How Many Square Metres Fit in a Container?
| Thickness | Slate weight | Typical m² loaded | Crates in the box |
|---|---|---|---|
| 5–6 mm | ~14–16 kg/m² | ~1,700–2,030 m² | 28–34 crates (60 m² each) |
| 6–8 mm | ~16–22 kg/m² | ~1,270–1,700 m² | 21–28 crates (60 m² each) |
| 7–9 mm | ~19–24 kg/m² | ~1,130–1,450 m² | 19–24 crates (60 m² each) |
| 9–11 mm | ~24–30 kg/m² | ~920–1,130 m² | 15–19 crates (60 m² each) |
| 13–15 mm (thick board) | ~35–41 kg/m² | ~670–780 m² | 33–39 crates (20 m² each) |
Read the range this way: the formula behind every row is the same — a 20ft box works on about 27.5 tonnes of payload, and slate weighs 2.7 t per cubic metre. Divide 27,500 kg by the thickness's kg/m² and you get the m². Thin 5–6 mm slate stretches the box towards 2,000 m², the standard 6–8 mm band lands around 1,300–1,700 m², and the heavy 9–11 mm slate drops to just under 1,000–1,130 m². Thick 13–15 mm board is the special case: it ships in smaller 20 m² crates, about 33–39 of them. The exact number for your spec is the formula on this page, run in about thirty seconds.
Why 20ft only? Slate is a weight cargo, and the loading plan stops at the 20ft box. A full slate load reaches the weight ceiling before the box is full, and a longer container adds space, not weight — so the extra cube would just sit empty and cost freight. The 20ft is the container the trade uses for roofing slate, and this page's numbers are all 20ft numbers.
The short answer: a 20ft container of roofing slate carries roughly 670–2,030 m², and the spread is pure thickness. The rule of thumb is two numbers: slate weighs 2.7 t per m³, and a 20ft box works to about 27.5 t of slate. Divide 27.5 t by your slate's kg/m² and you have the m² that will load. This page shows you that division, thickness by thickness, so you can check any quote you're holding in about five minutes.
Every month someone writes to us with the same one-line question: "20ft container, 500×250, 7–9 mm — how many square metres?" When I started in the packing office, I answered with a single number, the way I'd been answered myself. It took about two claims to learn that a single number is a promise the stone was never asked to keep.
Now I answer with the density rule. One line: the stone is 2.7 t per cubic metre, so every millimetre of thickness adds 2.7 kg per square metre. Once you hold that line, the whole container arithmetic is one division. This page is that division, run for the thicknesses people actually order, with the arithmetic left visible so you can run your own.
What the Question Is Really Asking
A container is not a room that holds a pile of tiles. It's a box with a hard ceiling: a weight ceiling. Loading slate means asking one question — how many tonnes of stone can the box carry — because for slate the weight always gives out before the space does.
Start with the box itself. A standard 20ft container measures about 5.9 m long, 2.35 m wide and 2.39 m high on the inside — about 33 m³ of space. Its payload, the weight the shipping line will carry, is around 28 tonnes (26,500–28,500 kg depending on the operator and the route). We load to about 27.5 tonnes, leaving a working margin under the ceiling.
Now take the slate. The number that does all the work is the density: slate weighs about 2.7 tonnes per cubic metre. A 6 mm slate therefore weighs about 6 × 2.7 = 16.2 kg per square metre; an 8 mm slate about 8 × 2.7 = 21.6 kg/m²; a 10 mm slate about 27 kg/m². The roofing slate weight guide carries the full table, but this one rule is all you need for a container.
So the first thing to unlearn is the phrase "how much fits." The useful question is "how many kilos per square metre is my slate?" — because the container's 27.5 tonnes divided by that number is the square metres you can load. That's why the trade number always lands somewhere in the 670–2,030 m² band: it's not a mystery, it's a division.
Ceiling One — Weight: The Number That Pays the Bill
Weight is the ceiling every shipment meets, so it's the one to understand first.
Work through a 500×250 mm slate, 7–9 mm thick, the standard of the trade. At about 8 mm mean thickness, one square metre weighs about 8 × 2.7 = 21.6 kg. That's the number the whole container turns on — nothing else matters as much as the thickness you specify.
