What Is the Best Thickness for an Inflatable Paddle Board?

This article examines the relationship between the length and width of a SUP board relative to the height of the rail.  We explain the influence this has on rider stability and weigh in on the types of paddling environments and board types that are best suited for a 5" or 6" inflatable SUP.

SUP Board Stability Explained

The thickness of an inflatable paddle board affects its performance in many important ways. For this reason, thickness should be one of the main criteria in selecting a board, as it is at least as important as length, width, and outline shape.

The inflatable SUP industry has shifted hard in the direction of producing 6-inch thick boards, but not always for the best reasons.  The reason is simple: Making a board thicker is an easy way to make it feel more rigid, even if the materials it is made from are not the strongest.  While rigidity is an attribute we look for in a paddle board, it can be better achieved with proper use of materials and construction methods, without having to resort to the shortcut of making the board thicker than it should optimally be. 

An excessively thick board has some notable disadvantages for riders of all experience levels.  It raises your center of gravity while riding the board, resulting in a wobbly feeling which can make it challenging for beginners to gain confidence. Extra board thickness makes it harder to get back on the board when you fall in the water.  A thicker board takes longer to inflate, is more difficult to carry under your arm, and has a bouncier ride than a thinner board. Boards designed for all-around paddling by people in average weight ranges will have the best combination of ride feel, stability, and rigidity when designed with a thickness of 5 inches, provided the board is well constructed using the best materials and is properly inflated to the manufacturer’s recommended pressure, usually 15 PSI for a performance level inflatable SUP. 

6-inch board thickness has its place in longer touring boards, which benefit from the extra rigidity due to their length, in some whitewater/crossover boards where extra volume helps keeps the rider above rough water, and in boards designed for multiple riders, or certain sport-specific models.

Before we proceed we need to take the step of defining how the thickness of a SUP board is measured, and how the commonly used specifications are arrived at. Stand up paddle boards are generally specified in inches, but manufactured in centimeters. This distinction gives some rounding considerations between imperial and metric conversions.

  • A 5” SUP is manufactured at 12cm which equals a converted thickness of 4.72”
  • A 6” SUP is manufactured at 15cm which equals a converted thickness of 5.91”

Less common core constructions are 4" (10cm / 3.93") and 5.5" (14cm / 5.51") which follow the same principle of specifying in inches though manufacturing in centimeters.

The 5” and 6” distinctions are widely accepted in use among most SUP manufacturers but it leads to some minor differences when comparing the two in terms of volume and performance.  This measurement takes into account the core PVC and not additional material such as deck pad EVA or material additions. This leads to rounding to a whole number in from the technical manufacturing spec and perceived differences even though the board core is essentially the same, albeit with minor variance depending on the actual material composition.

When using the technical measurement and manufacturing (metric) specification (12cm / 15cm) of an inflatable paddle board, a 6 inch SUP is actually 25% larger than an equivalent 5 inch SUP, where the widely used marketing specification in inches (5” / 6”) would indicate it's only 20%.

girl paddling a lake on a 5" SUP

Board Stability Ratio (BSR) and How It Affects Inflatable SUP Performance

When prospective inflatable paddle board buyers think about board size, the focus tends to be on length, which is a key determinant of speed, and width, which contributes greatly to stability.  What most buyers fail to take into account is board thickness, which has a huge impact on every aspect of inflatable SUP performance.  Here, we will go into some depth on how board thickness interacts with other design parameters to influence the stability of the board.

One of the guiding principles we use when designing a board is what we call Board Stability Ratio, or BSR for short.  It’s part of the basic process that goes into shaping an inflatable SUP to achieve the specific performance characteristics we’re targeting.  

Before we get into detail, we should state that we have several formulas we use in the construction of our boards that relate to aspects beyond stability, and our shaping procedure requires a lot of on-water testing of our prototypes.  But we felt the discussion of BSR was an interesting insight to illustrate how a relative stability parameter can be quantified with a basic mathematical model. 

This formula has limitations, specifically that it can only be applied to inflatable SUP boards due to the uniformity of the rail.  Epoxy or hard SUP boards taper the rail which leads to less stability.  Hardboard designers do this to decrease weight or to tune specific surfing characteristics so BSR can not be applied with any equivalency to a hardboard SUP.

BSR goes beyond the concept of stability being a pure function of board width and encompasses additional parameters including board thickness and outline shape. The formula we have developed to relate board width, thickness, and outline shape to stability is this:

Board Stability Ratio = Max width x (.5 tail width + .25 nose width) / thickness.

Let's define the terms used in the formula:

Board Stability Ratio is a relative measure of the stability of the board, used for comparing one board to another.

