diff --git a/src/backend/utils/adt/dbsize.c b/src/backend/utils/adt/dbsize.c
index 768b04ee8d1..a6094ceaddb 100644
--- a/src/backend/utils/adt/dbsize.c
+++ b/src/backend/utils/adt/dbsize.c
@@ -427,9 +427,15 @@ pg_size_pretty(PG_FUNCTION_ARGS)
 							 (size + mult / 2) / mult);
 				else
 				{
+					/* Here we have to worry about avoiding overflow */
+					int64	val;
+
 					mult *= 1024;
+					val = size / mult;
+					if ((size % mult) >= (mult / 2))
+						val++;
 					snprintf(VARDATA(result), 50, INT64_FORMAT " TB",
-							 (size + mult / 2) / mult);
+							 val);
 				}
 			}
 		}