diff --git a/doc/ncdu.pod b/doc/ncdu.pod
index ef2d4b0f3527e6a7385f21df5ffb56aaa5383f6b..3d8df5b335ce93af91fe21550a5c7a2c3096aaac 100644
--- a/doc/ncdu.pod
+++ b/doc/ncdu.pod
@@ -276,6 +276,10 @@ links, and will thus be scanned and counted multiple times.
 Some minor glitches may appear when displaying filenames that contain multibyte
 or multicolumn characters.
 
+All sizes are internally represented as a signed 64bit integer. If you have a
+directory larger than 8 EiB minus one byte, ncdu will clip its size to 8 EiB
+minus one byte.
+
 Please report any other bugs you may find at the bug tracker, which can be
 found on the web site at http://dev.yorhel.nl/ncdu
 
diff --git a/src/dir_mem.c b/src/dir_mem.c
index 3001c014afa943b430c58c4973b3b34e7983d56f..d13ba24b8561bfaf0d50604b4710f540488dd7fb 100644
--- a/src/dir_mem.c
+++ b/src/dir_mem.c
@@ -84,8 +84,8 @@ static void hlink_check(struct dir *d) {
           if(pt==par)
             i=0;
     if(i) {
-      par->size += d->size;
-      par->asize += d->asize;
+      par->size = adds64(par->size, d->size);
+      par->asize = adds64(par->size, d->asize);
     }
   }
 }
diff --git a/src/global.h b/src/global.h
index 7249bcd0c664dbcaf70eda216834c48c095d4dd6..d6eb2cb4a41ab45ca07cee5939973c63cc98de98 100644
--- a/src/global.h
+++ b/src/global.h
@@ -29,6 +29,7 @@
 #include "config.h"
 #include <stdio.h>
 #include <stddef.h>
+#include <limits.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 
diff --git a/src/util.c b/src/util.c
index d519581970f56deb429e87b5aec7f7e22659e5a8..ce000e6498a09103010105f2ffca5931cef25b7e 100644
--- a/src/util.c
+++ b/src/util.c
@@ -287,8 +287,8 @@ struct dir *getroot(struct dir *d) {
 
 void addparentstats(struct dir *d, int64_t size, int64_t asize, int items) {
   while(d) {
-    d->size += size;
-    d->asize += asize;
+    d->size = adds64(d->size, size);
+    d->asize = adds64(d->asize, asize);
     d->items += items;
     d = d->parent;
   }
diff --git a/src/util.h b/src/util.h
index 8b1b959a3cb30175dc1bea2c4d17a2c1148cfbf0..5c638766f1c4d7306f96530221d7d66dec0ecb58 100644
--- a/src/util.h
+++ b/src/util.h
@@ -81,6 +81,12 @@ char *getpath(struct dir *);
 /* returns the root element of the given dir struct */
 struct dir *getroot(struct dir *);
 
+/* Add two positive signed 64-bit integers. Returns INT64_MAX if the result
+ * would overflow.
+ * I use uint64_t's to detect the overflow, as (a + b < 0) relies on undefined
+ * behaviour, and (INT64_MAX - b >= a) didn't work for some reason. */
+#define adds64(a, b) ((uint64_t)(a) + (uint64_t)(b) > (uint64_t)INT64_MAX ? INT64_MAX : (a)+(b))
+
 /* Adds a value to the size, asize and items fields of *d and its parents */
 void addparentstats(struct dir *, int64_t, int64_t, int);