How do I calculate the size of an equipment case?

Work outwards: load outline plus foam, finger gap and tolerance for the cavity, then add walls.

Is thicker foam always better?

No. Thickness is buffer travel, but each step costs cavity volume and packed weight. Match it to load weight and drop height.

Dividers or a cut foam cavity?

Fixed items and tight location favour foam; changing counts favour dividers, and the two combine well.

Why do equipment cases with the same outside size differ inside?

Walls, ribs, stacking faces and latch keepers take space, so compare cavity size at acceptance.

Equipment case first or liner first?

Liner first. Fix the load and liner, derive the cavity, then pick the shell that delivers it. ---

How to Choose an Equipment Case: Work Back from the Load

Quick answer: The size of an equipment case is not picked from a catalogue; it is derived from the largest outline of the load and the posture used to lift it out. Fix the load type first, then the liner, then the internal clear size, and only then the outside dimensions. In that order the size chain closes once. In reverse, the case arrives looking right and fails on the first lift.

Equipment case selection: start with the load
Three liner strategies and what each costs
Sizing backwards from the load envelope
How case-side specs follow the load
Six data points to send with an RFQ
FAQ

1. Equipment case selection: start with the load

The proportions of an equipment case follow the load, not the look of the shell. Sort the load into three types and the ratio and lid direction settle themselves; only tolerances and liner thickness remain. Reverse that order and the shell is chosen blind.

Sand-coloured case open, a square cavity cut into thick black foam

Long shafts set the internal length; width and height pull close and pad out. Panel instruments set the footprint from the faceplate, with depth following connectors and cable space. Loose loads are sized by count and largest piece, then divided rather than cut as one block.

2. Three liner strategies and what each costs

An equipment case has three liner options: a full foam cavity, adjustable dividers, or no liner with soft stowage. They trade cost, weight, access speed and cleaning effort; none wins on all four.

Olive case open with no foam, holding rolled bags, a stove and coiled rope

A cut foam cavity gives the tightest location and holds the load in a drop, at the price of volume. Dividers re-cut the space when counts change, losing precision but suiting mixed batches. A bare cavity maximises volume and cuts weight, at the cost of contact and slower counts. Thickness is buffer travel: ten extra millimetres add twenty to the cavity.

3. Sizing backwards from the load envelope

Internal clear size is an addition: load outline, plus foam on both sides, plus a finger gap, plus tolerance. Outer dimensions add wall thickness, ribs and the stacking face on top.

A gloved hand measuring the open cavity with a tape measure, foam compartments either side

A load of 400 by 300 by 150 millimetres with 20 millimetres of foam and a 30 millimetre finger gap needs about 500 by 350 by 190 inside; walls add 15 to 25 per side. The finger gap is most often left off a drawing; without it an equipment case passes inspection and still cannot be unloaded. For volume orders, one outside size with several internal layouts keeps pallets uniform.

4. How case-side specs follow the load

Heavier and more delicate loads push case-side values up; the driver is packed weight, drop exposure and handling.

Navy case on a stainless bench, folding side handles and worn corners

Load trait	Case-side response	Reason
Under 10 kg	Single handle, two latches	One person, fast opening
10 to 25 kg	Paired side handles, three latches	Two-person lift, shared load
Over 25 kg	Side handles plus wheels	Less carrying off the ground
Panel instruments	Thicker wall, separate liner bay	Impact kept off the faceplate

Weight bands are a starting point: twenty kilograms of metal and twenty of instrument face need different walls and buffer. Sealing and stacking follow the same logic and change structure, so settle them before the cavity is frozen.

5. Six data points to send with an RFQ

Six data points remove two rounds of sampling, and with all six the quote covers cavity, liner, hardware and price.

Largest outline and unit weight, with projections
Item count and fixed positions
Lifting posture and finger access
Stacking tiers and handling
Storage environment, seal grade
Labelling or numbering method

Lifting posture is most often missing: given only an outline, a supplier quotes a plain cavity, and the finished equipment case needs a scallop that eats into the size chain. The other omission is the refresh cycle, where a replaceable liner outlasts a cut cavity.

Conclusion

The order for an equipment case is load, liner, cavity, outside size, hardware. Follow it and the dimensions close on the first drawing; reverse it and problems surface at sampling.

Test any proposal on three points: load fits, hand fits, and the packed equipment case still stacks and carries.

This article covers container-side specifications and design implications only.