Baluster Calculator
Getting baluster spacing right means meeting building code and having an even, professional look. This calculator figures out how many balusters you need per railing section and the exact gap between each one, making sure no gap exceeds the 4-inch code maximum.
Most building codes (IRC R312.1.3) require that a 4-inch sphere cannot pass through any opening in a railing. That means the clear gap between balusters must stay under 4 inches. This calculator uses your railing length, baluster width, and post dimensions to find the minimum number of balusters that satisfies the code, then distributes them evenly for a clean layout.
Standard balusters are 1.5 inches wide (actual dimension of a 2x2). Metal balusters for aluminum or wrought-iron railings are typically 0.75 inches. The wider the baluster, the fewer you need per section, but the look changes. Most deck builders use 2x2 wood or 3/4-inch round metal.
The "on-center" spacing is the distance from the center of one baluster to the center of the next. This is the measurement you will actually mark on the rail during installation. For standard 2x2 balusters with a 4-inch maximum gap, on-center spacing works out to about 5.25-5.5 inches depending on the exact section width.