diff --git a/example/box_example.cpp b/example/box_example.cpp
index 00923a81..79c93a4a 100644
--- a/example/box_example.cpp
+++ b/example/box_example.cpp
@@ -1,83 +1,87 @@
-#include <units/physical/si/derived/acceleration.h>
 #include <units/physical/si/constants.h>
+#include <units/physical/si/derived/area.h>
+#include <units/physical/si/derived/speed.h>
 #include <units/physical/si/derived/density.h>
 #include <units/physical/si/derived/force.h>
-#include <units/physical/si/base/length.h>
 #include <units/physical/si/base/mass.h>
 #include <units/physical/si/base/time.h>
 #include <units/physical/si/derived/volume.h>
 #include <units/format.h>
-#include <units/quantity_io.h>
 #include <cassert>
+#include <iostream>
 
 namespace {
 
 using namespace units::physical;
-using namespace units::physical::si::literals;
 
 using m = si::metre;
+using m2 = si::square_metre;
+using m3 = si::cubic_metre;
 using kg = si::kilogram;
 using N = si::newton;
-using m3 = si::cubic_metre;
 using kgpm3 = si::kilogram_per_metre_cub;
 
 inline constexpr auto g = si::si2019::standard_gravity<>;
+inline constexpr si::density<kgpm3> air_density(1.225);
 
-}  // namespace
 
-struct Box {
-  static constexpr auto air_density = 1.225_q_kg_per_m3;
+class Box {
+  si::area<m2> base_;
+  si::length<m> height_;
+  si::density<kgpm3> density_ = air_density;
+public:
+  constexpr Box(const si::length<m>& length, const si::length<m>& width, si::length<m> height) : base_(length * width), height_(std::move(height)) {}
 
-  si::length<m> length;
-  si::length<m> width;
-  si::length<m> height;
-
-  struct contents {
-    si::density<kgpm3> density = air_density;
-  } contents;
-
-  constexpr Box(const si::length<m>& l, const si::length<m>& w, const si::length<m>& h) : length{l}, width{w}, height{h} {}
-
-  constexpr si::force<N> filled_weight() const
+  [[nodiscard]] constexpr si::force<N> filled_weight() const
   {
-    const si::volume<m3> volume = length * width * height;
-    const si::mass<kg> mass = contents.density * volume;
+    const si::volume<m3> volume = base_ * height_;
+    const si::mass<kg> mass = density_ * volume;
     return mass * g;
   }
 
-  constexpr si::length<m> fill_level(const si::mass<kg>& measured_mass) const
+  [[nodiscard]] constexpr si::length<m> fill_level(const si::mass<kg>& measured_mass) const
   {
-    return height * (measured_mass * g) / filled_weight();
+    return height_ * measured_mass * g / filled_weight();
   }
 
-  constexpr si::volume<m3> spare_capacity(const si::mass<kg>& measured_mass) const
+  [[nodiscard]] constexpr si::volume<m3> spare_capacity(const si::mass<kg>& measured_mass) const
   {
-    return (height - fill_level(measured_mass)) * width * length;
+    return (height_ - fill_level(measured_mass)) * base_;
   }
 
   constexpr void set_contents_density(const si::density<kgpm3>& density_in)
   {
     assert(density_in > air_density);
-    contents.density = density_in;
+    density_ = density_in;
   }
 };
 
-#include <iostream>
+}  // namespace
+
 
 int main()
 {
-  auto box = Box(1000.0_q_mm, 500.0_q_mm, 200.0_q_mm);
+  using namespace units;
+  using namespace si::literals;
+
+  const si::length<m> height(200.0_q_mm);
+  auto box = Box(1000.0_q_mm, 500.0_q_mm, height);
   box.set_contents_density(1000.0_q_kg_per_m3);
 
-  const auto fill_time = 200.0_q_s;      // time since starting fill
-  const auto measured_mass = 20.0_q_kg;  // measured mass at fill_time
+  const auto fill_time = 200.0_q_s;        // time since starting fill
+  const auto measured_mass = 20.0_q_kg;    // measured mass at fill_time
+
+  const Length auto fill_level = box.fill_level(measured_mass);
+  const Dimensionless auto fill_percent = quantity_cast<percent>(fill_level / height);
+  const Volume auto spare_capacity = box.spare_capacity(measured_mass);
+  const auto input_flow_rate = measured_mass / fill_time;    // unknown dimension
+  const Speed auto float_rise_rate = fill_level / fill_time;
+  const Time auto fill_time_left = (height / fill_level - 1) * fill_time;
 
   std::cout << "mp-units box example...\n";
-  std::cout << "fill height at " << fill_time << " = " << box.fill_level(measured_mass) << " ("
-            << (box.fill_level(measured_mass) / box.height) * 100 << "% full)\n";
-  std::cout << "spare_capacity at " << fill_time << " = " << box.spare_capacity(measured_mass) << '\n';
-  std::cout << "input flow rate after " << fill_time << " = " << measured_mass / fill_time << '\n';
-  std::cout << "float rise rate = " << box.fill_level(measured_mass) / fill_time << '\n';
-  const auto fill_time_left = (box.height / box.fill_level(measured_mass) - 1) * fill_time;
-  std::cout << "box full E.T.A. at current flow rate = " << fill_time_left << '\n';
+  std::cout << fmt::format("fill height at {} = {} ({} full)\n", fill_time, fill_level, fill_percent);
+  std::cout << fmt::format("spare_capacity at {} = {}\n", fill_time, spare_capacity);
+  std::cout << fmt::format("input flow rate after {} = {}\n", fill_time, input_flow_rate);
+  std::cout << fmt::format("float rise rate = {}\n", float_rise_rate);
+  std::cout << fmt::format("box full E.T.A. at current flow rate = {}\n", fill_time_left);
 }