Now take a 20ft box with a 27.5-tonne working payload:
- At 5–6 mm: ~14–16 kg/m² → about 1,700–2,030 m² — the thin end, where the m² climbs fastest.
- At 6–8 mm: ~16–22 kg/m² → about 1,270–1,700 m² — the everyday band most roofing orders sit in.
- At 7–9 mm: ~19–24 kg/m² → about 1,130–1,450 m² — the 500×250 trade standard.
- At 9–11 mm: ~24–30 kg/m² → about 920–1,130 m² — the heavy end of ordinary board.
- At 13–15 mm (thick board): ~35–41 kg/m² → about 670–780 m² — ships in ~20 m² crates, 33–39 per box.
Notice what this means for your order: the thickness line is not a minor spec detail. Between 5 mm and 15 mm, the same box carries three times more at the thin end than at the heavy end. If you're comparing quotes, that's the difference between a full roof and a roof plus two gable ends. It's also the reason the thickness guide and the supplier's thickness measurement matter more than the price per tile.
Line Two — the Crate That Carries the Metres
Volume is the second line of the picture, and it's where the crate comes in. The container's 33 m³ of space is never the binding limit for slate — but the crate is what the m² actually rides in, so it's worth understanding how it works.
On our line, a standard crate for 6–8 mm slate holds about 60 m² of roof, weighs about 0.8–1.2 tonnes gross, and measures roughly 1.05 m long by 0.6–0.8 m wide by 0.6–0.8 m high. The m² per crate changes with thickness: at 6 mm, 60 m² of slate is about 16.2 kg/m² × 60 = ~970 kg of stone plus the wood — a one-tonne crate. At 8 mm it's 21.6 × 60 = ~1,300 kg, edging toward the 1.2 t top of the band.
Run that through a 20ft: 33 m³ of space sounds generous, but a container is not a crate-shaped room. Rows of crates leave a centre aisle, a gap at the door, and 5–8% of the cube you simply can't use — the honest usable cube is about 27–28 m³. A 0.6 × 0.8 × 0.8 m crate is about 0.38 m³, so the box fits roughly 70 crates by volume alone. That's 70 × 60 = 4,200 m² — far more than the 27.5-t weight ceiling allows. Which is exactly the point: for slate, the volume ceiling is never the binding one.
And here's the surprise that a tape measure alone won't tell you: the crate size doesn't set the m² — the weight does. Whether your slate ships in 60 m² crates (the standard bands) or in 20 m² crates (thick 13–15 mm board), the container still stops at ~27.5 tonnes. That's why a supplier's crate count for the same thickness is a fingerprint of their loading plan, not a different product.
Two things to keep from this line. First, when a supplier quotes a round number like "1,400 m² in a 20ft," ask what thickness produced it — the answer tells you whether they've done the density division this month. Second, don't take a short crate to gain m²: the crates carry the stone either way, and your freight bill follows the total weight no matter how small the box gets.
The Crate Is the Real Metre
Here's the mental habit that saves every buyer time: think in crates, not in tiles. A container plan is written in crates — the count, the footprint, the stack height. The m² is just the crate count multiplied by the m² inside each crate.
On our line there are two crate sizes in regular use:
- Standard crates (6–8 mm slate): about 60 m² per crate, 0.8–1.2 t gross, crates about 1.05 m × 0.6–0.8 m × 0.6–0.8 m. This is the everyday crate for roofing slate.
- Thick-board crates (13–15 mm slate): about 20 m² per crate — the board is twice as heavy per m², so a crate carries a third of the area.
So the arithmetic becomes:
- a 20ft box of 5–6 mm → about 28–34 crates × 60 m² → roughly 1,700–2,030 m²
- a 20ft box of 6–8 mm → about 21–28 crates × 60 m² → roughly 1,270–1,700 m²
- a 20ft box of 7–9 mm → about 19–24 crates × 60 m² → roughly 1,130–1,450 m²
- a 20ft box of 9–11 mm → about 15–19 crates × 60 m² → roughly 920–1,130 m²
- a 20ft box of 13–15 mm → about 33–39 crates × 20 m² → roughly 670–780 m²
Once you can think in crates, you can sanity-check any packing list in ten minutes with a pocket calculator: crates × m² per crate = m², and the total weight must sit under ~27.5 tonnes. If the two numbers don't fit together on paper, they won't fit together in the box either — the receiving-side check in the inspection checklist turns this into a routine.
