Sizing rectangular tube end caps is a three-number job: width, height, and wall thickness. Miss any one of those readings and a CNC aluminum cap will feel loose, sit proud, or refuse to start - even when the cut list sounded right on paper.
This how-to shows fabricators how to measure rectangle (and rectangular) aluminum tube for a permanent interference fit, how stock titles map to those three numbers, and when to leave the rectangle end caps collection for a custom size end cap.
Shop the rectangle collection first
If the section is rectangle - two different outside dimensions - start in rectangle end caps. That collection is filtered for rectangle profiles so you are not sorting past square or round SKUs. Use the full aluminum end caps range when a job mixes shapes, or jump to square and round when the cut list is not rectangle.
Search phrases like rectangular tube end caps, rectangle tube end caps, aluminum rectangle tube end caps, and rectangle tube end cap all point at the same product: CNC-machined aluminum caps sized to close open rectangle tube ends. Nelsons does not run a separate "pipe-only" rectangle line - if drawings say pipe or hollow section, translate the label into measured width, height, and wall the same way as tube.
Width, height, and wall - the three size keys
Stock rectangle titles lead with the two outside faces, then wall thickness. A true match means the outer face of the cap finishes the end cleanly while the insert engages the internal opening created by those outside sizes minus the wall on each side.
Before you order:
- Outside width - measure across the short or long face at the open end with digital calipers; do not trust a packing-list habit like "2 by 4 tube"
- Outside height - measure the other outside axis at the same cut; rectangle means the two outside numbers differ
- Wall thickness - measure the wall at the open end; the insert is sized to the internal opening, not to a single generic nickname
- Orientation - write which number is width vs height the way the shop will install the tube so the order form matches the cut list
- Cut quality - square, deburred ends install more predictably than rough saw cuts or crushed tips
- Coating build-up - heavy powder or paint inside the mouth can shrink the effective opening even when bare outside sizes look correct
Full caliper method: how to measure aluminum tube for end caps. Wrong width, height, or wall is the usual reason a rectangle cap feels loose, rocks on one axis, or will not start.
How stock sizes are written on nelsonscnc.com
On the rectangle collection, stock is listed as outside x outside x wall (inches) - for example 1" x 2" x 0.125", 2" x 4" x 0.125", or 3" x 6" x 0.188". Read all three numbers. Matching only the two outside faces and ignoring wall is how near-size mistakes get ordered at quantity.
Two tubes can share the same outside width and height and still need different caps. A 2" x 4" section with a thin wall has a larger internal opening than a 2" x 4" section with a heavy wall. The interference-fit insert is machined for that internal geometry, not for a single generic "2 by 4" label.
A correct rectangular tube end cap should:
- Start squarely on both axes with controlled rubber-mallet taps
- Seat fully so the outer face finishes the end cleanly on width and height
- Hold under normal handling without adhesive, screws, or welds
- Track aluminum thermal movement on outdoor and architectural work better than a soft plastic plug
That is the practical meaning of an interference fit on rectangle tube. Soft multi-fit covers solve temporary protection; they are not a substitute when the end is visible or outdoors. Material trade-offs: aluminum vs plastic tube end caps.
Square edge vs radius edge on rectangle tube
Rectangle stock is sold in two edge families:
- Square edge - for tube with sharp or near-sharp outside corners
- Radius edge - for tube with radiused outside corners (the radius value is part of the size match)
Outside width, height, and wall can look right and still fail if the corner family is wrong. If the extrusion has a visible radius on the outside corners, confirm radius-edge stock rather than forcing a square-edge cap. Detail guide: square edge vs radius edge aluminum end caps.
Rectangle vs square - do not swap them
Square tube has matching outside axes. Rectangle tube does not. A published square size that shares one familiar number with your cut list is still the wrong shape if the other axis differs.
