Guides / Modular Buildings
Almost every container house enquiry opens with a size, and almost every size that comes back is the outside of the box. The number you can put a bed or a desk in is smaller, and how much smaller is decided long before the drawing is issued: by the panel thickness, the floor build-up and the ceiling void. This page takes the three envelopes we build most often, converts each one to square metres, and shows what the wall, the road and the loading plan do to the figure.
A container house is quoted by its external envelope. The standard 20 ft unit is 6,058 × 2,438 × 2,591 mm, which is 14.77 m² of plan area; the 40 ft unit is 12,192 × 2,438 × 2,591 mm, which is 29.72 m². Neither is floor area. A 50 mm panel on each side takes 0.84 m² out of the 20 ft unit and 100 mm panels take 1.66 m², so the usable plan is 13.93 to 13.11 m² before a single internal wall goes in.
The flat-pack range is a different envelope altogether: 5,950 × 3,000 × 2,800 mm, or 17.85 m². It is 562 mm wider than a shipping container, which is an advantage on a site plan and a permitting question on the road. The rest of this page is the arithmetic behind those numbers, and the list of sizes to put on an enquiry so that two suppliers are quoting the same box.
When a quotation says 6,058 × 2,438 × 2,591 mm it is describing one specific thing, and it is not the one a buyer usually has in mind. Four measurements are in play on every project, and they answer four different questions.
| Measurement | What it covers | 20 ft example |
|---|---|---|
| External envelope | The outside of the frame, corner posts and skins. This is what a quotation names and what travels. | 6,058 × 2,438 × 2,591 mm |
| Gross plan area | Length × width of that envelope. Used for a first comparison and for freight, not for layout. | 14.77 m² |
| Internal clear size | Finished wall face to finished wall face, and finished floor to ceiling. This is what a layout fits into. | On the drawing: 13.93 to 13.11 m² depending on panel |
| Packed size and stack height | The unit knocked down, as it sits in the container. This is what the loading plan counts. | Quoted per project per container |
External sizes are the standard figures on our detachable container house page; plan areas are length × width of those figures, divided by 10&sup6; to convert mm² to m². The internal figures are the arithmetic in the wall build-up section below, and the drawing governs.
The gap between the first and third line is where most sizing arguments start. A buyer measures a room on a plan and expects the room to be 14.77 m². The supplier quotes 14.77 m² and delivers a box whose inside is 13.11 m² with 100 mm panels. Both are correct, and the conversation only gets somewhere when the enquiry asks for the internal clear size as well.
Twenty feet is 6,096 mm. The external length of a 20 ft container is 6,058 mm, which is 38 mm short, and the 40 ft length is 12,192 mm, which is exactly 40 feet. The difference is not rounding. ISO 668:2020 fixes the external dimensions of series 1 freight containers so that they interchange between ships, rail wagons, road chassis and cranes, and the 20 ft length carries a deliberate 38 mm allowance at each unit.
Put two 20 ft units end to end and you get 12,116 mm. Put them in the 40 ft slot they are designed for and you have 12,192 mm to fill, which leaves 76 mm — two allowances of 38 mm — for the corner fittings and the coupling between them. That single detail explains a number buyers notice and suppliers rarely explain: a 40 ft container house has more plan area than two 20 ft units bolted end to end, 29.72 m² against 29.54 m², because the 40 ft envelope has no joint inside it.
| Unit | Nominal | External length | Difference |
|---|---|---|---|
| 20 ft | 6,096 mm | 6,058 mm | −38 mm |
| 40 ft | 12,192 mm | 12,192 mm | 0 mm |
| Two 20 ft end to end | 12,192 mm slot | 12,116 mm of unit | 76 mm of clearance |
Nominal lengths are 20 × 304.8 mm and 40 × 304.8 mm. External lengths are ISO 668:2020 series 1 dimensions, which are also the figures on our own product page.
Two other dimensions come from the same standard and matter just as much. The width is 2,438 mm, fixed by road and ship handling rather than by anything a house needs. The height is 2,591 mm, or 8 ft 6 in; the high-cube envelope is 2,896 mm, 305 mm taller. Our flat-pack range uses both: 5,950 × 3,000 × 2,800 mm for the wider module and 6,055 × 2,438 × 2,896 mm for the one that keeps the container footprint and the high-cube height.
