diff --git a/test/excache/memory_test.exs b/test/excache/memory_test.exs
new file mode 100644
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+++ b/test/excache/memory_test.exs
@@ -0,0 +1,194 @@
+defmodule Excache.MemoryTest do
+  use ExUnit.Case, async: false
+
+  alias Excache.Memory
+  alias Excache.SkipList
+  alias Excache.Store
+  alias Excache.Stream, as: CacheStream
+
+  setup do
+    Store.flushdb()
+    on_exit(&Store.flushdb/0)
+    :ok
+  end
+
+  test "entry overhead matches the real cost of an ETS insert" do
+    entries = for index <- 1..2000, do: {"probe:#{index}", :string, "value-#{index}", nil}
+
+    table = :ets.new(:memoryProbe, [:set, :public])
+    before = :ets.info(table, :memory)
+    Enum.each(entries, &:ets.insert(table, &1))
+    measured = (:ets.info(table, :memory) - before) * Memory.wordSize()
+    :ets.delete(table)
+
+    estimated = Memory.usedMemory(entries)
+
+    assert measured > 0
+    assert_in_delta estimated / measured, 1.0, 0.02
+  end
+
+  test "reference counted binaries are counted beyond the flat term size" do
+    value = :binary.copy("x", 4096)
+    flat = :erts_debug.size(value) * Memory.wordSize()
+
+    assert flat < 200
+    assert Memory.estimateValue(:string, value) == flat + 4096
+
+    nested = [:binary.copy("a", 1024), :binary.copy("b", 1024)]
+    assert Memory.estimateValue(:list, nested) >= 2048
+
+    heapBinary = :binary.copy("y", 64)
+
+    assert Memory.estimateValue(:string, heapBinary) ==
+             :erts_debug.size(heapBinary) * Memory.wordSize()
+  end
+
+  test "estimates grow with the element count of every aggregate type" do
+    pairs = [
+      {:list, listValue(2), listValue(40)},
+      {:hash, hashValue(2), hashValue(40)},
+      {:set, setValue(2), setValue(40)},
+      {:zset, zsetValue(2), zsetValue(40)},
+      {:stream, streamValue(2), streamValue(40)}
+    ]
+
+    for {kind, small, large} <- pairs do
+      assert Memory.estimateValue(kind, small) > 0, "#{kind} small estimate is not positive"
+
+      assert Memory.estimateValue(kind, large) > Memory.estimateValue(kind, small),
+             "#{kind} estimate did not grow with the element count"
+    end
+  end
+
+  test "element counts and payload bytes are reported per type" do
+    assert Memory.elementCount(:string, "abc") == 1
+    assert Memory.payloadBytes(:string, "abc") == 3
+
+    assert Memory.elementCount(:list, ["a", "bb"]) == 2
+    assert Memory.payloadBytes(:list, ["a", "bb"]) == 3
+
+    hash = %{"f1" => "v1", "f2" => "vvv2"}
+    assert Memory.elementCount(:hash, hash) == 2
+    assert Memory.payloadBytes(:hash, hash) == 2 + 2 + 2 + 4
+
+    set = MapSet.new(["a", "bb", "ccc"])
+    assert Memory.elementCount(:set, set) == 3
+    assert Memory.payloadBytes(:set, set) == 6
+
+    zset = zsetValue(2)
+    assert Memory.elementCount(:zset, zset) == 2
+    assert Memory.payloadBytes(:zset, zset) == 2 * (byte_size("member1") + Memory.wordSize())
+
+    stream = streamValue(2)
+    assert Memory.elementCount(:stream, stream) == 2
+
+    assert Memory.payloadBytes(:stream, stream) ==
+             2 * (2 * Memory.wordSize() + byte_size("field") + byte_size("value1"))
+  end
+
+  test "usedMemory sums every entry and usageByType splits them by kind" do
+    entries = [
+      {"s", :string, "value", nil},
+      {"l", :list, listValue(3), nil},
+      {"h", :hash, hashValue(3), nil},
+      {"st", :set, setValue(3), nil},
+      {"z", :zset, zsetValue(3), nil},
+      {"x", :stream, streamValue(3), nil}
+    ]
+
+    assert Memory.usedMemory(entries) ==
+             Enum.reduce(entries, 0, fn entry, total -> total + Memory.entryBytes(entry) end)