- Rectangle end caps when width and height differ
- Square end caps when both outside axes match (and edge type is confirmed)
- Round end caps for round OD sections
- Aluminum end caps when you need the full shape map in one place
Square buyers can also use how to choose aluminum end caps for square tube. Do not substitute a square cap onto rectangle tube because one published dimension looks similar.
Pipe language vs tube language on rectangle sections
Pipe and tube are the same end-cap product family once you stop treating the word as the size. Nominal labels and schedule nicknames often do not equal the numbers on the cut end. Imported or coated stock still drifts even when the drawing looked clean.
Work the same checklist either way:
- Ignore the word on the cut list long enough to put calipers on the open end
- Record outside width, outside height, and wall in inches (or convert carefully if the drawing is metric)
- Confirm corner family - square edge or radius edge
- Match a published rectangle end caps size to those readings
- If nothing matches, open custom rather than forcing a near size
If your search started as pipe caps or hollow-section covers, keep the same path - measure, then shop rectangle stock. See also aluminum pipe end caps vs tube end caps and tube end covers vs end caps when the RFQ used finish language instead of catalog language.
Buying checklist for rectangular tube end caps
- Confirm shape - rectangle only. Square and round are different collections even when one dimension looks familiar.
- Record width, height, and wall - write caliper numbers on the cut list, not a memory of "standard 2x4".
- Confirm edge type - square edge or radius edge to match the extrusion corners.
- Match stock in rectangle end caps - prefer exact outside x outside x wall over "close enough".
- Order quantity with a buffer - extra ends, site damage, and future extensions beat a second rush shipment.
- Plan install - square clean cuts, rubber mallet, follow how to install aluminum end caps.
When stock width x height x wall is wrong
Skip forcing a near size when:
- Measured width, height, or wall sits between published stock sizes
- The extrusion is die-specific, metric, or mixed-unit
- Outside axes were swapped on the order form relative to how the tube installs
- Corner radius does not match the square-edge or radius-edge family on the shelf
- Coating inside the mouth changes the effective opening across a full batch
- Quantity is high enough that a forced near-size would fail on every end
Use custom size end caps with caliper measurements (and a drawing if you have one). Custom work is still the same product type: CNC aluminum, interference-fit, sized to your rectangle section - not a different catalog philosophy. Spec checklist: what to specify for a fast accurate quote. Common fit questions also sit on the FAQ.
Quick decision path
- Do the two outside axes differ? If yes, stay in rectangle end caps.
- Measure outside width, outside height, and wall on the actual cut ends (measuring guide).
- Confirm square edge vs radius edge on the extrusion corners.
- Match published outside x outside x wall. If pipe or hollow-section language started the search, still use the caliper trio - not the nickname.
- If nothing matches, open Custom Size with real numbers - do not force a loose or crushed fit.
- Install per how to install once the size is correct.
FAQ - rectangular tube end cap sizing
Is outside width and height enough to order rectangle end caps?
No. The two outside faces pick the right size family, but wall thickness decides the internal opening the insert must fit. Order width x height x wall together, and confirm edge type.
Do rectangle pipe and rectangle tube need different caps?
No separate product family. Measure outside width, height, and wall on the section you will close, then match rectangle end caps or request custom. The word on the drawing does not replace calipers.
What if my outside sizes match stock but the wall does not?
Do not buy the outside-only near size. Either find the published width x height x wall trio that matches all three readings, or use custom size end caps with the measured set.
Can I use a square end cap on rectangle tube?
Only if both outside axes truly match - and then it is square tube, not rectangle. If width and height differ, stay in the rectangle collection or go custom.
Bottom line
Aluminum rectangular tube end caps are sized by measured outside width, outside height, and wall thickness - plus the correct square or radius edge - not by a nominal nickname. Start in the rectangle end caps collection, confirm all three numbers on the batch you will install, and reserve custom size for non-stock sections. When the match is right, you get a clean aluminum finish and a true interference fit that holds without adhesive.
Browse rectangular tube end caps for stock sizes, keep the measure guide next to the calipers, and use installation guidance when the caps arrive on site.