that can be stored inside the object,
+ * without additional heap allocation.
+ *
+ * Depending on a platform's pointer size, pointer alignment requirements,
+ * and struct padding, the compiler will usually round up sizeof(UnicodeString)
+ * to 4 * sizeof(pointer) (or 3 * sizeof(pointer) for P128 data models),
+ * to hold the fields for heap-allocated strings.
+ * Such a minimum size also ensures that the object is easily large enough
+ * to hold at least 2 UChars, for one supplementary code point (U16_MAX_LENGTH).
+ *
+ * sizeof(UnicodeString) >= 48 should work for all known platforms.
+ *
+ * For example, on a 64-bit machine where sizeof(vtable pointer) is 8,
+ * sizeof(UnicodeString) = 64 would leave space for
+ * (64 - sizeof(vtable pointer) - 2) / U_SIZEOF_UCHAR = (64 - 8 - 2) / 2 = 27
+ * UChars stored inside the object.
+ *
+ * The minimum object size on a 64-bit machine would be
+ * 4 * sizeof(pointer) = 4 * 8 = 32 bytes,
+ * and the internal buffer would hold up to 11 UChars in that case.
+ *
+ * @see U16_MAX_LENGTH
+ * @draft ICU 56
+ */
+#ifndef UNISTR_OBJECT_SIZE
+# define UNISTR_OBJECT_SIZE 64
+#endif
+
 /**
  * UnicodeString is a string class that stores Unicode characters directly and provides
- * similar functionality as the Java String and StringBuffer classes.
+ * similar functionality as the Java String and StringBuffer/StringBuilder classes.
  * It is a concrete implementation of the abstract class Replaceable (for transliteration).
  *
  * The UnicodeString class is not suitable for subclassing.
  *
  * <p>For an overview of Unicode strings in C and C++ see the
- * <a href="http://icu-project.org/userguide/strings.html">User Guide Strings chapter</a>.</p>
+ * <a href="http://userguide.icu-project.org/strings#TOC-Strings-in-C-C-">User Guide Strings chapter</a>.</p>
  *
  * <p>In ICU, a Unicode string consists of 16-bit Unicode <em>code units</em>.
  * A Unicode character may be stored with either one code unit
@@ -3474,9 +3511,12 @@ private:
 
