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Deck Calculator

Calculate decking materials and cost

Units
Deck Dimensions
Decking Material
Framing & Structure
Stairs & Railing
Cost Estimation
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Step 1 of 5: Deck Dimensions

Deck Dimensions

Choose your deck shape and enter dimensions.

Shape

Steps

Results
Deck Area

240 ft²

Boards

60

Total Cost

$1,013

Cost/ft²

$4.22

Cost Breakdown

Decking$510
Framing$312
Hardware$81
Concrete$110
Total$1,013

Shopping List

Deck Boards60
Joists16
Beams2
Posts6
Concrete Bags (80lb)20
Deck Screws840
Joist Hangers16

What You'll Need

Simpson Strong-Tie Joist Hanger 2x6 20pk

Simpson Strong-Tie Joist Hanger 2x6 20pk

$30-$454.8
View on Amazon
Grip-Rite PrimeGuard Deck Screws 2-1/2 inch

Grip-Rite PrimeGuard Deck Screws 2-1/2 inch

$8-$124.9
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SDSNTE Heavy-Duty Deck Board Straightening Tool

SDSNTE Heavy-Duty Deck Board Straightening Tool

$25-$354.4
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Stanley FatMax 25ft Magnetic Tape Measure

Stanley FatMax 25ft Magnetic Tape Measure

$18-$254.8
View on Amazon
IRWIN Carpenter Square 8x12"

IRWIN Carpenter Square 8x12"

$10-$154.7
View on Amazon
SWANSON Tool 7 Inch Speed Square Blue

SWANSON Tool 7 Inch Speed Square Blue

$8-$124.8
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Simpson Strong-Tie Joist Hanger 2x6 20pk

Simpson Strong-Tie Joist Hanger 2x6 20pk

$30-$454.8
View on Amazon
Grip-Rite PrimeGuard Deck Screws 2-1/2 inch

Grip-Rite PrimeGuard Deck Screws 2-1/2 inch

$8-$124.9
View on Amazon
SDSNTE Heavy-Duty Deck Board Straightening Tool

SDSNTE Heavy-Duty Deck Board Straightening Tool

$25-$354.4
View on Amazon
Stanley FatMax 25ft Magnetic Tape Measure

Stanley FatMax 25ft Magnetic Tape Measure

$18-$254.8
View on Amazon
IRWIN Carpenter Square 8x12"

IRWIN Carpenter Square 8x12"

$10-$154.7
View on Amazon
SWANSON Tool 7 Inch Speed Square Blue

SWANSON Tool 7 Inch Speed Square Blue

$8-$124.8
View on Amazon

As an Amazon Associate, we earn from qualifying purchases.

Frequently Asked Questions

Q

How do you calculate decking materials?

Calculate deck area (length × width). Determine boards needed based on board width and length. Calculate joists based on spacing (typically 16" on center). Estimate posts every 6-8 feet along the perimeter.

  • Area formula: Length (ft) × Width (ft) = square footage
  • A 12 × 20 ft deck = 240 sq ft needs ∶52 boards (5.5" wide, 12 ft long)
  • Joists: divide deck length by spacing (16") and add 1 for the end joist
  • Posts: 1 post every 6–8 ft along each beam line, minimum 4×4 treated lumber
  • Add 10–15% waste factor for cuts, defects, and end-of-board trimming
MaterialCost per Board (12 ft)LifespanMaintenance
Pressure-treated pine$8–1510–15 yearsStain every 2–3 years
Cedar$15–2515–20 yearsSeal every 2–3 years
Composite (Trex)$25–4525–30 yearsMinimal – wash only
PVC / Cellular$35–6030+ yearsNone required
Q

What is standard deck board spacing?

Deck boards are typically installed with 1/8" to 1/4" gap between boards for drainage. Standard board width is 5.5" (5/4 board) or 5.5" with spacing.

  • 1/8" gap is standard for dry wood that will expand with moisture
  • 1/4" gap works for pre-dried or composite boards that won't expand
  • Actual 5/4×6 board measures 1" thick × 5.5" wide
  • 16d nails or 2.5–3" deck screws, 2 per joist per board
  • Composite boards may require hidden fastener systems ($0.50–$1.00 per clip)
Q

How far apart should deck joists be?

Standard joist spacing is 16 inches on center (OC) for most decking materials. Some composite decking allows 24" OC spacing. Check manufacturer specifications for your decking material.

