Discussion on the “volumetric efficiency” of a tent might immediately cause some heavy eyelids, but it simply refers to how weight efficient a tent’s geometry is [I can already tell this post isn’t going to rank in my top 10]. Thus any outdoors person interested in lightweight gear should possess at least a cursory familiarity.
If the goal of a lightweight shelter is to provide living space at the lowest weight, then starting with a basic shape (or “geometry”) that uses the least material to provide that space should clearly be desirable (amongst numerous other criteria). So we are looking to optimize the volume:surface area ratio.
To introduce the topic and demonstrate how it can be non-intuitive, I will compare this volume:surface area ratio for two popular styles of trekking pole shelters against a tent of my own design – the X-Mid. The first popular tent geometry is a single pole pyramid with a rectangular base (the MLD DuoMid being a classic example). Single pole mids have the most simple possible base shape (a rectangle) and the most simple possible structure (1 vertical pole) so intuitively many folks think this is the most efficient or the “lightest” design (if other attributes like fabric choice are equalized).
The second comparison is a standard “pup tent”. This tent is also based around a rectangle but uses two poles set nearly as far apart as possible. Obviously this maximizes the volume you can get from two poles but also increases the surface area, so it’s unclear if it is more efficient in terms of the material required for the amount of living space (again the volume:surface area ratio).
The last tent is my X-Mid design, which again uses a rectangle shape at the base but places the two poles at an internal position along a diagonal axis.
These poles are inset from the perimeter of the fly so the tent can pitch without the guylines required for the pup tent, and the poles are on a diagonal which avoids conflict in the doorways present on these other two shelters. The layout is like this:
Readers with a fair recollection of high school geometry might already be able to guess the most efficient shape. As a hint, the best possible volume:surface area ratio is possessed by a sphere, as the graph below shows (area on the y-axis, volume on the x, lower lines are more efficient). The core message of this graph is that for any given volume, a sphere always has the least surface area. A second notable takeaway is how much worse the three sided shape is (which is partly why you don’t see 3 sided tents).
One last takeaway is that all of these shapes improve as the tent grows larger (e.g. for any shape you could double the volume without doubling the surface area). That can be seen by the downward curving lines for all shapes and is why any comparison needs to standardize one of these things to see how the other varies.
So what is the most efficient shape for a tent? It’s a dome tent because a floorless dome is just half of a sphere. A dome has half the volume of a sphere and also half the surface area (if we are not including the base since the floor could occupy any portion of that dome). Thus the ratio is just as good as a sphere and unlike a sphere, you can actually build a dome shaped tent.
But of course creating a tent that is both a perfect dome and trekking pole supported is impossible. You’d need to have a perfectly round base (requiring an infinite number of stakes) and a perfectly domed surface (requiring an infinite number of support poles). The interior would be an impenetrable cluster of trekking poles with no usable space (but amazingly robust).
At the other end of the spectrum we have the 3 sided triangular pyramid highlighted earlier for being distinctly poor. Is the simplest possible 3D shape as it has only 3 sides (and therefore 3 stakes) and requiring just 1 pole but with a dreadful surface area: volume ratio. Somewhere in between the minimal stake and structure requirements of the triangle and the maximal such requirements of a dome there is an optimum for a trekking pole tent. This optimum exists where you add more sides, stakes and seams until the diminishing returns no longer give a net weight savings on that investment (of course there are many other considerations as well, like how many poles a hiker might have and how a hiker might sleep within a maze of poles).
Back to our case study – below I have sketched out these three shelters to scale to show their geometry. To approximately equalize I have used the same base dimensions (100″ x 67″) for all of them and assigned the pole height at 54″ for the single pole shelter and 45″ for the dual pole shelters since these are typical values.
From these dimensions I calculated the volume and surface area of each shelter to get the volume:surface area ratio as you can see above. Note that the surface area excludes the floor since I am only calculating the material you would need to build a fly for this tent. Any floor wouldn’t necessarily have to use the full area of the base.
First, the single pole mid has a volume of 70 cubic feet and requires 78 square feet of fabric to build that, for a ratio of 0.89:1 (or 0.89 cubic feet of space from each square foot of fabric). Unsurprisingly the pup tent possesses more volume (by 24%) at 87 cubic feet but also requires 26% more fabric (99 vs 78 square feet) so it actually has slightly worse ratio of 0.88:1.
