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 are widely referred to as being the most simple tent, 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 rather than at the perimeter. This is roughly halfway in between the previous two shelters.
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, recall that 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.
Below I have sketched out these shelters to scale to show their geometry. 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.
Before we get into this though, I’ll give away the theoretical answer right now and then we’ll see how close these tents come. The most efficient possible shape in terms of the volume:surface area ratio is a dome tent since it has half the volume of a sphere and also half the surface area (if we are not including the base). 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 because you’d need to have a perfectly round base (requiring an infinite number of stakes) and an infinite number of support poles (for a perfectly domed fly). Thus the closer a trekking pole design gets to a dome, the more stakes and seams it requires. Somewhere there is an optimum where you minimize fabric with a semi-dome shape while avoiding too much seam and stake weight. So we’re looking to optimize the volume:total weight ratio, not just the volume:surface area ratio.
Okay onward to comparing these tents and then at the end we’ll compare them to both domes and shapes that are closer to a dome, such a hexagon based shelters.
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 an impressive 24% more volume (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 about 10% 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.
However, the dimensions I’ve used thus far for the single pole mid aren’t that realistic because single pole mids have such low wall angles on the ends 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, 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″), both of these accommodate the same length of floor (87″) and 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). Thus X-Mid accommodates taller hikers while also having a shorter footprint.
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 isn’t hardly much of a difference. 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 ends were the canopy is very low. This is why the DuoMid is 10″ longer than the X-Mid, yet actually has a shorter floor length in the inner tent. So 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. That why it needs to be 10″ longer for a similar length of sleeping area. Conversely the X-Mid avoids the very low slopes of the end walls such that virtually 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”.
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 much 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 it is mostly close to the ground so 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″), a single pole mid has about 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 more than double that of a single pole mid and which is why it feels and is much more spacious. You can sit up anywhere inside. So to sum it up, the X-Mid has slightly more total volume, substantially more “useable volume” and way 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 realize some of those savings, without adding more weight than it saves in additional seams, stakes and struts. This task is not easy.
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 hard but this would be roughly halfway between the X-Mid and a dome in terms of fabric requirements. This would save 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 is indisputably more efficient. Overall there is an optimum at four to five sides because seams and stake weight increases linearly for each side you add, while savings in fabric area are diminishing (four is a big improvement over three, five a modest one over four, anything beyond is slight). 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. You 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. So offset single pole designs with struts are low efficiency.
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 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. I’m not aware of anything 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.