How to Calculate Fence Posts, Pickets, Deck Boards & Footings Accurately
Exterior wood and composite carpentry projects—including privacy fences, picket fences, split-rail borders, ground-level floating decks, and elevated patio decks—require translating overall linear footage or square footage into individual dimensional lumber pieces, hardware counts, and concrete footing bags. A key pitfall for DIY builders is confusing nominal lumber names (like "1×6 picket" or "5/4×6 deck board") with actual dressed mill dimensions standardized by NIST PS 20 (where a 1×6 or 2×6 actually measures 5.5 inches wide, and a 1×4 or 2×4 actually measures 3.5 inches wide).
When planning a fence line, start with post spacing—almost universally 8 feet on-center for standard wood and vinyl fencing, or 6 feet on-center in high-wind coastal and plains regions. Divide your total fence length by the post spacing, round up to get the number of fence bays (sections), and add 1 for the terminal end post (plus extra posts for gate framing). A 6-foot-tall privacy fence requires 3 horizontal 2×4 rails per bay for warp resistance, whereas a 4-foot picket fence requires 2 rails per bay.
To calculate fence pickets or deck boards without coming up 10% short, always divide by the actual board width plus the gap spacing. For a solid zero-gap privacy fence using 1×6 dog-ear pickets (5.5" actual width), every 8-foot (96-inch) bay requires 96 ÷ 5.5 = 17.45 pickets. For a deck using 5/4×6 boards (5.5" actual width) spaced with a 1/8-inch (0.125") drainage gap, each board row covers 5.625 inches (0.46875 feet) of width.
Finally, every fence post and deck pier requires below-frost-line support. Setting a 4×4 fence post inside a 10-inch diameter hole dug 30 inches deep requires roughly 1.25 cubic feet of concrete—just over two 80-lb bags per hole. Underneath low-profile decks, spreading 3 inches of #57 crushed stone over landscape fabric stops weeds and mud splash-back while keeping framing joists dry.
Lumber Yard Ordering & Fastener Rules
Order 10% Extra Pickets and Deck Boards for Cull & Cut Waste
Pressure-treated pine and cedar bundles always contain a few boards with wane, bark edges, or bow. Including a 10% lumber allowance ensures you can set aside twisted boards and handle miters cleanly.
Use Hot-Dipped Galvanized or 304/316 Stainless Fasteners in Treated Wood
Modern ACQ and copper-azole pressure-treated lumber corrodes standard electro-galvanized nails and interior drywall screws rapidly. Always specify exterior structural deck screws, hot-dipped ring-shank nails, or stainless hidden clips.
Match Deck Board Length to Deck Dimensions to Eliminate Butt Joints
If your deck is 12 or 16 feet wide, ordering full-length 12-foot or 16-foot deck boards lets every board span the full width in a single piece with zero water-trapping butt joints.
Standard Bulk Material Densities & Coverage Reference
Compare typical weights per cubic yard and recommended application depths from our verified engineering and university extension reference library:
| Material | Planning Density | Typical Range | Tons / yd³ | Rec. Depth |
|---|---|---|---|---|
| Shredded Hardwood Bark Mulch (Dry/Seasoned) | 600 lbs/yd³ | 400–800 lbs | 0.3 tons | 2"–3" |
| Cedar or Pine Bark Mulch / Nuggets | 500 lbs/yd³ | 400–600 lbs | 0.25 tons | 2"–4" |
| Fresh / Damp Arborist Wood Chips | 800 lbs/yd³ | 700–1000 lbs | 0.4 tons | 3"–4" |
| Recycled Rubber Mulch Nuggets | 1100 lbs/yd³ | 1000–1250 lbs | 0.55 tons | 1.5"–3" |
| #57 Crushed Stone (3/4" Clean Angular Limestone/Granite) | 2700 lbs/yd³ | 2600–2900 lbs | 1.35 tons | 3"–4" |
| Driveway Crusher Run / Quarry Process (DGA Base with Fines) | 2800 lbs/yd³ | 2700–3100 lbs | 1.4 tons | 4"–6" |
| Pea Gravel (3/8" Rounded Washed Stone) | 2600 lbs/yd³ | 2500–2800 lbs | 1.3 tons | 2"–3" |
| River Rock Cobbles (1" to 3" Smooth Stone) | 2600 lbs/yd³ | 2400–2800 lbs | 1.3 tons | 3"–4" |
| Decomposed Granite (DG Pathway Fines) | 2800 lbs/yd³ | 2700–3000 lbs | 1.4 tons | 3"–4" |
| Volcanic Lava Rock (Scoria Landscape Stone) | 1300 lbs/yd³ | 1100–1500 lbs | 0.65 tons | 2"–3" |
Frequently Asked Questions (Fence & Deck)
Why does a 6-inch fence picket or deck board only cover 5.5 inches?
Under the American Softwood Lumber Standard (NIST PS 20), rough-sawn "6-inch" lumber is planed smooth and kiln-dried at the mill to an actual finished width of 5.5 inches. Using 6 inches instead of 5.5 inches in your math leaves you 9% short on boards.
How many 80 lb bags of concrete do I need per fence post?
A standard 10-inch diameter hole dug 30 inches deep around a 4×4 post requires approximately 2.1 bags of 80-lb concrete (or 2.8 bags of 60-lb concrete). Deeper 36-inch holes in northern frost zones require 2.5 to 3 bags per post.
How many deck screws do I need per 100 square feet of decking?
With standard 5.5-inch-wide deck boards and joists spaced 16 inches on-center (using 2 screws at every joist intersection), you need approximately 350 screws per 100 square feet of deck area.