  • 16" OC: required for standard wood decking and most composites
  • 12" OC: recommended for diagonal deck board patterns
  • 24" OC: only allowed with specific composite brands on straight runs
  • Common joist sizes: 2×6 (up to 8 ft span), 2×8 (up to 12 ft), 2×10 (up to 14 ft)
  • Double the rim joist where the ledger attaches to the house
Joist SpacingBest ForBoards per 12 ft Width
12" OCDiagonal patterns, heavy loads13 joists
16" OCStandard wood and composite10 joists
24" OCSelect composites, straight only7 joists
Q

How many posts do I need for a deck?

Posts are typically spaced 6-8 feet apart along the perimeter. For a 20ft deck, you'd need posts at 0ft, 6ft, 12ft, 18ft, and 20ft (5 posts per side).

  • Maximum post spacing: 8 ft for 2×8 beams, 6 ft for 2×6 beams
  • Minimum post size: 4×4 for decks under 6 ft high, 6×6 for taller
  • Footing depth: below frost line (24–48" depending on region)
  • Typical footing diameter: 12" for low decks, 16–20" for elevated decks
  • A 20 × 12 ft deck typically needs 8–10 posts total

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Example Calculations

1Standard Deck (20 × 12 ft)

Inputs

Deck Length20 ft
Deck Width12 ft
Board Width5.5 in
Board Length12 ft
Joist Spacing16 in
Board Price$8.50

Result

Deck Area240.0 sq ft
Boards Needed52
Joists Needed16
Posts Needed10
Estimated Cost$442

Area = 20 × 12 = 240 sq ft. Board width in feet = 5.5/12 = 0.458 ft. Boards per row = ceil(12/0.458) = 27. Rows = ceil(20/12) = 2. Total boards = 27 × 2 = 54. Joists = ceil(20/(16/12)) + 1 = 16. Posts = ceil(20/6 + 1) × 2 = 10. Cost = 54 × $8.50 = $459.

2Large Deck (30 × 16 ft)

Inputs

Deck Length30 ft
Deck Width16 ft
Board Width5.5 in
Board Length16 ft
Joist Spacing16 in
Board Price$12.00

Result

Deck Area480.0 sq ft
Boards Needed70
Joists Needed24
Posts Needed12
Estimated Cost$840

Area = 30 × 16 = 480 sq ft. Board width in feet = 5.5/12 = 0.458 ft. Boards per row = ceil(16/0.458) = 35. Rows = ceil(30/16) = 2. Total boards = 35 × 2 = 70. Joists = ceil(30/(16/12)) + 1 = 24. Posts per side = ceil(30/6) + 1 = 6. Total posts = 6 × 2 = 12. Cost = 70 × $12 = $840.

Formulas Used

Deck Area

Deck Area = Length × Width

Total deck surface area.

Where:

Length= Deck length in feet
Width= Deck width in feet

Boards Needed

Boards = ceil(Width / Board Width) × ceil(Length / Board Length)

Number of boards to cover the deck based on board width (in inches, converted to feet) and board length.

Where:

Board Width= Width of each board in inches
Board Length= Length of each board in feet

Joists & Posts

Joists = ceil(Length / Joist Spacing) + 1, Posts = ceil(Length / 6 + 1) × 2

Joists spaced at regular intervals. Posts placed every 6 ft along both sides of the deck.

Where:

Joist Spacing= Distance between joists in inches (typically 16")
6= Post spacing in feet

Complete Guide to Deck Materials, Costs, and Planning

1

2026 Decking Material Costs and Lifespan

Pressure-treated pine remains the most affordable decking option at $8–$15 per 12-foot board, but its 10–15 year lifespan means two full replacements over the life of a composite deck. Composite boards from brands like Trex and TimberTech run $25–$45 per board yet last 25–30 years with virtually zero maintenance beyond occasional washing.

PVC and cellular decking sits at the premium end at $35–$60 per board with a 30+ year warranty and complete immunity to moisture, rot, and insects. For a standard 12×20 ft deck (240 sq ft), material costs range from roughly $450 for pressure-treated pine to over $3,000 for PVC—but the lifecycle cost tells a different story.

When you factor in staining every 2–3 years at $1–$3 per square foot, pressure-treated pine costs $2,400–$4,300 over 30 years while composite costs $1,300–$2,300 total. Use our deck staining calculator to estimate your recurring refinishing costs.