+
+    byType = Memory.usageByType(entries)
+    assert Map.keys(byType) |> Enum.sort() == Enum.sort(Memory.kinds())
+    assert Enum.all?(byType, fn {_kind, usage} -> usage.keys == 1 end)
+
+    assert byType.string.bytes == Memory.entryBytes({"s", :string, "value", nil})
+
+    assert Enum.sum(Enum.map(byType, fn {_kind, usage} -> usage.bytes end)) ==
+             Memory.usedMemory(entries)
+
+    withoutStreams = Enum.reject(entries, fn {_key, kind, _value, _ea} -> kind == :stream end)
+    assert Memory.usageByType(withoutStreams).stream == %{keys: 0, bytes: 0}
+  end
+
+  test "stats separate overhead from dataset and point at the largest key" do
+    entries = [
+      {"tiny", :string, "a", nil},
+      {"huge", :string, :binary.copy("z", 8192), 1_700_000_000_000},
+      {"mid", :list, listValue(10), nil}
+    ]
+
+    stats = Memory.stats(entries)
+
+    assert stats.keys == 3
+    assert stats.volatileKeys == 1
+    assert stats.usedMemory == Memory.usedMemory(entries)
+    assert stats.datasetBytes == stats.usedMemory - stats.overheadBytes
+    assert stats.overheadBytes == 3 * Memory.entryOverhead() + overheadKeyBytes(entries)
+    assert stats.bytesPerKey == div(stats.usedMemory, 3)
+    assert {"huge", huge} = stats.largestKey
+    assert huge >= 8192
+    assert Memory.stats([]).largestKey == nil
+    assert Memory.stats([]).bytesPerKey == 0
+  end
+
+  test "describe reports the volatile flag and separates payload from overhead" do
+    value = :binary.copy("p", 512)
+    described = Memory.describe("session:1", {:string, value, 1_700_000_000_000})
+
+    assert described.kind == :string
+    assert described.volatile
+    assert described.payloadBytes == 512
+    assert described.bytes == described.keyBytes + described.valueBytes + Memory.entryOverhead()
+    assert described.overheadBytes == described.bytes - 512
+    assert Memory.describe("k", {:string, "v", nil}).volatile == false
+  end
+
+  test "usage reads a live key and reports nil for a missing one" do
+    :ok = Store.set("alpha", "value", [])
+    {:ok, 3} = Store.rpush("beta", ["a", "b", "c"])
+
+    assert Memory.usage("alpha") == Memory.estimateEntry("alpha", Store.watch_snapshot("alpha"))
+    assert Memory.usage("ghost") == nil
+    assert Memory.describeKey("ghost") == nil
+    assert Memory.describeKey("beta").kind == :list
+    assert Memory.describeKey("beta").elements == 3
+
+    assert Memory.usedMemory() == Memory.usage("alpha") + Memory.usage("beta")
+    assert Memory.usageByType().list.keys == 1
+    assert Memory.stats().keys == 2
+  end
+
+  test "keyBytes grows with the key length" do
+    assert Memory.keyBytes(String.duplicate("k", 200)) > Memory.keyBytes("k")
+    assert Memory.entryOverhead() == 10 * Memory.wordSize()
+  end
+
+  defp overheadKeyBytes(entries) do
+    Enum.reduce(entries, 0, fn {key, _kind, _value, _ea}, total ->
+      total + Memory.keyBytes(key)
+    end)
+  end
+
+  defp listValue(count), do: Enum.map(1..count, &"item#{&1}")
+
+  defp hashValue(count), do: Map.new(1..count, fn index -> {"field#{index}", "value#{index}"} end)
+
+  defp setValue(count), do: MapSet.new(1..count, &"member#{&1}")
+
+  defp zsetValue(count) do
+    Enum.reduce(1..count, SkipList.new(), fn index, zset ->
+      SkipList.insert(zset, "member#{index}", index * 1.0)
+    end)
+  end
+
+  defp streamValue(count) do
+    Enum.reduce(1..count, CacheStream.new(), fn index, stream ->
+      {:ok, updated, _id} =
+        CacheStream.add(stream, {:full, index, 0}, ["field", "value#{index}"], 0)
+
+      updated
+    end)
+  end
+end