diff --git a/folly/IPAddressV6.h b/folly/IPAddressV6.h
index ed64a75..d4876af 100644
--- a/folly/IPAddressV6.h
+++ b/folly/IPAddressV6.h
@@ -82,6 +82,10 @@ class IPAddressV6 : boost::totally_ordered<IPAddressV6> {
   // Binary prefix for 6to4 networks
   static const uint32_t PREFIX_6TO4;
 
+  // Size of std::string returned by toFullyQualified.
+  static constexpr size_t kToFullyQualifiedSize =
+    8 /*words*/ * 4 /*hex chars per word*/ + 7 /*separators*/;
+
   /**
    * Create a new IPAddress instance from the provided binary data.
    * @throws IPAddressFormatException if the input length is not 16 bytes.
diff --git a/folly/test/IPAddressTest.cpp b/folly/test/IPAddressTest.cpp
index 9f39b5a..2e62f5d 100644
--- a/folly/test/IPAddressTest.cpp
+++ b/folly/test/IPAddressTest.cpp
@@ -409,6 +409,11 @@ TEST(IPAddress, ToFullyQualifiedLocal) {
   EXPECT_EQ("0000:0000:0000:0000:0000:0000:0000:0001", ip.toFullyQualified())
       << ip;
 }
+TEST(IPAddress, ToFullyQualifiedSize) {
+  auto actual = IPAddressV6::kToFullyQualifiedSize;
+  auto expected = IPAddress("::").toFullyQualified().size();
+  EXPECT_EQ(expected, actual);
+}
 
 // test v4-v6 mapped addresses
 TEST_P(IPAddressMappedTest, MappedEqual) {