The Beta

Discussing the technical background and innovation of SlingFin.

Volume: Adding a New Dimension

Volume: Adding a New Dimension
Richard Ying - Monday, October 29, 2018


(note: areas marked with a red X mean that the occupant’s body extends past the spatial boundary of the tent body)




Can you really call it a two-person tent if it’s not usable by 2 people? The rush to make lighter and lighter tents has led manufacturers to make tents that are so small they are often not livable or functional for their stated capacity.

Weight is important but so are livability and function.

The users should ask themselves what functional features they are willing to give up in order to reach their target weight. What is the balance between having a light tent versus a livable or functional tent? Is the vestibule large enough for your pack, boots and gear? Is the vestibule large enough to cook in? Does the tent have a dry entry? Is the stated square footage usable or are the sidewalls and tent corners so steep that the measurements are, for all practical purposes, useless?

In the era prior to the Internet, a tent buyer would shop for a tent by first looking at catalogs then visiting a store and getting inside the tent before picking the “right” one. These days the average tent buyer (of technical, ultralight or lightweight tents) spends 4 to 6 hours researching their options online prior to making a purchase. The trouble with relying on current online tent metrics is that the livability and functionality of a tent are not known until you get inside the tent and see for yourself.

To help the tent buyer, SlingFin has added several new specification metrics to its current list of tent specifications: Tent Body Volume, Vestibule Volume and Weight. We encourage the outdoor specialty tent manufacturers to include tent volume specifications to their list of tent specs. Some companies like Nemo and Sierra Designs have already begun similar efforts and we really hope the momentum continues leading to an industry-wide consensus. We also want to encourage gear testers to include tent and vestibule volumes with their reviews.



Calculate the tent body and vestibule volumes.

With the improvements in technology and 3D imagery, it is now much easier to create 3D models, calculate volumes of complex shapes and convey spatial images to customers virtually.

Tent and flysheet measurements can be manually entered into a 3D solid modeling CAD program or, if you have access to a 3D scanner, then by all means scan away. When the tent and flysheet are modeled, the 3D CAD program can calculate the volume of the tent body and/or vestibule(s). You can also calculate the area or volume at any height or cross section of the tent/vestibule.

We have 3D modeled our two TreeLine tent style (2Lite and CrossBow 2) as well as a few popular models from other manufacturers.


Calculating the weight-volume index

Once we have the weight and volume measurements of the tent body and vestibule(s) we can calculate the weight to volume ratio, giving us what we call the weight-volume index, which is the total minimum tent weight divided by the total volume (tent body and vestibules). This measurement, in ounces per cubic foot, will allow users to compare tent models based on how much living and storage space the tent offers for the weight. This number is a valuable piece of information because it describes numerically how efficient the weight is that you are carrying.

For example, a tent with a weight-volume index of 0.61 oz/cu-ft is a better use of weight than a tent with an index of 0.82 oz/cu-ft, even though the latter tent may be lighter overall. In the example pictured and above, both tents have a square footage of approx. 28 sq. ft, but have much different weight and volume measurements. Overall weight is important, but for those who count ounces, understanding that an extra 13 oz may also translate into an additional 34 cubic feet of usable volume, it could easily justify the weight (stated trail weight of 44 oz vs. 31 oz). This means there is an additional 34 cubic feet of volume for yourself, your gear, your partner, dog, etc. Note, that there is some ambiguity in the term “weight“ and that will need to be precisely clarified in any index using “weight“ (see *1 below).




3D imaging software has the ability to “map” the inside of the tent or vestibule with shapes and objects. Human figures (lying down or sitting up), pads, sleeping bags, etc. can be easily added to scale. Using 3D models to visualize, or map, interior spaces is the next step in evaluating tent specs, particularly when the tent is not physically in front of the shopper. Mapping will be especially helpful for tall individuals who are looking for lightweight tents. It will also help communicate how a tent solves, or fails to solve, other factors impacting tent livability.




Every individual situation has its own optimum shelter ranging from a simple tarp (or no shelter at all!) to mountaineering tents that must block spindrift, blunt extreme temperatures and withstand winds that would take many houses apart . We believe it is our job to provide the information a consumer needs to make the purchase that is best for them and their intended use.

When looking for lightweight shelters, buyers need to find that balance between the weight of a tent, its livability and how functional it is. By adding a new dimension into the provided specifications (the tent and vestibule volumes as opposed to just square footage), we hope to help the buyer make the best informed purchasing decision. Better yet, we hope to help pair them with the right gear so that they can best enjoy the great outdoors.

We will leave you with 2 thoughts our friends have shared with us.
“Pick the right tent for extreme conditions because you are only a millimeter away from spending the night outside” (Robert Link , and “The heaviest tent in the world is a lightweight tent that failed” (Phil Scott, designer with 50+ years of outdoor design and production experience).

