Should I use internal or external dimensions when choosing a case?

Use internal dimensions to decide what fits, and external dimensions to decide where the case will go. They differ by two wall thicknesses plus lid depth. If a supplier only publishes external dimensions, ask for the internal drawing for that specific model rather than estimating the difference yourself.

How do I measure an item with a lens hood, knobs or connectors?

Measure it in the state it will travel in. A lens hood that can be removed is still usually shipped fitted, so measure it fitted. Knobs, sliders and screens determine top clearance. Connectors and plugs determine the real protrusion once mated. Measure each one and write the height down - do not eyeball it.

How thick should the foam wall be?

Thick enough that the contents cannot bottom the foam out against the shell in the worst orientation. Required thickness depends on content weight, drop height and foam density, which is why it is set by weight class and then confirmed by a fit trial and drop test, not by a rule of thumb alone. Heavier contents need thicker walls and denser foam.

Will the lid foam press on my equipment?

It can. Lid foam is there to hold the top of the contents down, and if the top of your item has knobs, a screen or a connector, closed-lid pressure will land on exactly those parts. Check with the lid actually closed and latched, and relieve the lid foam where needed. This is the step most often skipped and most often the cause of damage.

Should I leave extra room for future accessories?

Yes, but leave it as planned unused foam, not as slack around the current item. A cavity that is slightly too large lets the item move, and movement wastes the crush distance the foam was providing. Plan a spare bay or leave a block uncut, and keep every cavity fitting snugly.

How to Size a Protective Case: Measuring Guide

The most common sizing failure is not picking the wrong material. It is picking the wrong size. Too small and the item does not go in; too large and the item moves inside the cavity, the foam's crush distance is wasted, and impact energy goes straight into the contents. Measuring sounds trivial, but what you measure and how you measure it determines everything downstream.

Measure the object, not the idea of it

Most people picture the item, estimate a size, then look for a model that is close. Do it the other way round: place the item in the orientation it will actually travel in, measure the maximum extent on all three axes, and then find the case.

Place it as it will travel. With the lens fitted, the cable plugged in and the battery installed.
Measure maximums, not averages. The widest point is the one that decides fit.
Measure all three axes. Height is the one people forget, and it is the one the lid foam lands on.
Measure irregular shapes in several places and record the largest value on each axis.

Instrument-grade case showing internal volume for measuring reference — For fitting, the internal dimensions are what matter; external only matters for vehicle fit.

Protrusions are the real dimensions

What decides whether something fits is rarely the body. It is the parts that stick out - and these are exactly the parts people leave out of the measurement.

Protrusion	Why it gets missed	How to handle it
Lens hood, filter, side handle	It can be removed, so people assume it does not count	Measure it fitted, or design a separate bay for it
Knobs, sliders, switches, screens	Small in height but they decide top clearance	Measure to the highest point; relieve the lid foam or slot it
Connectors and fitted cables	They look like they fold away	Measure the real outward reach of the mated plug
Handles, brackets, antennas	Awkward shapes, awkward to measure	Measure separately and mark them on the drawing
Battery pack or base	Often both the heaviest and the most protruding part	Sets the weight class and the required wall thickness

Internal and external dimensions are different numbers

Always check which one the specification sheet is giving you. Internal dimensions decide what fits. External dimensions decide whether the case goes into the vehicle, onto the shelf, or into a container. The two differ by two wall thicknesses plus a lid depth, and that difference is not the same across ranges.

Question	Which dimension to use	Why
Will my equipment fit?	Internal	This is the usable envelope
Will it fit in the vehicle or rack?	External	This is the space it occupies
How many fit on a pallet?	External	Pallet planning uses the outside footprint
How much foam do I need?	Internal	Foam is cut to the internal envelope

Ask for a drawing with tolerances, not a round number on a marketing page. Round numbers are usually nominal, and nominal is not what you fit against.

Foam wall thickness is crush distance

The foam between the cavity edge and the shell wall is the travel that absorbs the impact. It has to be thick enough that, in the worst orientation, the contents never compress the foam all the way to the shell. How much travel is required depends on content weight, drop height and foam density - it is not something you can reliably work out with a simple formula, which is why the practical route is a recommended value by weight class, then confirmation by fit trial and drop test.

Heavier contents need thicker walls and denser foam. Same geometry, more energy to absorb.
Thin walls put the contents close to the shell, where corner and edge impacts are most severe.
Zone by weight inside one case. Heavy items low and towards the centre; light, fragile items isolated.
Lid foam does not replace side-wall travel. It holds the top down; the walls absorb the impact.

Clearance, slack and tolerance

A cavity needs a little clearance so the item can be lifted out - but clearance beyond that becomes movement, and movement is lost protection. Precision and fragile items want a snug fit that still releases with one hand; tools and consumables can be looser.

Reserve space for future accessories as a planned empty bay, not by enlarging current cavities.
Do not run cavities out to the boundary. Leave uncut foam at the edges to keep the sheet structurally intact.
Keep clearance around thin, breakable parts - probes, antennas, tube necks - so the foam takes the load instead of them.
Absorb manufacturing tolerance. Both the moulded shell and the cut foam have tolerances, and foam moves slightly with temperature and humidity.

Designing to zero clearance means that once tolerances stack up in the wrong direction, some units will not close or will press too hard. Leave room and verify with a first article as well as a production sample.

Lid foam interference

When the lid closes, the convoluted or flat foam in the lid presses on the top of the contents. If the top of your item carries knobs, a screen or a connector, that is exactly where the pressure lands. Close the lid, latch it, and check - then relieve the lid foam where it interferes.

One dimension sheet beats ten emails

Consolidate your measurements into a single sheet before contacting a supplier or cutting foam. It removes most of the back-and-forth and gives you something to check against when you reorder.

List every item with its maximum outside dimensions.
Name each protrusion and record its height.
State the transport orientation - which face is down.
Give the target internal dimensions and the tolerance you can accept.
Record total weight and how the weight is distributed.
Say whether the lid foam needs relieving, and where.
Attach photos, especially close-ups of the protrusions.

Common measuring mistakes

Mistake	Consequence	Correct approach
Measuring the item stripped down	Accessories then have nowhere to go	Measure in the shipping configuration
Using the carton the item came in	Transit packaging is always larger than the product	Measure the product itself
Ignoring top clearance	Lid foam presses on knobs and screens	Measure height and do a closed-lid check
Copying a nominal catalogue size	Real internal envelope differs	Ask for a tolerance drawing
Designing to zero clearance	Tolerance stack-up makes some units unclosable	Leave absorbable margin, verify on a first article
Planning no spare capacity	Any new accessory means a new foam set	Leave a planned empty bay or uncut block

Once the dimensions are settled, freeze them as a drawing and attach it to the purchase order. When you reorder, replace the foam, or change supplier, that drawing is worth more than any written description.

How to Size a Protective Case: Protrusions, Wall Thickness and Foam Clearance