How many deck boards do I need?

The number of deck boards depends on your deck dimensions, board width, board length, and the gap between boards. For a typical 16'×20' deck using 5½" wide × 12' long boards with ⅛" gap and 10% waste, you'll need approximately 95 boards. Use the calculator above for an exact count based on your specific measurements.

How do I calculate deck boards with gaps?

Add the gap width to the board's nominal width to get the effective board width. Then divide the deck dimension across the board run by the effective width to find boards per row. Multiply by the number of rows (deck dimension along the boards ÷ board length) and round up. Our calculator does this automatically — use the Board Direction toggle to choose which way boards run.

How much extra decking should I buy for waste?

For a straight (90°) layout, add 10% extra for off-cuts and mistakes. For diagonal (45°) layouts, add at least 15%. If you're using a complex pattern like herringbone, add 20–30%. The waste factor slider in our calculator lets you adjust this based on your project's complexity.

What is the standard size for deck boards?

The most common residential deck board is 5½" wide (nominal 2×6 or 5/4×6) in 12' or 16' lengths. 3½" boards (2×4) are used for narrower profiles. Composite decking also comes in 7¼" wide planks for a modern look. Board thickness is typically 1" (5/4) or 1½" (2×).

Composite vs wood decking — which costs more?

Pressure-treated wood is the most affordable at $8–15 per 12' board. Capped composite costs $25–45 per board but requires virtually no maintenance. PVC decking is the most expensive at $35–55 per board but is fully waterproof. Over 25 years, composite and PVC often cost less than wood when maintenance (staining, sealing, replacement) is factored in.

How many screws do I need per deck board?

For face-screwed decking, you need 2 screws at every joist intersection. For a 12' board on 16" OC joists, that's about 18–20 screws per board. Hidden fastener systems use 1 clip per joist intersection. The calculator automatically computes this based on your board count, joist spacing, and fastener type.

What joist spacing should I use for decking?

16" on center is standard for residential wood decking. 12" OC is recommended for composite decking (required by most manufacturers) and for decks with diagonal board layouts. 24" OC is only suitable for 2× thick wood decking running perpendicular to joists. Always check your local building code and manufacturer specifications.

How do I measure my deck for replacement boards?

Measure the full length and width of the deck surface. For existing decks, you can measure board count × board width as a cross-check. For irregular shapes, divide into rectangular sections and calculate each separately. Don't forget to account for stairs and any picture frame border if you're doing a full replacement.

How much does decking cost per square foot?

Pressure-treated wood decking costs $2–4 per sq ft for materials only. Composite decking runs $5–12 per sq ft. PVC ranges from $8–15 per sq ft. Labor adds $15–35 per sq ft for professional installation. Our calculator estimates material costs only — enter your price per board and per fastener for a detailed material estimate.

Do I need a permit to build or replace a deck?

Most jurisdictions require a building permit for new decks, especially if attached to the house or above a certain height (typically 30"). Deck board replacement on an existing substructure usually does not require a permit, but always check with your local building department. Structural changes (joists, beams, footings) almost always require permits and inspections.

Decking Calculator

Units

Deck Dimensions

Length (ft)

Width (ft)

Board Specifications

Material

Board Width (in)

Board Length (ft)

Joist Spacing (in)

Board Gap (Spacing)

2/16"

Gap between boards for drainage and expansion. Typical: 1/8". Leave 0 for butt-joint.

Waste Factor

10%

Extra material for cuts, mistakes, and defects. 10% is standard for straight layouts.

Decking Pattern

Board Direction

Fastener Type

Picture Frame / Border Boards

Add perimeter border boards

Cost Estimate

Estimate material costs for your project.

Price per Board

Price per Fastener

boards

320 sq ft+9 waste

43 boards/row × 2 rows

Material Breakdown

Board count and dimensions

Boards Per Row

Number of Rows

Effective Board Width

5.625in

Total Linear Feet

1,140ft

Fasteners

Per joist intersection count

Screws (16" OC, 13 joists)

1,204

Deck Area

Total surface area

Area (sq ft)

320.0ft²

Area (sq m)

29.73m²

What is a Decking Calculator?

A quick, accurate way to estimate decking materials before you buy.

A Decking Calculator helps homeowners, DIY builders, and contractors determine exactly how many deck boards, fasteners, and materials are needed for a deck project. Instead of guessing or doing manual math, you enter your deck dimensions, board size, and spacing preferences — and get an accurate count in seconds.

320 sq ft

Avg. Deck Size

16' × 20'

5½" × 12'

Common Board

2×6 pressure-treated

⅛"

Standard Gap

Wood decking

10%

Typical Waste

Straight layouts

Used by: DIY homeowners planning weekend builds, contractors estimating material orders, and lumber yards helping customers size purchases.