The X-Mid on the other hand possesses 16% more volume than the single pole mid despite only using 7% more material, so its volumetric efficiency is much better at 0.97:1. It gets almost a cubic foot of space from each square foot of fabric. In other words, if you were to scale these tents so they all have the same volume, the X-Mid would require 10% less material. Or if you built all of these shapes from the same amount of fabric, the X-Mid would have about 10% more volume. Thus of these three shapes, the X-Mid is easily the most weight efficient. This is also intuitively true now that we know that the shape closest to a dome is the most efficient and the X-Mid appears to be that.
While I’m biased as the X-Mid designer, it is objectively the pinnacle of efficiency for a trekking pole tent because rectangular tents have superior efficiency over the more common hexagonal sided shape (discussed later), two poles can beat one pole as they can better approximate a dome, and the X-Mid layout is the only way to approximate a dome with two poles (e.g. not have the poles along the perimeter) while keeping the poles off the floor. I’m confident the efficiency of this geometry will never be exceeded because it’s rooted in first principles of geometry. It is surprising no one thought of this prior to the X-Mid, unless the examiner working on my patent application discovers otherwise.
Back to the comparison – the dimensions I’ve used thus far for the single pole mid aren’t realistic because single pole mids have such low wall angles on the ends compared to the other two designs that they need a longer floor to allow for a decent length of sleeping area. Shown below are the actual dimensions of perhaps the most popular single pole pyramid (MLD DuoMid), which is 10″ longer than the X-Mid:
Here, we see that the DuoMid has less volume (79 vs 81 cubic feet) despite using more fabric (85 vs 84 square feet). Again this shows that single pole mids are less efficient designs. The X-Mid achieves more volume while using less fabric.
As a side point, while the longer length of the DuoMid looks better suited for tall hikers but this is not the case because the heavily sloping end walls result in very low and unusable area at the ends. Despite the DuoMid being 10″ longer (100″ vs 110″), the X-Mid is actually about 6″ longer if you look at the height about 15″ off the ground (where the top of a sleeping bag would be).
Now you might agree that the X-Mid is more volumetrically efficiency on paper but wish to point out that gleaning an extra 2.3 cubic feet from 1.5 square feet less of fabric is hardly a difference substantial enough to celebrate. Indeed it would hardly show on the scale. However if we go beyond the total volume and look at where that volume is located, we can see that the X-Mid is far more spacious.
As mentioned, a single pole pyramid “wastes” about 5% of its volume at the perimeter of the base were the canopy is very low. A portion of a single pole mids volume isn’t “useable”. This is admittedly a subjective term, but for a definition let’s turn to Henry Shires – the most prolific trekking pole designer of the 21st century – for one he provided in his patent for the TarpTent StratoSpire:
“Useable space is space where the canopy walls are high enough above the ground so that the occupants and their gear are not pressing against or distorting the canopy walls while inside. I prefer to define useable space as the interior volume where all canopy fabric is 12” (inches) or more above the ground.”
Based on both subjective experience and that definition, the single pole pyramid has a lot of non-useable space. Conversely the X-Mid avoids the very low slopes of the end walls such that nearly all its volume is useable. Thus while the X-Mid has about 5% more total volume, it really has closer to 10% more “useable volume” – all in a smaller footprint.
But even this doesn’t really reflect how the space feels in these shelters because if you actually sit in both of these shelters, you’ll find the X-Mid feels far larger. This occurs because the volume in a single pole mid is heavily biased towards the lower half. It does have a lot of volume but very little of it is in the top half, which is why headroom is sparse (you can only sit up adjacent to the pole).
Looking only at the volume in the upper half (let’s say above 24″), the single pole mid has a mere 15% of its volume above this point (about 12 cubic feet out of its 78 cubic feet total), whereas the X-Mid has about 35% of its volume above 24″ (28 of 81 cubic feet). Thus the X-Mid volume in the upper half (aka “headroom”) is 2.5x that of a single pole mid and which is why it feels and is much more spacious. So to sum it up, the X-Mid has 5% more total volume, 10% more “useable volume” and 250% more “headroom” – all from less fabric.
