How many deck boards do you need? Enter your deck size and material to get an exact board count, joist layout, footing plan, and cost estimate for pressure treated, cedar, composite, and PVC decking.
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Last updated March 31, 2026 by our expert review team
| Material | Board Size | Cost/sqft | Lifespan | Maintenance |
|---|---|---|---|---|
| Pressure Treated | 5/4×6" | $3.5/sqft | 10-15 years | High |
| Cedar | 5/4×6" | $6/sqft | 15-20 years | Medium |
| Composite Standard | 1"×5.5" | $8/sqft | 25-30 years | Low |
| Composite Premium | 1"×5.5" | $13/sqft | 30-50 years | Very Low |
| PVC | 1"×5.5" | $10/sqft | 25-30 years | Very Low |
Ehsan Ghazanfari
Licensed Structural Engineer
FISE-certified structural engineer with 11+ years designing bridges, retaining walls, and foundations. MSc from Aalto University.
See full profileCherry Jane Limbago
Licensed Civil Engineer & Master Plumber
Licensed civil engineer and master plumber with 10+ years in structural design, estimation, and project management. Currently Project Engineer at Universal Robina Corporation.
See full profileUpdated March 2026
How we verify our calculatorsSpace joists at 16" OC for straight-laid decking, but drop to 12" OC for diagonal patterns or composite boards. Both Trex and TimberTech void warranties above 12" OC on angles.
Use stainless steel or ACQ-compatible coated screws, never standard galvanized nails. Copper in modern PT lumber corrodes plain galvanized hardware within 2 to 3 years.
Install self-adhesive joist tape on the tops of all joists before laying decking. A $60 roll of tape can add 10+ years to your substructure.
Always install ledger board flashing where the deck meets the house. Per IRC Chapter 5, improper ledger attachment is the leading cause of deck collapses.
Leave a 1/8" gap between boards for drainage. Use a 16d nail as a spacer, and increase to 3/16" for composite installed below 40 degrees F.
Pour footings below the frost line (36 to 48" in northern states, 12 to 18" in the South). Use our concrete cylinder calculator to estimate concrete for your deck's sonotube footings, the stair calculator to plan deck stairs, and the board foot calculator for framing lumber.
Need a deck beam? Our beam calculator sizes beams for your deck span and load requirements.
This calculator mirrors how a professional deck builder actually estimates a job. Start by entering your deck dimensions and selecting a decking material. The tool computes total square footage, then works outward through each structural layer: decking boards, joists, beams, posts, and concrete footings.
Board count uses an effective width of 5.625" (the actual 5.5" board plus a 1/8" drainage gap) rather than nominal dimensions, because using nominal 6" would leave you short by roughly one board every 4 feet of deck width. Waste factors are built in at 10% for straight layouts and 15% for diagonal patterns. Diagonal cuts produce more off-cuts that are too short to reuse, especially at the perimeter.
Joist count is derived from your chosen on-center spacing (12", 16", or 24"), and the tool sizes beams based on span tables from the IRC R507 code section. Post spacing follows the conservative 8-foot maximum, with three 80-lb bags of concrete per footing, enough for a 12" diameter sonotube at 36" depth, which satisfies most northern frost-line requirements. Stairs use the IRC standard 7.5" rise per step, and railing calculations assume code-compliant 36" height with balusters at 4" maximum spacing.
Calculate Area
Area = Length x Width (+ secondary section if L-shape)
Board Count
Rows = ceil(width_in / 5.625) x boards/row x (1 + waste%)
Framing & Footings
Joists = floor(length_in / spacing) + 1 | Posts = ceil(length / 8) + 1
Key Formulas
Boards = ceil(width_in / 5.625) x ceil(length / board_len) x (1 + waste)
Joists = floor(length_in / joist_spacing) + 1
Concrete bags = posts x 3 (80lb each)
Stair steps = round(height_in / 7.5)
How much does a deck cost per square foot?
PT lumber: $3.50 to $6/sqft. Cedar: $6 to $8. Composite: $8 to $13. Professional installation adds $20 to $28/sqft for labor.
Is composite decking worth the extra cost?
For decks you plan to keep 15+ years, yes. Composite requires only soap-and-water cleaning and lasts 25 to 50 years with warranties.
How long does a wood deck last?
PT pine: 10 to 15 years with annual maintenance. Cedar/redwood: 15 to 20 years. Regular staining and flashing are critical for longevity.
Do I need a permit to build a deck?
Any deck attached to a house or over 30 inches above grade typically requires a permit per IRC code. Freestanding ground-level decks are often exempt.
What is the best size for a backyard deck?
12x16 (192 sqft) fits a dining set for six plus a grill. Plan at least 20 sqft per person for comfortable seating.
How far apart should joists be spaced?
16" OC is standard for straight-laid boards. Use 12" OC for diagonal patterns or PVC decking. Never exceed 24" OC.
How deep do deck footings need to be?
Below the frost line: 12 inches in the South to 48+ inches in northern states. Check your local building code for the exact depth.
Can I build a deck directly on the ground?
Ground-level floating decks using deck blocks work for platforms under 30". Use ground-contact-rated lumber (UC4A min) and a gravel pad.
12' x 16' Rectangle | Pressure Treated | 3 ft Height
A straightforward rectangular deck with stairs and railing. PT 5/4x6 boards at 16" OC keeps waste at 10%.
~$2,800 DIY materials · $6,500-$7,500 installed
16' x 20' Rectangle | Trex Select Composite | 4 ft Height
320 sqft with mid-grade composite for a maintenance-free space. The 4-foot height requires railing on all open sides.
~$7,500 DIY materials · $14,000-$17,000 installed
20' x 14' + 10' x 10' L-Shape | TimberTech Premium | 3.5 ft Height
380 sqft L-shaped wraparound with premium capped composite boards. L-shape adds framing complexity and increases waste to 12-15%.
~$11,000 DIY materials · $20,000-$25,000 installed
Skipping ledger board flashing
Unflashed ledgers are the #1 cause of deck collapses. Install Z-flashing or peel-and-stick membrane between ledger and rim joist.
Setting footings above the frost line
Shallow footings heave in freeze-thaw cycles. Check local frost depth (36 to 48" in the Midwest, 12" in the South) and dig 6 inches deeper.
Using interior-rated hardware on treated lumber
ACQ/CA-B preservatives corrode standard galvanized hardware within 2 to 3 years. Use Simpson ZMAX or stainless steel connectors.
Not accounting for board end waste
Mix board lengths strategically (alternate 16' and 12' boards) to minimize waste. Always add 10 to 15% to your board count.
Ignoring local permit requirements
Attached decks or any deck over 30 inches high typically require a permit. Unpermitted decks can block home sales and void insurance.
Spacing joists at 24" OC for composite
Virtually every composite manufacturer requires 16" OC max for straight boards and 12" OC for diagonal. Exceeding this voids the warranty.
Important Disclaimer
These estimates are for planning purposes only. Actual costs vary by location, material availability, and project complexity. Always get at least 3 local quotes. This calculator does not replace professional advice.