Buildy Buddy

When to Use This Calculator

Use the Deck Board Calculator when planning a deck build or deck board replacement. Proper gap spacing between boards is critical for drainage, air circulation beneath the decking, and allowing for thermal expansion and contraction — especially with composite decking materials that expand and contract significantly more than natural wood throughout the seasons.

How It Works

Enter your total deck width, board width, and desired gap size, and the calculator tells you exactly how many boards you need and whether the last board will require a rip cut. It can also suggest an adjusted gap size that makes all boards fit evenly without any rip cut at all, giving you a cleaner finished look and saving time at the table saw.

Tips for Best Results

• Most natural wood decking needs 1/8" gaps between boards, while composite decking typically requires 3/16" to 1/4" — always check the manufacturer's installation specifications for your specific product.
• Install deck boards crown-side up (the curve of the growth rings facing up) so water sheds off the surface rather than pooling in the center of each board.
• If the calculator shows the last board needs a significant rip cut, consider using a slightly wider or narrower gap to eliminate it — a 1/32" change per gap is invisible to the eye but adds up across the full deck width.
• Account for the fascia board thickness if your deck has one, as it reduces the usable width for decking by the fascia stock thickness on each side.