diff --git a/libcxx/include/locale b/libcxx/include/locale
--- a/libcxx/include/locale
+++ b/libcxx/include/locale
@@ -1458,10 +1458,12 @@
     char __fmt[6] = {'%', 0};
     const char* __len = "l";
     this->__format_int(__fmt+1, __len, true, __iob.flags());
-    const unsigned __nbuf = (numeric_limits<long>::digits / 3)
-                          + ((numeric_limits<long>::digits % 3) != 0)
-                          + ((__iob.flags() & ios_base::showbase) != 0)
-                          + 2;
+    // Worst case is octal, with showbase enabled. Note that octal is always
+    // printed as an unsigned value.
+    const unsigned __nbuf = (numeric_limits<unsigned long>::digits / 3)          // 1 char per 3 bits
+                            + ((numeric_limits<unsigned long>::digits % 3) != 0) // round up
+                            + ((__iob.flags() & ios_base::showbase) != 0)        // base prefix
+                            + 1;                                                 // terminating null character
     char __nar[__nbuf];
     int __nc = __libcpp_snprintf_l(__nar, sizeof(__nar), _LIBCPP_GET_C_LOCALE, __fmt, __v);
     char* __ne = __nar + __nc;
@@ -1485,10 +1487,12 @@
     char __fmt[8] = {'%', 0};
     const char* __len = "ll";
     this->__format_int(__fmt+1, __len, true, __iob.flags());
-    const unsigned __nbuf = (numeric_limits<long long>::digits / 3)
-                          + ((numeric_limits<long long>::digits % 3) != 0)
-                          + ((__iob.flags() & ios_base::showbase) != 0)
-                          + 2;
+    // Worst case is octal, with showbase enabled. Note that octal is always
+    // printed as an unsigned value.
+    const unsigned __nbuf = (numeric_limits<unsigned long long>::digits / 3)          // 1 char per 3 bits
+                            + ((numeric_limits<unsigned long long>::digits % 3) != 0) // round up
+                            + ((__iob.flags() & ios_base::showbase) != 0)             // base prefix
+                            + 1;                                                      // terminating null character
     char __nar[__nbuf];
     int __nc = __libcpp_snprintf_l(__nar, sizeof(__nar), _LIBCPP_GET_C_LOCALE, __fmt, __v);
     char* __ne = __nar + __nc;
@@ -1512,10 +1516,11 @@
     char __fmt[6] = {'%', 0};
     const char* __len = "l";
     this->__format_int(__fmt+1, __len, false, __iob.flags());
-    const unsigned __nbuf = (numeric_limits<unsigned long>::digits / 3)
-                          + ((numeric_limits<unsigned long>::digits % 3) != 0)
-                          + ((__iob.flags() & ios_base::showbase) != 0)
-                          + 1;
+    // Worst case is octal, with showbase enabled.
+    const unsigned __nbuf = (numeric_limits<unsigned long>::digits / 3)          // 1 char per 3 bits
+                            + ((numeric_limits<unsigned long>::digits % 3) != 0) // round up
+                            + ((__iob.flags() & ios_base::showbase) != 0)        // base prefix
+                            + 1;                                                 // terminating null character
     char __nar[__nbuf];
     int __nc = __libcpp_snprintf_l(__nar, sizeof(__nar), _LIBCPP_GET_C_LOCALE, __fmt, __v);
     char* __ne = __nar + __nc;
@@ -1539,10 +1544,11 @@
     char __fmt[8] = {'%', 0};
     const char* __len = "ll";
     this->__format_int(__fmt+1, __len, false, __iob.flags());
-    const unsigned __nbuf = (numeric_limits<unsigned long long>::digits / 3)
-                          + ((numeric_limits<unsigned long long>::digits % 3) != 0)
-                          + ((__iob.flags() & ios_base::showbase) != 0)
-                          + 1;
+    // Worst case is octal, with showbase enabled.
+    const unsigned __nbuf = (numeric_limits<unsigned long long>::digits / 3)          // 1 char per 3 bits
+                            + ((numeric_limits<unsigned long long>::digits % 3) != 0) // round up
+                            + ((__iob.flags() & ios_base::showbase) != 0)             // base prefix
+                            + 1;                                                      // terminating null character
     char __nar[__nbuf];
     int __nc = __libcpp_snprintf_l(__nar, sizeof(__nar), _LIBCPP_GET_C_LOCALE, __fmt, __v);
     char* __ne = __nar + __nc;