diff --git a/a8/fifobuffer.cc b/a8/fifobuffer.cc
index bb48349..1629c41 100644
--- a/a8/fifobuffer.cc
+++ b/a8/fifobuffer.cc
@@ -44,7 +44,10 @@ namespace a8
             BufHead* buf_head = (BufHead*)(p - sizeof(BufHead));
             head_ += sizeof(BufHead) + buf_head->len;
             if (head_ >= buf_len_) {
+                capacity_ += sizeof(BufHead) + buf_head->len + (buf_len_ - head_);
                 head_ = 0;
+            } else {
+                capacity_ += sizeof(BufHead) + buf_head->len;
             }
         } else {
             free(p);