How It Works

Step-by-step calculation with formulas

Measure deck area

Multiply length × width to get the total square footage. For imperial, measurements are in feet. For metric, measurements are in meters.

Area = Length × Width

Calculate boards per row

Divide the deck span across the board run by the effective board width, accounting for the gap only between boards (not after the last). Always round up.

Boards per row = ⌈ (Span + Gap) ÷ (Board width + Gap) ⌉

Add border & apply waste

Add border boards to field count, then apply your waste percentage. Switching to diagonal recommends 15% waste (auto-set from the default 10%).

Total = ⌈ (Field boards + Border boards) × (1 + Waste%) ⌉

Count fasteners

Each board line crosses every joist, plus extra fastening at butt joints between board pieces. Face screws use 2 per intersection; hidden clips use 1.

Fasteners = Board lines × (Joists + Seams) × Fasteners per intersection

Worked Example — 16' × 20' Deck

Area = 16 × 20 = 320 sq ft

Effective board width = 5.5″ + 0.125″ = 5.625″

Boards per row = ⌈(240″ + 0.125″) ÷ 5.625″⌉ = 43 boards

Rows = ⌈16′ ÷ 12′⌉ = 2 rows

Total = ⌈(86 field + 0 border) × 1.10⌉ = 95 boards

Fasteners = 43 lines × (13 joists + 1 seam) × 2 = 1,204 screws

Board Materials & Sizes

Wood, composite, and PVC compared

Choosing the right decking material affects cost, maintenance, and lifespan. Here's how the three main options compare:

Pressure-Treated Wood

Price:$8–15 per boardLifespan:15–20 yearsMaintenance:Annual staining/sealing requiredBest for:Budget builds, traditional look

Capped Composite

Price:$25–45 per boardLifespan:25–30 yearsMaintenance:Occasional cleaning onlyBest for:Low maintenance, hidden fasteners

PVC / Cellular PVC

Price:$35–55 per boardLifespan:30+ yearsMaintenance:Virtually none, fully waterproofBest for:Pool decks, wet climates, premium

Board widths: 3½″ (2×4, 5/4×4) for narrow profiles · 5½″ (2×6, 5/4×6) the residential standard · 7¼″ composite wide planks for a modern look with fewer fasteners. Standard lengths are 8′, 10′, 12′, 16′, and 20′.

Waste Factor & Board Spacing

Why gaps and extra material matter

10%

Straight Layout

Standard waste for perpendicular boards

15–20%

Diagonal Layout

Angled end cuts — recommended minimum

20%

Complex Patterns

Herringbone, picture frame with miters

Why board gap matters

Board gap (spacing between deck boards) allows for drainage and thermal expansion. Wood decks typically use ⅛″ gap. Composite and PVC expand more with temperature changes and need 3⁄16″ gap. Most online calculators ignore board gap entirely, overestimating material by 2–5%. Our calculator includes an adjustable gap slider for accurate counts.

Common Mistakes to Avoid

Don't let these errors cost you time and money

Skipping the waste factor

Always add at least 10% for off-cuts and mistakes. Running short mid-project means delays, extra delivery fees, and potential dye-lot mismatches on composite products.

Butt-jointing boards with zero gap

Boards expand and contract with temperature and humidity. Zero-gap installations buckle within the first season.

Using 24" OC spacing with composite

Composite decking requires 12" or 16" OC joist spacing. 24" OC is only suitable for 2× thick wood. Always check manufacturer specs.

Forgetting border / picture frame boards

A perimeter border adds 10–20 extra linear feet of decking. Enable the picture frame toggle to include border boards in your estimate.

Ordering exact count with no contingency

Keep an extra 2–3 boards on hand. Damaged boards, hidden defects, and measurement errors happen. It's cheaper to return unopened boxes than to reorder mid-project.

Pro Tips for Better Results

Advice from experienced deck builders

Buy all boards from the same lot

Composite and PVC dye lots can vary slightly. Order all your boards at once from the same batch to ensure color consistency across the entire deck.

Acclimate boards before installation

Let composite and PVC boards sit on-site for 24–48 hours before installing. This allows them to adjust to local temperature and humidity, reducing post-installation expansion.

Stagger your seams for a natural look

Avoid lining up board ends in a single line. Stagger end joints by at least 12" across rows. This looks better and distributes structural load more evenly.

Use the calculator in both imperial and metric

If you're ordering materials from suppliers who list in different units, toggle between ft/in and m/cm. The calculator handles unit conversion automatically so you can compare prices accurately.

Frequently Asked Questions

Common questions and detailed answers

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Last updated May 25, 2026