We only provide the specifications that Gregory gives us as far as pack volume. In general, a total volume (in this case 24 liters) includes all available storage compartments (both the main compartment and the helmet compartment sounds accurate). The dimensions we provided are also an approximate dimension. The pack has kind of a trapezoidal shape/non-rectangular shape, so the main compartment may have a little more space than just the 16 liters from the dimensions we measured (we measured in the middle of the back-panel for the width, and the bottom of the pack for depth).