diff --git a/include/xo/arena/MemorySizeInfo.hpp b/include/xo/arena/MemorySizeInfo.hpp
index 540453f..764eae6 100644
--- a/include/xo/arena/MemorySizeInfo.hpp
+++ b/include/xo/arena/MemorySizeInfo.hpp
@@ -15,15 +15,18 @@ namespace xo {
         struct MemorySizeInfo {
             using size_type = std::size_t;
 
-            MemorySizeInfo(std::string_view name, std::size_t a, std::size_t c, std::size_t r)
-              : resource_name_{name}, allocated_{a}, committed_{c}, reserved_{r}
+            MemorySizeInfo() = default;
+            MemorySizeInfo(std::string_view name, std::size_t u, std::size_t a, std::size_t c, std::size_t r)
+              : resource_name_{name}, used_{u}, allocated_{a}, committed_{c}, reserved_{r}
             {}
 
-            static MemorySizeInfo sentinel() { return MemorySizeInfo("", 0, 0, 0); }
+            static MemorySizeInfo sentinel() { return MemorySizeInfo(); }
 
             /** resource name **/
             std::string_view resource_name_;
-            /** memory in-use **/
+            /** memory used (excluding wasted space) **/
+            std::size_t used_  = 0;
+            /** memory allocated (including wasted space e.g. empty slots in hash tables **/
             std::size_t allocated_ = 0;
             /** memory committed (backed by physical memory) **/
             std::size_t committed_ = 0;
diff --git a/include/xo/arena/hashmap/HashMapStore.hpp b/include/xo/arena/hashmap/HashMapStore.hpp
index 436e88e..6393bb0 100644
--- a/include/xo/arena/hashmap/HashMapStore.hpp
+++ b/include/xo/arena/hashmap/HashMapStore.hpp
@@ -20,6 +20,7 @@ namespace xo {
                 using control_vector_type = xo::mm::DArenaVector<uint8_t>;
                 using slot_vector_type = xo::mm::DArenaVector<value_type>;
                 using MemorySizeVisitor = xo::mm::MemorySizeVisitor;
+                using MemorySizeInfo = xo::mm::MemorySizeInfo;
 
             public:
                 /** group_exp2: number of groups {x, 2^x} **/
@@ -51,8 +52,20 @@ namespace xo {
                 float load_factor() const noexcept { return size_ / static_cast<float>(n_slot_); }
 
                 void visit_pools(const MemorySizeVisitor & visitor) const {
-                    control_.visit_pools(visitor);
-                    slots_.visit_pools(visitor);
+                    // complexity here in service of HashMapStore-specific value for MemorySizeInfo.used 
+
+                    MemorySizeInfo ctl_info;
+                    MemorySizeInfo slot_info;
+
+                    control_.visit_pools([&ctl_info](const auto & x) { ctl_info = x; });
+                    slots_.visit_pools([&slot_info](const auto & x) { slot_info = x; });
+
+                    // control: 1 byte per (key,value) pair
+                    ctl_info.used_ = size_;
+                    slot_info.used_ = size_ * sizeof(value_type);
+
+                    visitor(ctl_info);
+                    visitor(slot_info);
                 }
 
                 void resize_from_empty(const std::pair<size_type,
diff --git a/src/arena/DArena.cpp b/src/arena/DArena.cpp
index 81a0cae..32cb7c8 100644
--- a/src/arena/DArena.cpp
+++ b/src/arena/DArena.cpp
@@ -167,7 +167,12 @@ namespace xo {
 
         void
         DArena::visit_pools(const MemorySizeVisitor & fn) const {
+            /** arena can't tell purpose of allocated memory;
+             *  must assume it's all used
+             **/
+
             fn(MemorySizeInfo(config_.name_,
+                              this->allocated() /*used*/,
                               this->allocated(),
                               this->committed(),
                               this->reserved()));
diff --git a/src/arena/DCircularBuffer.cpp b/src/arena/DCircularBuffer.cpp
index 372462f..e5b1c72 100644
--- a/src/arena/DCircularBuffer.cpp
+++ b/src/arena/DCircularBuffer.cpp
@@ -83,6 +83,7 @@ namespace xo {
         DCircularBuffer::visit_pools(const MemorySizeVisitor & visitor) const
         {
             visitor(MemorySizeInfo(config_.name_,
+                                   occupied_range_.size() /*used*/,
                                    occupied_range_.size(),
                                    mapped_range_.size(),
                                    reserved_range_.size()));