Enjoy the journey as you discover what works best for you!

Martin Zemitis- Designer
Richard Ying- COO and Mechanical Engineer
Scott Chenoweth- Mechanical Engineer & CAD Specialist

Copyright SlingFin 2018 under Creative Commons License: okay to use with attribution. Please link to this article if quoting or reusing.



What are some of the shortcuts to making lighter tents?

To achieve ultralight weights, tents are made with very lightweight (and expensive) fabrics that aren’t necessarily strong, small tent pole diameters, very lightweight waterproof coatings, small zippers and they are making the tents and vestibules very small. Many models have “wet” entries (where the flysheet does not cover the doorway) and nearly non-existent vestibules. Many manufacturers are only stating the floor area, but the usable floor area is significantly smaller than the stated spec.

What else should you consider when determining tent livability?

Tent floor weight and footprints: Nearly all ultralight tents need to be used with a footprint or tarp to protect the tent floor. Footprint weights are not included in tent “minimum or trail “weights” but the weight should be considered when comparing tent specs.

Floor Area: The floor area of a tent is stated in square feet, but the floor area specification does not state how much of the floor area is usable. Many lightweight 2 person tents have so little usable floor area that they are often used as one person tents and, in turn, 3 person tents are used as 2 person tents. Buying a lightweight 3 person tent so that 2 people have enough livable space is expensive and defeats the purpose of buying a lightweight tent.

Vestibule Area: Vestibule area is also stated in square feet. This measurement represents the area covered by the flysheet. This is one of the most misleading specifications used by tent manufacturers. The square footage of a vestibule may have no relation to the usability of the vestibule. Is there enough room in the vestibule to get in and out of your tent without crawling over your gear? Do 2 peoples’ gear fit into the vestibule or vestibules? Is there enough room to cook in the vestibule? Will all the gear fit in one vestibule so you can cook in the other vestibule? Usable vestibule space is even more important as tents are becoming smaller, lighter and have less interior space (in the tent body) to store gear.

Dry Entry: The purpose of a tent is to keep the weather and bugs out and allow the user to camp in safety and comfort. Having rain enter the tent when the door and vestibule are open defeats the purpose of having a shelter. Make sure that any tent chosen should have a dry entry if the intended area of use is a climate where rain is expected. Rain should not directly enter the tent when the flysheet is open even when getting in or out of the tent.

Interior Peak Height: This measurement is clear – floor to the highest point on the interior of the tent. Unfortunately, this measurement can be misleading depending on the geometry of the tent. The usable height of Pyramid tents and a large number of dome tents is often much less that the stated interior peak height.

Tent length: This measurement is taken on the floor of the tent from one end of the tent to the other. Depending on the sidewall angle the usable floor area may be significantly shorter than the stated length. An 85” long tent with near vertical endwalls will fit people better that a 92” long tent with low sidewall angles. Do not assume that longer tents will fit better. One reason for low sidewall angles is that shorter tent poles and the use of hub designs allow for weight savings but the downside is that many designs have very low sidewall angles which significantly reduce usable interior floor area and volume. There is little worse that having the a low tent wall draped down onto your down sleeping bag, running condensation all over it.

Where is the balance of competing variables?

No matter which variables you prioritize there are other variables that will be compromised. Make the tent strong and you add weight and cost with more tent poles and heavier fabrics. Make the tent lighter and smaller and you save weight and cost but the tent may not hold up in high winds and heavy rain. Make the tent ultralight and you add cost (because of expensive ultralight fabrics) and you reduce the tent size and volume to save even more weight. Soon, that ultralight tent becomes more of a bivy tent than a livable structure you can enjoy.

Each individual will have their own optimum balance. Each user will determine their own set of priorities for a tents’ performance, function, livability, weight and cost. Intended use and the target environment are key features. It is important that each user assesses 1) the intended environment of use, 2) the possible impact of gear failure (which ranges from mild inconvenience to significant physical danger) and 3) their own comfort thresholds.

It is up to manufactures and retailers to provide the information consumers need to make the best choice for their individual needs!



The backpacking industry has adopted 2 weight categories to help consumers compare tent models based on weight.

Minimum weight: Tent body, flysheet (if applicable) and tent poles (if
applicable) are included in the minimum weight. This weight is for comparative purposes only.

Trail Weight: Tent body, flysheet, tent poles, stuff sacks, tent pegs and any other accessories that come with the tent are included in the trail weight. The trail weight is the weight of the tent as it comes from the retailer less packaging. The actual trail weight may be less than stated if the tent comes withoptional items or if the tent has convertible features the user may not want to bring.

Please note: There are some retailers that have their own definitions of what the minimum and trail weights are. If you are comparing specs make sure you are comparing apples to apples.