diff --git a/xo-alloc2/include/xo/alloc2/AAllocator.hpp b/xo-alloc2/include/xo/alloc2/AAllocator.hpp
index bcb75fa8..4cb2d4e3 100644
--- a/xo-alloc2/include/xo/alloc2/AAllocator.hpp
+++ b/xo-alloc2/include/xo/alloc2/AAllocator.hpp
@@ -19,6 +19,12 @@ namespace xo {
          *
          **/
         struct AAllocator {
+            /** @defgroup mm-allocator-type-traits allocator type traits **/
+            ///@{
+            /** @brief type used for allocation amounts **/
+            using size_type = std::size_t;
+            ///@}
+
             /*
              *    <----------------------------size-------------------------->
              *    <------------committed-----------><-------uncommitted------>
@@ -44,19 +50,20 @@ namespace xo {
              *  Includes committed + uncommitted memory.
              *  Cannot be increased.
              **/
-            virtual std::size_t reserved(Copaque d) const = 0;
-            /** allocator size in bytes (up to reserved limit)
-             *  for allocator @p d.
-             *  Includes all committed memory.
+            virtual size_type reserved(Copaque d) const = 0;
+            /** Synonym for @ref committed.
              *  Can increase on @ref alloc
              **/
-            virtual std::size_t size(Copaque d) const = 0;
+            virtual size_type size(Copaque d) const = 0;
             /** committed size (physical addresses obtained)
              *  for allocator @p d.
+             *  @ref alloc may auto-increase this
              **/
-            virtual std::size_t committed(Copaque d) const = 0;
+            virtual size_type committed(Copaque d) const = 0;
             /** unallocated (but committed) size in bytes for allocator @p d **/
-            virtual std::size_t available(Copaque d) const = 0;
+            virtual size_type available(Copaque d) const = 0;
+            /** allocated (i.e. in-use) amount in bytes for allocator @p d **/
+            virtual size_type allocated(Copaque d) const = 0;
             /** true iff allocator @p d is responsible for memory at address @p p.
              **/
             virtual bool contains(Copaque d, const void * p) const = 0;
diff --git a/xo-alloc2/include/xo/alloc2/IAllocator_Any.hpp b/xo-alloc2/include/xo/alloc2/IAllocator_Any.hpp
index e344403a..18f06f87 100644
--- a/xo-alloc2/include/xo/alloc2/IAllocator_Any.hpp
+++ b/xo-alloc2/include/xo/alloc2/IAllocator_Any.hpp
@@ -36,6 +36,7 @@ namespace xo {
             [[noreturn]] size_type           size(Copaque) const override { _fatal(); }
             [[noreturn]] size_type      committed(Copaque) const override { _fatal(); }
             [[noreturn]] size_type      available(Copaque) const override { _fatal(); }
+            [[noreturn]] size_type      allocated(Copaque) const override { _fatal(); }
             [[noreturn]] bool            contains(Copaque, const void *) const override { _fatal(); }
 
             [[noreturn]] bool              expand(Opaque, std::size_t) const override { _fatal(); }
diff --git a/xo-alloc2/include/xo/alloc2/IAllocator_DArena.hpp b/xo-alloc2/include/xo/alloc2/IAllocator_DArena.hpp
index 2a0a7036..bd5551ab 100644
--- a/xo-alloc2/include/xo/alloc2/IAllocator_DArena.hpp
+++ b/xo-alloc2/include/xo/alloc2/IAllocator_DArena.hpp
@@ -35,6 +35,7 @@ namespace xo {
             static size_type size(const DArena &);
             static size_type committed(const DArena &);
             static size_type available(const DArena &);
+            static size_type allocated(const DArena &);
             static bool contains(const DArena &, const void * p);
 
             /** expand committed space in arena @p d
diff --git a/xo-alloc2/include/xo/alloc2/IAllocator_Xfer.hpp b/xo-alloc2/include/xo/alloc2/IAllocator_Xfer.hpp
index 838cc0db..cedc37a2 100644
--- a/xo-alloc2/include/xo/alloc2/IAllocator_Xfer.hpp
+++ b/xo-alloc2/include/xo/alloc2/IAllocator_Xfer.hpp
@@ -40,6 +40,9 @@ namespace xo {
             size_type      available(Copaque d) const override {
                 return I::available(_dcast(d));
             }
+            size_type      allocated(Copaque d) const override {
+                return I::allocated(_dcast(d));
+            }
 
             bool            contains(Copaque d, const void * p) const override {
                 return Impl::contains(_dcast(d), p);
diff --git a/xo-alloc2/include/xo/alloc2/RAllocator.hpp b/xo-alloc2/include/xo/alloc2/RAllocator.hpp
index a47f4828..3701d997 100644
--- a/xo-alloc2/include/xo/alloc2/RAllocator.hpp
+++ b/xo-alloc2/include/xo/alloc2/RAllocator.hpp
@@ -28,6 +28,7 @@ namespace xo {
             size_type size() const { return O::iface()->size(O::data()); }
             size_type committed() const { return O::iface()->committed(O::data()); }
             size_type available() const { return O::iface()->available(O::data()); }
+            size_type allocated() const { return O::iface()->allocated(O::data()); }
             bool contains(const void * p) const { return O::iface()->contains(O::data(), p); }
             bool expand(size_type z) { return O::iface()->expand(O::data(), z); }
             std::byte * alloc(size_type z) { return O::iface()->alloc(O::data(), z); }
diff --git a/xo-alloc2/src/alloc2/IAllocator_DArena.cpp b/xo-alloc2/src/alloc2/IAllocator_DArena.cpp
index 73bce9d3..77b3718e 100644
--- a/xo-alloc2/src/alloc2/IAllocator_DArena.cpp
+++ b/xo-alloc2/src/alloc2/IAllocator_DArena.cpp
@@ -40,6 +40,11 @@ namespace xo {
             return s.limit_ - s.free_;
         }
 
+        size_t
+        IAllocator_DArena::allocated(const DArena & s) {
+            return s.free_ - s.lo_;
+        }
+
         bool
         IAllocator_DArena::contains(const DArena & s,
                                           const void * p)
diff --git a/xo-alloc2/utest/arena.test.cpp b/xo-alloc2/utest/arena.test.cpp
index 02912fe8..5006c7bc 100644
--- a/xo-alloc2/utest/arena.test.cpp
+++ b/xo-alloc2/utest/arena.test.cpp
@@ -103,6 +103,7 @@ namespace xo {
             REQUIRE(a1o.reserved() % cfg.hugepage_z_ == 0);
             REQUIRE(a1o.size() == 0);
             REQUIRE(a1o.committed() == 0);
+            REQUIRE(a1o.allocated() == 0);
         }
 
         TEST_CASE("allocator-expand-1", "[alloc2][AAllocator]")
@@ -115,6 +116,7 @@ namespace xo {
             obj<AAllocator, DArena> a1o{&arena};
 
             REQUIRE(a1o.available() == 0);
+            REQUIRE(a1o.allocated() == 0);
 
             size_t z2 = 512;
             REQUIRE(a1o.expand(z2));
@@ -126,6 +128,7 @@ namespace xo {
             REQUIRE(a1o.size() == a1o.committed());
             REQUIRE(a1o.available() >= z2);
             REQUIRE(a1o.available() == a1o.committed());
+            REQUIRE(a1o.allocated() == 0);
 
 #ifdef NOPE
             byte * m = a1o.alloc(1);