   // constants
   enum {
-    // Set the stack buffer size so that sizeof(UnicodeString) is,
-    // naturally (without padding), a multiple of sizeof(pointer).
-    US_STACKBUF_SIZE= sizeof(void *)==4 ? 13 : 15, // Size of stack buffer for short strings
+    /**
+     * Size of stack buffer for short strings.
+     * Must be at least U16_MAX_LENGTH for the single-code point constructor to work.
+     * @see UNISTR_OBJECT_SIZE
+     */
+    US_STACKBUF_SIZE=(int32_t)(UNISTR_OBJECT_SIZE-sizeof(void *)-2)/U_SIZEOF_UCHAR,
     kInvalidUChar=0xffff, // U+FFFF returned by charAt(invalid index)
     kGrowSize=128, // grow size for this buffer
     kInvalidHashCode=0, // invalid hash code
@@ -3544,9 +3584,10 @@ private:
    * (Padding at the end of fFields is ok:
    * As long as it is no larger than fStackFields, it is not wasted space.)
    *
-   * For some of the history of the UnicodeString class fields layout,
-   * see ICU ticket #11336 "UnicodeString: recombine stack buffer arrays"
-   * and ticket #8322 "why is sizeof(UnicodeString)==48?".
+   * For some of the history of the UnicodeString class fields layout, see
+   * - ICU ticket #11551 "longer UnicodeString contents in stack buffer"
+   * - ICU ticket #11336 "UnicodeString: recombine stack buffer arrays"
+   * - ICU ticket #8322 "why is sizeof(UnicodeString)==48?"
    */
   // (implicit) *vtable;
   union StackBufferOrFields {
@@ -3558,9 +3599,11 @@ private:
     } fStackFields;
     struct {
       int16_t fLengthAndFlags;          // bit fields: see constants above
-      UChar   *fArray;    // the Unicode data
-      int32_t fCapacity;  // capacity of fArray (in UChars)
       int32_t fLength;    // number of characters in fArray if >127; else undefined
+      int32_t fCapacity;  // capacity of fArray (in UChars)
+      // array pointer last to minimize padding for machines with P128 data model
+      // or pointer sizes that are not a power of 2
+      UChar   *fArray;    // the Unicode data
     } fFields;
   } fUnion;
 };
diff --git a/icu4c/source/common/unistr.cpp b/icu4c/source/common/unistr.cpp
index 9997aa7b57b..a995d76dacc 100644
--- a/icu4c/source/common/unistr.cpp
+++ b/icu4c/source/common/unistr.cpp
@@ -1,6 +1,6 @@
 /*
 ******************************************************************************
-* Copyright (C) 1999-2014, International Business Machines Corporation and
+* Copyright (C) 1999-2015, International Business Machines Corporation and
 * others. All Rights Reserved.
 ******************************************************************************
 *
@@ -367,8 +367,40 @@ UnicodeString::allocate(int32_t capacity) {
 //========================================
 // Destructor
 //========================================
+
+#ifdef UNISTR_COUNT_FINAL_STRING_LENGTHS
+static u_atomic_int32_t finalLengthCounts[0x400];  // UnicodeString::kMaxShortLength+1
+static u_atomic_int32_t beyondCount(0);
+
+U_CAPI void unistr_printLengths() {
+  int32_t i;
+  for(i = 0; i <= 59; ++i) {
+    printf("%2d,  %9d\n", i, (int32_t)finalLengthCounts[i]);
+  }
+  int32_t beyond = beyondCount;
+  for(; i < UPRV_LENGTHOF(finalLengthCounts); ++i) {
+    beyond += finalLengthCounts[i];
+  }
+  printf(">59, %9d\n", beyond);
+}
+#endif
+
 UnicodeString::~UnicodeString()
 {
+#ifdef UNISTR_COUNT_FINAL_STRING_LENGTHS
+  // Count lengths of strings at the end of their lifetime.
+  // Useful for discussion of a desirable stack buffer size.
+  // Count the contents length, not the optional NUL terminator nor further capacity.
+  // Ignore open-buffer strings and strings which alias external storage.
+  if((fUnion.fFields.fLengthAndFlags&(kOpenGetBuffer|kReadonlyAlias|kWritableAlias)) == 0) {
+    if(hasShortLength()) {
+      umtx_atomic_inc(finalLengthCounts + getShortLength());
+    } else {
+      umtx_atomic_inc(&beyondCount);
+    }
+  }
+#endif
+
   releaseArray();
 }
 
diff --git a/icu4c/source/test/cintltst/cintltst.c b/icu4c/source/test/cintltst/cintltst.c
index 1448e0633ef..5b883e2a09b 100644
--- a/icu4c/source/test/cintltst/cintltst.c
+++ b/icu4c/source/test/cintltst/cintltst.c
@@ -1,6 +1,6 @@
 /********************************************************************
  * COPYRIGHT:
- * Copyright (c) 1997-2013, International Business Machines Corporation and
+ * Copyright (c) 1997-2015, International Business Machines Corporation and
  * others. All Rights Reserved.
  ********************************************************************/
 /********************************************************************************
@@ -73,6 +73,10 @@ void ctest_setICU_DATA(void);
 static int gOrigArgc;
 static const char* const * gOrigArgv;
 
+#ifdef UNISTR_COUNT_FINAL_STRING_LENGTHS
+U_CAPI void unistr_printLengths();
+#endif
+
 int main(int argc, const char* const argv[])
 {
     int nerrors = 0;
@@ -233,6 +237,10 @@ int main(int argc, const char* const argv[])
 
     }  /* End of loop that repeats the entire test, if requested.  (Normally doesn't loop)  */
 
+#ifdef UNISTR_COUNT_FINAL_STRING_LENGTHS
+    unistr_printLengths();
+#endif
+
     endTime = uprv_getRawUTCtime();
     diffTime = (int32_t)(endTime - startTime);
     printf("Elapsed Time: %02d:%02d:%02d.%03d\n",
diff --git a/icu4c/source/test/intltest/intltest.cpp b/icu4c/source/test/intltest/intltest.cpp
index cfd944a1849..63ad6dc912b 100644
--- a/icu4c/source/test/intltest/intltest.cpp
+++ b/icu4c/source/test/intltest/intltest.cpp
@@ -1,6 +1,6 @@
 /********************************************************************
  * COPYRIGHT:
- * Copyright (c) 1997-2014, International Business Machines Corporation and
+ * Copyright (c) 1997-2015, International Business Machines Corporation and
  * others. All Rights Reserved.
  ********************************************************************/
 