Width is the measurement across the board from rail to rail, taken at the extreme end of the rounded edges

Tail Width is a measure of the width of the board taken 12 inches from the tail

Nose Width is a measure of the width of the board taken 12 inches from the nose.

Thickness is the total thickness of the board

Greg Miller paddling on a flat ocean

Examples of BSR in Use

Applying the BSR formula introduced above to three boards with the same length and width, but with different thicknesses and outline shapes provides a sense of how this all works.

Board A - A 34” width board measuring 20 inches wide 12 inches from the tail, 15 inches wide 12 inches from the nose, 5” thickness:  Applying the formula, the result is: BSR =  34 x (.5x20 + .25x15) / 5 = 93.5

To see how the BSR formula helps predict stability, let’s change some variables one at a time:

Board B - Same measurements as Board A, but with thickness increased from 5" to 6".  Applying the formula, we get:  BSR = 34 x (.5x20 + .25x15) / 6 = 77.9

Board C - Same measurements as Board B, except we narrow the tail from 20 inches to 16 inches. For this board, we get:  BSR = 34 x (.5x16 + .25x15) / 6 = 66.5

In the above examples, the BSR calculation predicts that Board B will be less stable than Board A, with the only difference in design being the board thickness of 6 inches vs. 5 inches.  Board C will be even less stable than Board B due to the effect of a narrower tail width.

Of course, a number derived from applying a formula is a simplification of a more complex issue and other design factors will influence how the board feels on the water.  However,  assuming a minimum level of positive  buoyancy is achieved the formula does help illustrate how changes in board thickness and shape affect stability, by putting a relative number to the reduction in stability from increasing thickness and reducing tail width.

Stability will also be influenced by the height of the rider, the type of paddling environment and most importantly the riders level of experience and skill. If all other factors are equal, the thickness of an inflatable SUP is a variable that can be isolated so it should be an important consideration when choosing the best inflatable SUP for your needs and paddling style.

Paddling a 5" SUP on a lake

What is the Right SUP Board Thickness Choice?

For the majority of paddlers, 5 inches of thickness is preferred over 6 inches thickness because it results in more stability for a given board width and outline shape.  For riders weighing over 225 lb, the extra volume may be helpful for keeping comfortably afloat, so a 6-inch thick board is generally called for, which is the kind of trade-off that is sometimes necessary.  But for riders who don’t need the extra volume, the extra inch of thickness of a 6-inch board is not always welcome, as it adds bulk, raises the rider's center of gravity, takes longer to inflate, makes it harder to get back on the board after falling off, and is more cumbersome to carry.

There are scenarios where a 6 inch board is a better design choice.  Longer touring or racing boards, generally 11’6” to 14’ in length, benefit from extra thickness to prevent bowing in the middle of the board.  Boards used primarily for whitewater are often 6 inches thick, as the volume helps the board skim over moving water keeping the nose from pearling with a well-designed rocker.  Some crossover boards used for a combination of running rapids, paddling upstream, surfing, and paddling on flat water are able to meet some specialized performance objectives by having some extra thickness. But for all-around paddling, we have found 5 inches of thickness to be the sweet spot for most paddlers, resulting in a board that feels properly balanced on the water. 

In general, quality inflatable SUP brands that focus on all-around paddling and use inherently rigid materials to construct their boards will have 5-inch thick all-around boards in their product line.  Whitewater-oriented or low-cost big box store brands skew toward 6-inch boards, but for very different reasons.

Why Are 6-Inch Boards So Prevalent in Most Paddle Board Brands?

It may seem surprising, but it is actually less expensive to produce a 6-inch board than a 5-inch board.  Why? Because adding thickness is a shortcut to achieving rigidity.  Thus, cheaper base materials can be used to produce a board that has adequate rigidity.  In making a 5-inch board, the base material must be stronger with less flexion to attain the required rigidity.  This is why almost all of the bargain-priced boards sold at Costco, Amazon, and high volume online dealers, are 6 inches thick, while higher-end all-around boards tend to be 5 inches thick. 

Unfortunately, a board being 5 inches thick doesn't give it a green light in terms of rigidity or performance either. There are many facets regarding board shape that need to be optimized to really get a board that can be rigid at 5 inches, but when you get it right the experience it provides a rider is second to none.

While board thickness is a critical factor in inflatable SUP board shaping, there are many other factors you should consider when assessing information on paddle board design.  For a comprehensive resource see the Ultimate Guide to Finding the Best Inflatable SUP which will examine facets of the industry and outline other criteria you should be looking at for your next paddle board purchase.

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