Worked Example — 500×250 at 7–9 mm, in a 20ft
Now the full calculation, the way it sits on the packing office desk, for the most ordered combination in the business.
| Line | Number | Where it comes from |
|---|---|---|
| Slate density | 2.7 t/m³ | the physical rule — 1 m³ of slate weighs 2.7 tonnes (see the weight guide) |
| Slate weight per m² | ~21.6 kg/m² | 8 mm mean × 2.7 kg/m² per mm |
| Working payload | ~27.5 t | we load to about 27.5 t, leaving a margin under the 28 t line |
| Crate content | 60 m² | standard 6–8 mm crate (see the packaging guide) |
| m² per container | ~1,270 m² | 27,500 kg ÷ 21.6 kg/m² = ~1,273 m² — take the practical 1,270 m² |
| Crates per container | ~21 crates | 1,270 m² ÷ 60 m² per crate = ~21 crates |
| Ceiling that lands first | Weight | the box stops at the 27.5 t line, not at the crate volume |
Read that bottom line carefully: the box stops at the weight line. For 500×250 at a mean 8 mm, one square metre is 21.6 kg, so 27,500 kg of payload divides to about 1,270 m². A quote of "1,400 m²" for this same spec usually means the supplier ran the arithmetic at 7 mm mean (18.9 kg/m² → 1,455 m²) or simply used a lighter average — that's the difference you can now check in your pocket. Across the full spec range, a 20ft box carries anywhere from 670 to 2,030 m², which is the band the table at the top of this page covers. If you want the same calculation for other thickness bands, the MOQ guide's container section runs the numbers for the whole range.
One more line, the one nobody writes on the packing list: the load is never as tight on the concrete as it is on the paper. A fork getting into the last row, a crate that bulges 2 cm, a door that needs 15 cm to shut — every loader knows this and builds in a margin. A good packing plan writes "about 21 crates" and loads 21; a suspicious one writes a round m² figure and hopes the scale doesn't notice. The scale always notices. That's why the invoice is settled at the weighbridge, not at the desk — it's the same reason the sorting-floor guide tells you to weigh the crate: weight is the witness.
How Size and Thickness Move the Number
Everything above is 500×250, because it's the middle of the market. But the arithmetic moves with the size, in a direction that surprises most buyers.
Bigger tiles don't load more area. A 600×400 tile is 0.24 m² — about 4.2 tiles per m². The square metres per tonne are the same, because the density rule doesn't care about the footprint: a 7 mm 600×400 tile weighs the same per m² as a 7 mm 500×250 tile. The crates are simply bigger and heavier, so the crate count per container drops, but the m² per container stays roughly the same for the same thickness. See how many slates per square metre for the per-size counts.
Smaller tiles are a subtle trap. At 400×200 mm the pieces get lighter, the handling gets costlier per m², and the crate volume per m² climbs because there are more tiles per crate and more cardboard between them. The m² figure barely moves — the density rule still rules — but the crate weight per m² creeps up, which can shave a few m² off the container if the crating is heavier.
Thickness is the one lever that really shifts it. That's the table at the top of this page, and it's worth repeating with the numbers: 13–15 mm → ~670–780; 9–11 mm → ~920–1,130; 7–9 mm → ~1,130–1,450; 6–8 mm → ~1,270–1,700; 5–6 mm → ~1,700–2,030. The weight per m² page is where the density numbers live.
Calculate the Container Quantity Yourself
Here's the two-step formula we actually use when a customer sends a spec and a roof area and asks "what fits" — you can run the same one on a phone's calculator.
- Find your slate's kg/m². Thickness in mm × 2.7 = kg/m². Example: 8 mm × 2.7 = 21.6 kg/m². For a band, use the middle: 7–9 mm ≈ 8 mm → ~21.6 kg/m².