A sandwich panel is fitted inside the line of the frame, so every millimetre of panel on one side comes off that dimension, and the same on the other side. A 50 mm panel therefore takes 50 mm off each end of the length and 50 mm off each side of the width, which is 100 mm off the length and 100 mm off the width in total. On a small envelope that is a visible amount of floor.
| Panel each side | 20 ft (6,058 × 2,438) | 40 ft (12,192 × 2,438) | Flat pack (5,950 × 3,000) |
|---|---|---|---|
| No wall (gross) | 14.77 m² | 29.72 m² | 17.85 m² |
| 50 mm | 13.93 m² (−5.7%) | 28.27 m² (−4.9%) | 16.96 m² (−5.0%) |
| 75 mm | 13.52 m² (−8.5%) | 27.55 m² (−7.3%) | 16.53 m² (−7.4%) |
| 100 mm | 13.11 m² (−11.2%) | 26.84 m² (−9.7%) | 16.10 m² (−9.8%) |
Formula: internal area = (L − 2t) × (W − 2t), with t the panel thickness on each side. 50, 75 and 100 mm are the panel options on our flat-pack range; on the detachable range the panel thickness is specified per project. The figure is an upper bound: it ignores the corner posts that intrude at the four corners and any internal partition, so the drawing is what a layout should be set out on.
The percentages run the other way from what most people expect. A 20 ft unit loses a larger share of its floor to the same panel than a 40 ft one does, because the wall runs round a smaller area. That is one reason a 40 ft unit feels disproportionately bigger than its length suggests, and one reason small units are usually specified with thinner panels where the climate allows it.
Height has the same subtraction. The external 2,591 mm is floor build-up, clear height and ceiling void added together. On the flat-pack range the floor is an 18 mm MgO board plus PVC, so the finished floor already sits below the external dimension, and the ceiling void is whatever the services need: surface conduit and a distribution box take far less than a ducted air-conditioning spine. Ask for the finished internal height in millimetres, not for the container height.
The flat-pack module is not a container, and its size list shows it. At 5,950 × 3,000 mm the envelope gives 17.85 m², which is 3.08 m² — about 21 per cent — more plan area than a 20 ft container from almost the same building length. The extra comes entirely from width: 3,000 mm against 2,438 mm.
That 562 mm changes what a building looks like. A 3 m module is deep enough for two rows of desks with a gangway, or a bed with a circulation strip beside it, without the corridor that a 2.44 m unit forces. Two 3 m modules side by side make a 5.95 × 6.00 m building with 35.70 m², which is a small classroom, a clinic room or a two-bedroom unit rather than a row of cells.
It also changes how the module travels, and this is the part that catches importers out. A 3,000 mm module cannot go inside a 2,438 mm container, so the flat-pack ships as panels and frames stacked flat, and the 3 m width only exists once the unit is assembled on site. When the module has to move by road as an assembled box, the limit is the road, not the ship: Directive 96/53/EC sets a general maximum vehicle width of 2.55 m for international transport in the EU, with 2.60 m for refrigerated vehicles. At 3,000 mm the load is 450 mm over that, which is an abnormal load with a permit, and in some countries an escort and a night movement. A 2,438 mm unit sits inside the limit with 56 mm to spare on each side.
| Envelope | Width | Against the 2.55 m general limit | How it usually moves |
|---|---|---|---|
| Container footprint | 2,438 mm | 112 mm inside | Standard flatbed or container chassis |
| Flat-pack wide module | 3,000 mm | 450 mm outside | Panels flat-packed, or an abnormal load permit when assembled |
Widths are the external figures on our product pages. The 2.55 m and 2.60 m limits are from Directive 96/53/EC; national rules differ and some countries allow more, so the route has to be checked with the haulier before the module size is fixed.
Height has a similar logic. The same directive caps vehicle height at 4.00 m in international traffic, and a 2,896 mm high-cube module leaves about 1.10 m for the trailer bed, the twist locks and the suspension travel. That is normally enough on a standard chassis and not always enough on a low-loader with a raised deck. It is a question for the haulier, and it is worth asking before the drawing is frozen rather than after.
