diff --git a/arena.h b/arena.h
index 7ec4c8f..0532dd8 100644
--- a/arena.h
+++ b/arena.h
@@ -297,7 +297,7 @@ void *arena_alloc(Arena *a, size_t size_bytes)
 
     if (a->end == NULL) {
         ARENA_ASSERT(a->begin == NULL);
-        size_t capacity = ARENA_REGION_DEFAULT_CAPACITY;
+        size_t capacity = ARENA_REGION_DEFAULT_CAPACITY / sizeof(uintptr_t);
         if (capacity < size) capacity = size;
         a->end = new_region(capacity);
         a->begin = a->end;
@@ -309,7 +309,7 @@ void *arena_alloc(Arena *a, size_t size_bytes)
 
     if (a->end->count + size > a->end->capacity) {
         ARENA_ASSERT(a->end->next == NULL);
-        size_t capacity = ARENA_REGION_DEFAULT_CAPACITY;
+        size_t capacity = ARENA_REGION_DEFAULT_CAPACITY / sizeof(uintptr_t);
         if (capacity < size) capacity = size;
         a->end->next = new_region(capacity);
         a->end = a->end->next;