@@ -1166,6 +1166,10 @@ IntlTest::run_phase2( char* name, char* par ) // supports reporting memory leaks
 #   define TRY_CNV_2 "sjis"
 #endif
 
+#ifdef UNISTR_COUNT_FINAL_STRING_LENGTHS
+U_CAPI void unistr_printLengths();
+#endif
+
 int
 main(int argc, char* argv[])
 {
@@ -1524,6 +1528,10 @@ main(int argc, char* argv[])
         u_cleanup();
     }
 
+#ifdef UNISTR_COUNT_FINAL_STRING_LENGTHS
+    unistr_printLengths();
+#endif
+
     fprintf(stdout, "--------------------------------------\n");
 
     if (execCount <= 0) {
diff --git a/icu4c/source/test/intltest/ustrtest.cpp b/icu4c/source/test/intltest/ustrtest.cpp
index 97151d956a2..a730be86f50 100644
--- a/icu4c/source/test/intltest/ustrtest.cpp
+++ b/icu4c/source/test/intltest/ustrtest.cpp
@@ -1,6 +1,6 @@
 /********************************************************************
  * COPYRIGHT: 
- * Copyright (c) 1997-2014, International Business Machines Corporation and
+ * Copyright (c) 1997-2015, International Business Machines Corporation and
  * others. All Rights Reserved.
  ********************************************************************/
 
@@ -2098,20 +2098,35 @@ UnicodeStringTest::TestUnicodeStringImplementsAppendable() {
 void
 UnicodeStringTest::TestSizeofUnicodeString() {
     // See the comments in unistr.h near the declaration of UnicodeString's fields.
+    // See the API comments for UNISTR_OBJECT_SIZE.
     size_t sizeofUniStr=sizeof(UnicodeString);
-    size_t expected;
-    switch(sizeof(void *)) {
-    case 4:
-        expected=32;
-        break;
-    case 8:
-        expected=40;
-        break;
-    default:
-        logln("This platform has neither 32-bit nor 64-bit pointers.");
-        return;
-    }
+    size_t expected=UNISTR_OBJECT_SIZE;
     if(expected!=sizeofUniStr) {
-        errln("sizeof(UnicodeString)=%d, expected %d", (int)sizeofUniStr, (int)expected);
+        // Possible cause: UNISTR_OBJECT_SIZE may not be a multiple of sizeof(pointer),
+        // of the compiler might add more internal padding than expected.
+        errln("sizeof(UnicodeString)=%d, expected UNISTR_OBJECT_SIZE=%d",
+              (int)sizeofUniStr, (int)expected);
+    }
+    if(sizeofUniStr<32) {
+        errln("sizeof(UnicodeString)=%d < 32, probably too small", (int)sizeofUniStr);
+    }
+    // We assume that the entire UnicodeString object,
+    // minus the vtable pointer and 2 bytes for flags and short length,
+    // is available for internal storage of UChars.
+    int32_t expectedStackBufferLength=((int32_t)UNISTR_OBJECT_SIZE-sizeof(void *)-2)/U_SIZEOF_UCHAR;
+    UnicodeString s;
+    const UChar *emptyBuffer=s.getBuffer();
+    for(int32_t i=0; i<expectedStackBufferLength; ++i) {
+        s.append((UChar)0x2e);
+    }
+    const UChar *fullBuffer=s.getBuffer();
+    if(fullBuffer!=emptyBuffer) {
+        errln("unexpected reallocation when filling with assumed stack buffer size of %d",
+              expectedStackBufferLength);
+    }
+    const UChar *terminatedBuffer=s.getTerminatedBuffer();
+    if(terminatedBuffer==emptyBuffer) {
+        errln("unexpected keeping stack buffer when overfilling assumed stack buffer size of %d",
+              expectedStackBufferLength);
     }
 }