Almost no project is one box. A camp block, a clinic or a site office is a number of modules plus the joints between them, and the arithmetic of that is worth doing before the layout is drawn, because the joint is where area quietly disappears.
| Arrangement | Envelope | Gross plan area |
|---|---|---|
| One 20 ft unit | 6.06 × 2.44 m | 14.77 m² |
| Two 20 ft, end to end | 12.12 × 2.44 m | 29.54 m² |
| Two 20 ft, side by side | 6.06 × 4.88 m | 29.54 m² |
| One 40 ft unit | 12.19 × 2.44 m | 29.72 m² |
| Two flat-pack, side by side | 5.95 × 6.00 m | 35.70 m² |
| Two flat-pack, end to end | 11.90 × 3.00 m | 35.70 m² |
| Expandable, opened on site | Container footprint when shipped | Double the floor area, per the model |
Envelopes and areas are arithmetic from the external dimensions above. Side-by-side and end-to-end arrangements are the same area because multiplication does not care which way the modules are turned; the difference is the building shape, the roof falls and the length of the joint.
Three things come out of that table. First, a 40 ft unit and two 20 ft units side by side land on almost the same area, so the choice between them is about the route, the crane and the number of joints rather than about floor space. Second, the flat-pack pair at 35.70 m² is the only arrangement in the list that gives a room wider than a container, which is why it is the one specified for classrooms and clinics. Third, stacking changes the arithmetic again: the corner posts on the flat-pack module carry a second storey, so two units stacked give the plan area twice over with one foundation and one roof.
What the joint costs is not captured by any of those numbers. A wall that comes out still leaves a frame line, the roof has to be detailed so water leaves the building at the junction, and the two envelopes have to be checked where they meet. On the detachable range the twin-unit combination is a bolted joint, and the bolted frame is designed to be taken apart again, which is the reason the joints are specified rather than improvised on site.
Size is usually discussed as an area question and settled as a logistics one. Five things follow directly from the envelope, and each of them can send a project back to the drawing.
The loading point deserves one more sentence, because it is the most common wrong assumption in a first enquiry. Buyers often expect a larger module to halve the number of units per container. It does not work that way: what fills a container is the stack of panels and frames, and a wider module that ships as flat panels can load at a similar count to a narrower one. The number is settled in the loading plan, not from the floor area. The container loading calculator is the quick way to see how volume, not area, drives that plan.
An enquiry that names these lines comes back as a scope rather than a guess, and two suppliers answering the same list can actually be compared.
| Item | What to state | Why it decides the answer |
|---|---|---|
| External envelope | Length × width × height in mm, or the model you want | The figure that travels, and the one a quotation usually names |
| Internal clear size | Finished internal length, width and height, if the layout is already fixed | The only figure a layout can be set out on |
| Panel thickness and core | 50, 75 or 100 mm, and EPS, rock wool or glass wool | Sets the wall build-up, and therefore the internal size and the insulation |
| Floor build-up and load rating | Floor finish, and the imposed load in kN/m² the use needs | Floor thickness comes off the internal height; the rating is per project |
| Roof and overhang | Roof type and whether the overhang is inside or outside the quoted envelope | An overhang outside the footprint changes the foundation and the transport width |
| Doors and windows | Positions and sizes, against a 900 × 2,000 mm standard door | Openings are set out on the wall that the internal size just defined |
| Stacking and storeys | Single storey or stacked, and the stair and landing if stacked | The corner posts and the foundation are designed for it or not |
| Route and destination | Destination, port, and any width, height or weight limit on the last leg | A 3 m module can be the right building and the wrong load |
Two lines on that list do most of the work. The internal clear size removes the wall build-up argument before it starts, and the route removes the transport argument before a drawing is issued. With both stated, the envelope, the panel and the joint can be chosen in that order rather than discovered one at a time.
Size is the first of three questions, and the other two have their own pages. The container house cost guide sets out the inputs behind a 20 ft and a 40 ft quotation, the detachable versus flat-pack comparison covers which construction suits which project, and the units-per-container guide takes the loading plan apart in the same way this page takes the envelope apart.
