Enter each item's weight and its length along the container. You get where to put each item so the load's center of gravity (CG) sits at the middle.

CG of item 1, measured from the left end. Pick which end is left and keep to it. Typing this turns off auto-centering.

ITEMWEIGHTLENGTHalong container,

Nothing to place yet. Enter an item's weight and length above; where to put it shows here as you type.

Terms
Container, flatrack
A closed box or an open platform. On a flatrack this tool keeps 8 in clear at each end (an unverified value).
20′, 40′
The nominal outside length. The inside is shorter and is not modeled.
Left end
The end you measure from. Pick which end of the container is left and keep to it.
Length along container
How much of the container's length an item takes up, not its width across.
CG, center of gravity
The point an item, or the whole load, balances on. This tool takes each item's CG to be at its middle.
First item CG
Type it to fix where item 1 goes. Leave it blank and the tool centers the load for you.
Center, from the left end
Where the middle of each item should sit, measured from the left end.
Packed end to end, then centered
Items are placed touching, in row order, then the group is slid so the combined CG is at the middle.
Combined CG
The balance point of all the items together.
Off center %
How far the combined CG is from the middle, as a share of the length. The 5% caution limit is this app's own, not from a standard.
Usable length
The length items may occupy: the full length, or the flatrack length less the clear ends.
US ton, metric ton
A US ton is 2,000 lb. A metric ton is 1,000 kg (about 2,205 lb).