Porch Railings Calculations Made Easier

Baluster Spacing and More

If building your own porch or putting up new railings, the porch railings calculations can make you scratch your head.



Let us take some mystery out of that.

Anatomy of Porch Railings

Porch railing anatomy diagram

  • Balusters: sometimes called 'pickets' run vertical between the top hand rail and bottom rail, the porch floor, or steps.
  • Balustrade: this is the combination of the top rail, the balusters, and bottom rail if used.
  • Bottom Rail: this is a horizontal piece (optional) to keep hole balusters in place and is also used for visual effect and for ease of cleaning. This needs to be supported every 36" from the floor.
  • Column: this is a supporting post that normally supports the roof.
  • Handrails: top piece of the balustrade; supports the balusters and provides a hand hold
  • Panels: can be glass or other material; used in lieu of balusters for both aesthetic and to minimize visual obstructions
  • Posts: these are required not usually to exceed every 72" to support the balustrade



Most building codes allow for a maximum gap of four inches between balusters for safety reasons. So, how do you calculate the spacing between balusters so that you don't end up with a gap that is too large?

Try this method for porch railings calculations. It's actually harder to explain it than it is to just do it. So read over it a few times and you'll discover how quickly you'll be able to make the computations!

Porch Railings Calculations

First, determine how many balusters you will need.
  • Add the width of one baluster to the maximum gap that is allowed (usually 4 inches). So, if your baluster is a 2 x 2, you'd add 1.5 inches to 4 inches to get 5.5 inches. (NOTE: Say what? You probably think the width of a 2x2 should be 2" - not 1.5". But remember, lumber measurements are based on the size of the board before it is cut so a 2x2 is actually 1.5 x 1.5 inches.)
  • Measure the distance between the two posts (let's say 68.5 inches - the max distance between posts - refer to diagram below) and divide it by the number you calculated above. So 68.5 divided by 5.5 = 12.45
  • Round up this number. So 12.45 rounded up = 13. This is the number of balusters you will need between the two posts. (How many spaces will there be? 14 spaces as there is always one more space than total number of balusters).

Now that you know you need 13 balusters (and have 14 spaces), it's time to figure the spacing distance between balusters.

To compute the spacing distance between balusters:
  • Measure the width of a baluster. A 2x2 is 1.5 inches.
  • Multiply 1.5 inches times the number of balusters. 1.5 inches x 13 = 19.5 inches
  • Subtract that from the distance between the posts: 68.5 inches - 19.5 inches = 49
  • Divide that by the total number of spaces. So that means 49 divided by 14 (there is always one more space than the total number of balusters). The answer is 3.5.
  • So now you know that the space between your balusters will be 3.5 inches.
  • Make a spacer that 3.5 inches wide. I like to use a scrap piece of 2x4. Tip: I always make at least two and sometimes four spacers. Why? I use one for the bottom and one for the top. I also am always dropping them. Your spacer should never be greater than the maximum allowed size (in this case, 4 inches)
  • There you have it! Like I stated earlier, it's actually easier to do than explain porch railings calculations.

  • You first determine how many balusters you need for the amount of space you have to fill.
  • Then you determine how much space to allow between balusters. Follow your local building codes.
  • If you have a small amount of variation in the spacing between balusters, it will probably not be noticed. So don't worry.
Be proud of your good work! Just be sure you stay true to the building codes for porch railings calculations. Be sure to refer to our general railing building code requirements below to get a better understanding of what's required.

Dave's Expedient Spacing Guide

Here's an easy way to determine both equal spacing and placement between two fixed points such as porch columns. You can also use this device for many different uses.
  • Purchase an elastic band approximately 1/2 to 1 inch wide by 6 to 8 feet long
  • Lay it on a flat surface and mark it in 2 inch increments
  • To determine both the number of balusters, hole, etc,, attach one end to fixed point
  • Stretch the band to the other fixed point until you reach your desired spacing, i.e., 3.5 inches, 4 inches, etc.
  • Place mark at the locations as indicated on your band.

You may be able to purchase a pre-marked band at your local supply store. I was lucky as Mary had an extra one in her craft room.


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