How to calculate decking
Decking is estimated from the deck's surface area and the size of each board:
Boards = deck area × (1 + waste %) ÷ (board coverage)
Board coverage accounts for the small gap left between boards for drainage and expansion: (board width + gap) ÷ 12 × board length. We add your waste allowance and round up. This covers the visible deck surface — framing is a separate calculation.
Typical board sizes
| Nominal size | Actual width |
|---|---|
| 5/4 × 6 (most common) | 5.5 in |
| 2 × 6 | 5.5 in |
| 2 × 4 | 3.5 in |
Don't forget
- Framing is separate. Joists, beams, posts, and footings depend on your span and local code.
- Screws add up. Budget about 3.5 per square foot (two per joist crossing at 16-inch spacing).
- Diagonal layouts waste more. Add 15%+ for diagonal boards or a picture-frame border.
Decking materials compared
The board you choose drives both the cost and how much upkeep the deck needs:
| Material | Relative cost | Lifespan | Upkeep |
|---|---|---|---|
| Pressure-treated pine | $ (lowest) | 15–20 yrs | Seal/stain every 1–3 yrs |
| Cedar / redwood | $$ | 15–25 yrs | Seal every 2–3 yrs |
| Composite (e.g. Trex) | $$$ | 25–30+ yrs | Wash only |
| PVC | $$$ | 30+ yrs | Wash only |
| Tropical hardwood (ipe) | $$$$ | 40+ yrs | Oil to keep color |
Pick a board length that fits
Decking comes in even lengths (8, 12, 16, 20 ft). Choosing a length that divides into your deck run reduces cuts, waste, and the number of end-to-end seams (butt joints). Where boards do meet, stagger the joints over a joist and plan the run direction before you buy.
Fasteners: screws or hidden clips
Budget about 3.5 screws per square foot for face-screwed boards (two per board at each joist, joists 16 in on-center). Grooved composite and PVC boards usually use hidden fasteners that clip into the side groove for a screw-free surface — those are sold by the square foot, so match them to your deck area.
How much does decking cost?
Boards run roughly $10–$60 each — pressure-treated at the low end, composite and PVC in the middle, hardwood at the top. Remember this calculator covers the boards and screws only; for a full project budget (framing, footings, railing, and labor) see our guide on how much it costs to build a deck.
Beyond the boards: structure and railing
The decking is just the surface — a safe deck stands on a structure this calculator doesn't size:
- Ledger & flashing. Where a deck attaches to the house, the ledger must be lag-bolted and flashed correctly — a failed ledger is the most common cause of deck collapse.
- Joists, beams, posts, footings. Sized to your span and load by code, with footings below the frost line.
- Railing. Required once the deck is more than ~30 inches above grade — usually 36 in tall in homes, with balusters less than 4 in apart.
- Stairs. Plan stringers, treads, and a graspable handrail — our stair calculator sizes the steps.