*Lifecycle cost includes replacement, staining, and repairs over 30 years for a 240 sq ft deck
MaterialCost per Board (12 ft)LifespanMaintenance30-Year Cost (240 sq ft)
Pressure-Treated Pine$8–$1510–15 yrStain every 2–3 yr$2,400–$4,300
Cedar$15–$2515–20 yrSeal every 2–3 yr$2,000–$3,500
Composite (Trex)$25–$4525–30 yrWash only$1,300–$2,300
PVC / Cellular$35–$6030+ yrNone required$1,800–$3,100
2

Complete Material Checklist for a Deck Build

A 12×20 ft deck at 16" on-center joist spacing requires approximately 52 deck boards (5.5" wide, 12 ft long), 16 joists (2×8 or 2×10 depending on span), 10 posts (4×4 minimum), and 2 ledger boards bolted to the house framing. Each board gets 2 screws per joist crossing, totaling roughly 1,600–1,700 deck screws for the surface alone.

Joist hangers are required at every joist-to-beam connection—budget $1.50–$3.00 per hanger and 10 joist-hanger nails each. Post bases ($6–$12 each) anchor posts to concrete footings, and post caps ($4–$8) connect posts to beams. Do not forget the flashing tape ($15–$25 per roll) for the ledger board to prevent water intrusion into the house.

Hardware costs add $150–$350 on top of lumber for a mid-size deck. Ordering 10–15% extra boards avoids project delays from warped or defective lumber.

*Prices reflect national averages as of early 2026
ComponentQuantity (12×20 ft Deck)Unit CostSubtotal
Deck boards (5.5" × 12 ft)52–57$8–$15$440–$855
Joists (2×8, 12 ft)16$9–$14$144–$224
Posts (4×4, 8 ft)10$8–$12$80–$120
Deck screws (5 lb box)3–4 boxes$28–$35$84–$140
Joist hangers16$1.50–$3$24–$48
3

Board Spacing: 1/8-Inch vs 1/4-Inch Gaps

Standard deck board spacing is 1/8" for freshly milled lumber that will expand as it absorbs moisture, and 1/4" for kiln-dried or composite boards that have already reached stable dimensions. Incorrect spacing causes either buckling (too tight) or excessive debris trapping (too wide).

For pressure-treated pine delivered wet from the mill, many contractors butt boards tightly with no gap—the wood shrinks as it dries over 4–6 weeks, naturally opening to roughly 1/8". Composite manufacturers typically specify 3/16" to 1/4" gaps, and most provide proprietary hidden fastener systems ($0.50–$1.00 per clip) that set spacing automatically.

Use a 16d nail as a 1/8" spacer or a carpenter’s pencil for a 1/4" gap. Consistent spacing across the entire deck is more important than the exact gap width—variations are visible from any angle and look unprofessional.

Board TypeRecommended GapSpacer ToolFastener Type
Wet pressure-treated0" (butt tight)None needed2 screws per joist
Dry/seasoned wood1/8"16d nail2 screws per joist
Composite3/16"–1/4"Manufacturer clipHidden fastener system
PVC1/4"Carpenter pencilHidden fastener system

Tip: Run a string line along the first installed board and measure to it every 5–6 boards. If cumulative gap error exceeds 1/4", adjust spacing on the next few boards to correct.

4

Annual Maintenance Costs by Material

A 240 sq ft pressure-treated pine deck costs $240–$720 per year to maintain when you factor in staining ($1–$3/sq ft every 2–3 years), board replacement (1–2 boards per year at $8–$15 each), and annual cleaning supplies ($15–$30). Over 15 years that totals $3,600–$10,800 in maintenance alone—often more than the original lumber cost.

Composite decks reduce annual maintenance to $30–$50 for a yearly power wash and occasional spot treatment for mold in shady areas. The beam size calculator helps you verify that structural members meet code for your specific span and load requirements, which is especially important when replacing a deteriorated substructure.

Inspect your deck each spring for loose fasteners, split boards, and post rot at the ground line. Catching a rotted post early costs $50–$100 to replace; ignoring it can lead to a $2,000–$5,000 structural failure. Check connections to the concrete footings for any signs of heaving or settling.

  • Pressure-treated pine — $240–$720/year (stain, replace 1–2 boards, clean)
  • Cedar — $200–$600/year (seal, occasional board swap, clean)
  • Composite — $30–$50/year (power wash only)
  • PVC — $15–$30/year (rinse with hose)
  • All materials — inspect fasteners, posts, and ledger flashing annually

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Last Updated: Mar 26, 2026

This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.

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