Joist Spacing Calculator
Decking dictates joist spacing, not the other way round. Tell it what boards you are laying and at what angle, and it returns the spacing the material requires, the number of joists, and whether your planned span works at that spacing.
Recommended products for this project
As an Amazon Associate we earn from qualifying purchases. Affiliate disclosure.
Outdoor Solid Composite Decking Boards 96"x5.43"x1", Polymer Composite Decking for All Weather Use,Poolside,Backyard,Patio(6 Pcs,Redwood and Maple,21.72sqft)
View on Amazon
Swanson Tool Co PL001M Magnetic Composite Post Level, Yellow, Includes Elastic Loop for Hands-Free Work and 3 Vials for Easy Plumb and Level Reads
View on Amazon
Ready Seal Exterior Wood Stain | Semi-Transparent Oil-Based Formula | Professional Grade Stain and Sealer in One | Ideal for Decks, Fences, Pergolas, Wood Siding & More | 5 Gallon | Natural Cedar
View on Amazon
SUNVIE All-in-One Low Voltage Deck Lights Kit, 8 Pack 5W Low Voltage Landscape Lighting with 60W Transformer 100FT Wire and Connectors, 3000K 12-24V AC/DC LED Deck Step Lights for Fence Post Patio
View on AmazonGet the Smart Decking Calculator app
Estimate decks on the go, save projects, and share with contractors.
Building something else? Try our sister calculators
Smart Fencing Calculator
Estimate fence cost by material and linear footage.
Smart Roofing Calculator
Free roof replacement cost estimates by material.
Smart Drywall Calculator
Android app — drywall sheets, mud, and tape estimates.
Smart Fencing Calculator
Android app — fence cost estimates on the go.
Frequently asked questions
- What joist spacing does composite decking need?
- Nearly every composite and PVC brand requires 16 inches on centre for straight boards and 12 inches for diagonal installations. The boards are heavier and less stiff than wood, so they telegraph any extra spacing as a soft, bouncy feel. It is also a warranty condition — installing at 24 inches typically voids the cover even if nothing visibly fails.
- Can I use 24 inch joist spacing?
- Only with 2x wood decking laid straight across. 5/4 wood, composite, PVC and hardwood all need 16 inches or tighter. If you are re-decking an old frame built at 24 inches and want to move to composite, you have to add joists between the existing ones — which is the hidden cost that catches most re-decking projects out.
- Why does diagonal decking need closer joists?
- A board crossing the joists at 45 degrees has to bridge a longer unsupported distance — about 41% further than the spacing itself. So 16 inch spacing effectively becomes 22.6 inches of unsupported board. Dropping to 12 inch spacing brings the effective distance back to about 17 inches, which the board can handle.
- How many joists do I need?
- Divide the width the joists are spaced along by the spacing in feet, then add one for the final joist. A 16 foot wide deck at 16 inch spacing needs 13 joists. Add extra for double joists under any butt joint in the decking and under anything heavy like a hot tub.
- Does joist spacing affect how far the joists can span?
- Yes, substantially. Closing the spacing shares the load between more joists, so each one carries less and can reach further. A 2x8 in southern yellow pine spans about 11 ft 10 at 16 inch centres but 13 ft 1 at 12 inch centres — so tightening the spacing for composite decking can remove the need for an extra beam.
