The Code of Safe Practice for Cargo Stowage and Securing (CSS Code, IMO A.714(17)) provides guidance for securing cargo on deck and in holds. Annex 13 provides a simplified method to calculate the external forces on cargo units using acceleration coefficients derived from the vessel's service route. The balance-of-forces method then checks that the total securing force from lashings and friction exceeds the computed external force.

MSL is the maximum force that a securing device may be subjected to. It is determined by the manufacturer and marked on the equipment. For wire rope, the MSL is typically 50% of the breaking strength. For lashing chains, MSL is often 50–75% of breaking strength. The CSS Code Annex 13 method requires that the sum of all lashing MSLs, plus friction, equals or exceeds the external forces on the cargo. Never confuse MSL with Breaking Strength (BS).

CSS Code Annex 13 Table 2 lists acceleration factors (a_x, a_y, a_z) as fractions of g for different sea areas and ship lengths. Short voyages in sheltered waters have lower coefficients (a_y ≈ 0.5g) while exposed North Atlantic winter passages can reach a_y ≈ 0.9g. Longer ships (L > 200 m) generally experience lower motion accelerations than short ships for the same sea area.

In the balance-of-forces method, all lashing forces are resolved into their vertical, transverse, and longitudinal components. The sum of upward lashing components plus the cargo weight must exceed the vertical external force; the sum of transverse lashing components plus friction must exceed the transverse external force; and so on. The CSS Code Annex 13 simplified method pre-computes the required securing force (CS) so the planner only needs to verify that total MSL × 1.5 ≥ CS.