Next lets discuss how this compares to a dome tent and a wider range of popular designs for trekking pole shelters. The elephant in the room here are the tents with a hexagon base since these are quite popular and seemingly closer to a dome than the X-Mid.
We can assess this by circling back to my earlier statement that a dome is the most efficient shape. If you were to build a dome tent with the same 81 cubic foot volume as the X-Mid, you could do it with 72 square feet of fabric (rather than 83.75). So that’s a potential savings of 11.75 square feet or 1.3 square yards of fabric if you built a perfect dome. Lightweight materials today are 0.5 – 1.4oz per square yard, so the possible savings are 0.65 – 1.8oz. Thus any design that aims to be more efficient than the X-Mid needs to add seams, stakes, poles and/or struts to realize some of those savings, without adding more weight than it saves.
The closest you might reasonably come to a trekking pole supported dome is an eight sided shelter supported by four trekking poles. The math here is complex but such a design turns out to be roughly halfway between the X-Mid and a dome in terms of fabric requirements. It would save about 6 sq ft of area (0.3 – 0.8oz of fabric) but also necessitate four more stakes which weight about 1.5oz – more than offsetting the fabric weight saved (plus there is further weight in the additional seams). Thus while the ratio of volume:surface area would improve, the volume:total shelter weight ratio would be worse. The same thing is true for a hexagonal shelters. They only marginally improve the volume:surface area ratio with a theoretical fabric weight savings of 0.2 – 0.5oz which will never translate into a net weight savings because two decent stakes weigh more than 0.5oz (and again, there are also additional seams). Thus any two pole hexagonal tent is going to be more complex and heavier for the same volume than a comparably sized rectangular based shelter.
On the flip side, what about about three sided shelter? If four is better than six, then perhaps three is better still? It is not. A possible three sided shelter would require a lot more fabric than a four sided one (scroll up to that graph earlier and notice how much worse a three sided shelter is than anything else) yet only save the weight of one stake, so four sides much more efficient. Overall there is an optimum at 4-5 sides because seams and stake weight increases linearly for each side you add, while savings in fabric area are diminishing (a negative asymptote). The key result is that four sides is a big improvement over three, five is about equal to four, anything beyond is worse). Whether four or five is better is hard to say as the fabric area savings roughly equals the added seam and stake weight.
Lastly, you may wonder about the efficiency of the myriad of other trekking pole shelter shapes out there. In short, anything with a single pole that is off center (aka asymmetrical such as the SMD Lunar Solo) is always going to be less volumetrically efficient than the same thing with the pole positioned centrally (just from the basics of geometry). This is only done to get the pole out of the sleeping area. That’s why you’ll often see this type of shelter often adding various struts to increase volume or improve the distribution of that volume (e.g. supplement headroom), such as the TarpTent Aeon and Zpacks Plexamid. But these struts will always cause a further decline in the overall weight efficiency of the shelter because the additional volume you get relative to the weight of a strut is always poor. Adding weight via struts is fundamentally less efficient than using a second trekking pole if you have one on hand anyways, so those design only make sense for single trekking pole hikers. Single trekking pole tents can add enough struts to achieve a reasonably sized living space but it would be much more weight efficient to start with a more efficient two pole geometry rather than tacking weight onto a less efficient one. Offset single pole designs with struts represent near the bottom of efficiency (but may have practical advantages like a smaller footprint).
The most weight efficient trekking pole tent will:
– Use a four or five sided shape
– 2 poles
– Position the poles a moderate distance apart to approximate a dome shape
– Avoid struts
– Avoid asymmetry
Take home message
The worst case scenario for weight efficiency is to start with a shape with very few (3) or many sides (e.g. hexagon or octagon), add a single pole in an offset position and then add struts to that.
Conversely, the best case scenario is a two pole shelter based around a 4-5 sided shape and with the poles located at a moderate distance apart. Nothing is more volumetrically efficient than the X-Mid but some other two pole designs do come close like the Black Diamond Beta Light (below). Thus if you find a tent that is lighter than the X-Mid, it’s almost certainly not because of the geometry but rather because it is smaller, using lighter materials or less fully featured.