How to Space Balusters Evenly Every Time

Balusters—the vertical posts along deck railings—are an architectural detail that you may not think about that often. When someone has spaced them unevenly, however, they provide a visual reminder of why you should take the time to get a project exactly right. Once you start looking at deck baluster spacing, you’re going to notice how often they are poorly spaced. “Don’t point it out at your friend’s house,” professional builder and craftsman Jordan Smith advises. “Just notice it and think, I’m not going to do that when I’m building stuff.”

Terminology: Balusters, Spindles, Balustrade

Balusters or spindles (another term for the same feature) are the vertical posts that fill the open section of a railing.

A “balustrade” was originally simply a series of balusters, though it also refers to the entire handrail—its top and bottom railings and the balusters too.

Supporting posts are located along a balustrade, but unlike balusters they hold up the railing and connect it to the floor of a building or a deck. These posts will typically be located no more than six feet from each other.

Baluster Spacing and Building Codes

US building codes require that the space between balusters be no more than four inches—assuring that small children can’t slip through or get stuck between them. They must also be able to resist up to 50 pounds of pressure exerted over a 1-square foot area. Many building inspectors carry a 4-inch ball—a quick way to check post spacing without getting out a tape measure.

Local building codes may dictate smaller spaces as well as the fasteners used to attach balusters to railings.

How to Calculate Even Deck Baluster Spacing

Spacing deck balusters evenly and according to code may seem simple—just start at one end and place a baluster every four inches, right? But because most deck or balustrade lengths are not evenly divisible by four, this method will leave you with an awkward space at one end, where you have to jam in an extra baluster less than four inches from the end of the balustrade.

To space balusters more evenly and elegantly, professional builder and craftsman Jordan Smith recommends a better method, in which you spread the extra space among the balusters at either end (or both ends), so that it’s practically invisible. 

Understanding basic construction math will help you execute every project more quickly and easily. Learn the fundamentals—including fractions, geometry, and trigonometry—in MT Copeland’s online Construction Math class

This approach to spacing will require some basic math and a calculator will come in handy. Here’s a step-by-step example: 

  1. Measure the length of the space between the end supports of the balustrade. For this example, assume it’s 13 feet 5 ½ inches, or 161 ½ inches. 
  2. Divide that length by four to determine the number of spaces you will need while having a minimum of 4 inches distance between balusters. These are being measured from the center of one baluster to the center of the next. The result is 40.375. 
  3. Round this down to the nearest whole number, 40, and you have the number of spaces along the balustrade. 
  4. Take the remaining fraction (0.375, or 3/8) and multiply it by 4 (to convert that number back into inches). The result is 1.5 inches. 
  5. Determine the smallest measurement that will still be easy to work with given the tools you have. Smith recommends 1/16  or ⅛  of an inch for projects like this one. 
  6. If you choose to work with ⅛ of an inch, then divide 1.5 inches by ⅛ (or .125). The result is 12. 
  7. Add ⅛ of an inch to a total of twelve of the spaces—locating the 4 ⅛ inch spaces together at one end of the balustrade is typically the most discreet way to incorporate them. The total length of the balustrade will be 161 ½ inches, with no awkward spaces left over. 

If you are concerned that 4 ⅛ inches is more than building codes allow, remember that this is the distance from the centerpoint of one baluster to the centerpoint of the next one. Given the width of the balusters, the space will be less than 4 inches. 

Smith promises that the math isn’t complicated, it just requires some practice. “Get really good at figuring out how many spaces there are total, what your remainder is, and be able to apply that remainder back into enough spaces that it’s a large enough number that you can easily add it, but it’s a small enough number that it fools the eye,” he suggests.

Another Method for Spacing Balusters

It is also possible to measure the entire length of the balustrade and then divide it evenly so that you get a spacing close to 4 inches. For example, if your balustrade is 161 ½ inches long, you might use 41 balusters evenly spaced 3.94 inches from each other—roughly 1/32 of an inch shorter than 4 inches. 

The downside: Locating the proper spot for each baluster will be a chore, because you’ll have to measure out to tiny increments for every baluster—and that’s assuming you even have a tape measure that includes marks for 32nds of an inch. Given the inevitable variance in the width of balusters, however, your precise measuring may be for nothing. 

MT Copeland offers video-based online classes that give you a foundation in construction fundamentals with real-world applications. Classes include professionally produced videos taught by practicing craftspeople, and supplementary downloads like quizzes, blueprints, and other materials to help you master the skills.

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