For the schedule, the lead time guide covers the five stages from deposit to on-site assembly, and the shipping and unloading guide covers what happens between the container being sealed and the unit standing on its foundation. If the order quantity is the open question, the MOQ guide explains why it is quoted per project rather than as a number. And if the brief is a pod rather than a box, the space capsule comparison covers what changes when the envelope is not rectangular.
Documents a project can ask for before dispatch are listed on the certifications page, and how they are checked is described under inspection. Send the site plan, the internal size you need and the destination, and the reply comes back as an envelope with the internal clear size and the loading plan attached. Send drawings and the destination, or use the contact page.
The standard 20 ft unit is 6,058 × 2,438 × 2,591 mm and the 40 ft unit is 12,192 × 2,438 × 2,591 mm. Those are ISO 668 external dimensions, so the length and width match a shipping container and the unit can travel on container equipment. Our flat-pack range adds two envelopes of its own: 5,950 × 3,000 × 2,800 mm and 6,055 × 2,438 × 2,896 mm. A supplier quoting a round 6,000 mm or 12,000 mm is quoting a nominal size, not the envelope that will arrive.
The external footprint is 6,058 × 2,438 mm, which is 14.77 m². The floor you can stand on is smaller: with a 50 mm panel on both sides the internal clear plan is 5,958 × 2,338 mm, or 13.93 m², and with 100 mm panels it is 13.11 m². That is a loss of 5.7 to 11.2 per cent before any internal partition. Ask for the internal clear dimensions on the drawing rather than converting the external figure yourself.
Twenty feet is 6,096 mm, and ISO 668 sets the 20 ft external length 38 mm shorter, at 6,058 mm. The 40 ft container is exactly 12,192 mm, which is two 6,058 mm units plus 76 mm. The 76 mm is the clearance two 20 ft units need when they stand end to end in a 40 ft slot, which is why a 40 ft unit has 0.19 m² more plan area than two 20 ft units joined end to end (29.72 m² against 29.54 m²).
It depends on the wall build-up, which is a specification choice rather than a fixed number. A sandwich panel of 50, 75 or 100 mm is fitted on each side, so each thickness step removes 100 mm from the length and 100 mm from the width. On a 20 ft unit that moves the internal clear plan from 13.93 m² to 13.52 m² to 13.11 m². Height works the same way: the finished internal height is the external height minus the floor build-up (18 mm MgO board plus PVC on the flat-pack range) and minus the ceiling void for services.
Almost, but not by the module count. One 40 ft unit has 29.72 m² of external footprint against 14.77 m² for a 20 ft unit, so it is slightly more than double, because a single 40 ft envelope loses nothing to a joint. Two 20 ft units joined end to end give 29.54 m² of footprint and 12.12 m of building length, but the joint has to be built and sealed. What decides between them is usually the route and the lifting plan rather than the area.
Yes. The twin-unit combination on our detachable range is two units joined with a bolted joint, and the expandable model ships at container footprint and expands sideways on site for double the floor area. Two 5,950 × 3,000 mm flat-pack modules side by side make a 5.95 × 6.00 m building with 35.70 m² of footprint. What the joint costs you is a frame line, a roof junction that has to be detailed for drainage, and a structural check where the two envelopes meet.
Very often, yes. Directive 96/53/EC sets a general maximum vehicle width of 2.55 m for international road transport in the EU, and 2.60 m for refrigerated vehicles. A 3,000 mm module is 450 mm over that limit, so it moves as an abnormal load in most European jurisdictions, which means a permit and sometimes an escort. This is why the 3,000 mm module is shipped flat-packed as panels and frames: the 3 m width exists on site, not on the road.
Not directly. The count is set by the folded stack height, not by the finished floor area, so a wider module is not automatically a smaller load. For our standard unit the figure is 4 sets in a 20 ft container and 8 sets in a 40HQ, flat-packed, and the count for your own order is confirmed in the loading plan before the container is sealed.
Every square metre and percentage on this page is arithmetic from the dimensions printed beside it, using the formulas shown, and can be recomputed by hand. No unit price is published here, and the panel thickness, the floor build-up and the final internal sizes for your own unit are confirmed against your drawing at quotation stage.