This is similar to how I do it, but much of my method is done in my head, so I copied this from another site. It's probably easier to understand than my way would be. :huh:
BTW.... he doesn't say it, (and I'm not positive of this), but I believe he is using a flat, level measurement in step 1. I, on the other hand, work with angled measurements, so that's why you aren't hearing too much about my method... it can get a bit confusing.
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Step1
Measure the staircase length between each rail post in inches, and write down the measurement. You may need someone to help hold the tape measure in place or take several measurements if your staircase turns.
Step2
Determine the local building code requirements for spacing balusters by calling your local building department or looking for the standards online. An industry standard is 4 inches, but your area may have different requirements so double-check to ensure accurate placement.
Step3
Refer to the balusters you've chosen and write down the width of the balusters in inches.
Step4
Add one baluster width to the staircase length.
Step5
Add the baluster width and the space between each baluster together. Then divide this number into the staircase length (plus the additional baluster length). For example, a 2-inch wide baluster with a required space of 4-inches and a 192-inch long staircase: 2 inches + 4 inches = 6 inches; 192 inches + 2 inches = 194 inches; 194 inches/6 inches = 32.33.
Step6
Round the fractional part of the number from Step 5 (after the decimal point) down to the nearest whole number. This is the number of balusters you'll need for the project and, for this example, you will need 32 balusters.
Step7
Multiply the number of balusters you need by the baluster width, and then subtract this number from the staircase length plus one baluster width. For example, 32 x 2 = 64; 194 – 64 = 130.
Step8
Use the final sum of 130 from Step 7 and divide the number of balusters you determined you'll need plus one more baluster. For this example, 130 / 32 (+1 baluster) = 3.93 inches between each baluster. If you feel 3.93 inches is too close, you can add another baluster.
Step9
For an odd number of balusters, start at the midpoint of the staircase run and install your first baluster. For an even number of balusters, you'll be off the midpoint by 1/2 the space amount. Working out from the center, install each subsequent baluster the distance as determined in Step 8.
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Here's a calculator that may help:
http://www.virginiarailingandgates.com/calculations_picketspace.asp
As a hint in ensuring equally spaced balusters...... most installers determine their spacer measurements beforehand, cut them all, and install the spacers as they go. This keeps everything tightened up and in line. But make sure the first baluster is perfectly plumb.... and check for plumb as you go. (You COULD have cut one or two spacers off just a little, and it will throw you off.) We who may not be all that accurate in all of our associated work often find that we probably should only cut a few of the fillet spacers at a time. :whistling2:

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