- Divide the working payload by it. 27,500 kg ÷ kg/m² = m² in the box. Example: 27,500 ÷ 21.6 = ~1,270 m².
That's it. The crates are just the packaging of that answer: divide the m² by 60 (standard 6–8 mm crates) to get the crate count, or by 20 for thick-board crates. Take the result and you'll be within a crate of the loading plan the factory sends you. When you ask a supplier "how many square metres," hand them the same two steps — the good ones will show you the arithmetic, the way this page does.
One last habit from the packing office, and it's the one that saves you more than the arithmetic: get the loading plan in the order, not in the surprise. A line in your contract that says "approx. 1,270 m², ~21 crates, gross ~27.5 t, weighbridge-verified" turns the whole discussion into a simple check — you weigh the crates on arrival and the number either matches or it doesn't. The arrival checklist tells you what to do on the second case. If a supplier can't write that line, ask what they can write instead.
And when the crate is opened at the other end, the same arithmetic works in reverse: count the bundles, weigh one crate against the line, and you'll know in ten minutes whether the container you got is the container you bought. The m² per container is a planning number on paper and a scale number in the warehouse — and the scale always has the last word.
FAQ
How many square metres of roofing slate fit in a 20ft container?
Roughly 670–2,030 m², and the range is pure thickness: 13–15 mm thick board loads to about 670–780 m², 9–11 mm about 920–1,130 m², 7–9 mm about 1,130–1,450 m², 6–8 mm about 1,270–1,700 m², and thin 5–6 mm reaches about 1,700–2,030 m². The rule is simple: slate weighs 2.7 t per cubic metre, so a millimetre of thickness is 2.7 kg per m²; the container's 27.5 t working payload divided by that number is the m² you can load.
Why do slate suppliers only ship in 20ft containers?
Because slate is a weight cargo. A full 20ft load already sits at the weight ceiling before the box is full, and a longer box adds space, not weight — so the extra cube would just sit empty and cost freight. The 20ft is the trade standard for roofing slate, and every number on this page is a 20ft number.
Why do two suppliers give different m² for the same size container?
Because the arithmetic is usually the difference. Ask each supplier for the slate thickness and the kg/m² they used: the same 20ft box divided at 7 mm gives about 1,450 m², at 8 mm about 1,270 m², at 10 mm about 1,010 m². A "1,600 m² in a 20ft" from one supplier and "1,000" from another usually means different thickness assumptions, not different honesty. Ask for the thickness and the density rule they applied — the number that matches your spec is the number you should trust.
Does slate thickness change how many m² fit in a 20ft container?
Yes, and it's the whole story. The density rule is 1 m³ = 2.7 t, so every millimetre of thickness adds 2.7 kg/m². A 20ft of 13–15 mm loads about 670–780 m²; 9–11 mm about 920–1,130 m²; 7–9 mm about 1,130–1,450 m²; 6–8 mm about 1,270–1,700 m²; and 5–6 mm about 1,700–2,030 m². That's why a quote that doesn't state thickness is a quote you can't compare.
How do I calculate the container quantity for my spec?
Two steps. First, your slate's kg/m² = thickness in mm × 2.7 (e.g. 8 mm → 21.6 kg/m²). Second, divide the container's working payload by it: 27,500 kg ÷ 21.6 = about 1,270 m² in a 20ft. For the crate count, divide the m² by 60 (standard crates) or by 20 (thick-board crates). The worked example in this article runs it for 500×250 at 7–9 mm.
My project needs about 500 m² — which container do I book?
That's the size where the decision moves off this page and into the MOQ guide. 500 m² is about a third to a half of a 20ft box, depending on thickness — you either pay for a container that's partly full, or you go LCL and pay the consolidation fees, or mix sizes in one box. The arithmetic of that choice is the MOQ guide's fourth number, and it's the cheapest conversation to have before you order.
Related Reading
Send Us Your Spec — We'll Send Back the Loading Plan
Every roofing slate quote we write includes the loading plan: m², crate count, gross weight and the container plan from your batch. Tell us the size, thickness and roof area — we'll run the density arithmetic for you and show you exactly how it fits. Ask for a loading plan · see the roofing slate range · view our test results