diff --git a/arrow-array/src/array/mod.rs b/arrow-array/src/array/mod.rs
index 50c5699bac32..e0ebd6678756 100644
--- a/arrow-array/src/array/mod.rs
+++ b/arrow-array/src/array/mod.rs
@@ -1026,17 +1026,6 @@ mod tests {
         );
     }
 
-    #[test]
-    fn test_memory_size_primitive_sliced() {
-        let arr = PrimitiveArray::<Int64Type>::from_iter_values(0..128);
-        let slice1 = arr.slice(0, 64);
-        let slice2 = arr.slice(64, 64);
-
-        // both slices report the full buffer memory usage, even though the buffers are shared
-        assert_eq!(slice1.get_array_memory_size(), arr.get_array_memory_size());
-        assert_eq!(slice2.get_array_memory_size(), arr.get_array_memory_size());
-    }
-
     #[test]
     fn test_memory_size_primitive_nullable() {
         let arr: PrimitiveArray<Int64Type> = (0..128)
diff --git a/arrow-buffer/src/buffer/immutable.rs b/arrow-buffer/src/buffer/immutable.rs
index 8d1a46583fca..b703bd7d54f8 100644
--- a/arrow-buffer/src/buffer/immutable.rs
+++ b/arrow-buffer/src/buffer/immutable.rs
@@ -164,7 +164,7 @@ impl Buffer {
     /// For externally owned buffers, this returns zero
     #[inline]
     pub fn capacity(&self) -> usize {
-        self.data.capacity()
+        self.length + self.data.capacity() - self.data.len()
     }
 
     /// Returns whether the buffer is empty.