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 estimate 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 from 0% to 30% based on your project's complexity.

How do I calculate square footage of a deck?

Multiply the deck's length by its width in feet. For example, a 12'×16' deck is 192 sq ft, a 10'×12' deck is 120 sq ft, and a 16'×20' deck is 320 sq ft. For L-shaped or irregular decks, divide into rectangular sections and add their areas together. The calculator's deck area section shows sq ft and sq m automatically.

How do I calculate linear feet for decking?

Multiply the total number of deck boards (including waste) by the board length in feet. For example, 95 boards × 12' each = 1,140 linear feet. If you're buying by the linear foot rather than per board, use this number. The calculator shows total linear feet (or meters in metric mode) in the Material Breakdown section.

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×).

Does this calculator include deck stairs, railings, posts, or footings?

No. This decking calculator only estimates deck boards, fasteners, linear feet, and material cost for the deck surface. It does not calculate stairs (use our Stair Calculator), joists or beams (use our Joist Span Calculator), posts, footings, railings, balusters, load capacity, labor, or permits. These require separate structural calculations or professional assessment.

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 do I calculate decking material cost?

Enable the Cost Estimate toggle and enter your price per board and per fastener. The calculator multiplies total boards × price per board and total fasteners × price per fastener. For example, 95 boards at $12 each = $1,140, plus 1,204 screws at $0.10 each = $120.40, for a total of $1,260.40. You can change the currency for international pricing.

How much does decking cost per square foot?

As of mid-2026, approximate US material-only ranges are: pressure-treated wood ~$2–4/sq ft, composite ~$5–12/sq ft, PVC ~$8–15/sq ft. Labor (when hiring a professional) varies widely by region. This calculator estimates material costs only — enter your actual price per board and per fastener for an estimate tailored to your local pricing.

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 / mm)

Board Length (ft / m)

Joist Spacing (in / mm)

Board Gap (in / mm)

2/16″ / 3mm

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. Switching to Diagonal auto-sets 15% — the pattern only adjusts waste; board count uses the same row formula.

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

Deck: 16.0ft × 20.0ft · Board: 5.5″ × 12′ · Gap: 2/16″ · Waste: 10%

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 estimate 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 a reliable estimate 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.

What It Estimates

Deck boards — Total count with waste & border boards
Fasteners — Screws or hidden clips by joist spacing
Linear feet — Total running length of all boards
Waste factor — Adjustable % for off-cuts & defects
Material cost — Board & fastener cost, multi-currency

What It Doesn't Estimate

Stairs, railings, posts, footings — use our Stair Calculator
Joists, beams, structural framing — use our Joist Span Calculator
Deck stain or sealant — use our Deck Stain Calculator
Load capacity
Labor costs or installation
Permits or building code

How to Calculate Deck Boards

Step-by-step calculation with formulas and real examples

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) ⌉

Multiply by rows & apply waste

Multiply boards per row by the number of rows (deck length ÷ board length, rounded up). Add optional border boards, then apply your waste percentage.

Total = ⌈ (Boards/row × Rows + Border) × (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 Examples — 5½″ × 12′ boards, ⅛″ gap, 10% waste

10′ × 12′ Deck = 29 boards

Area = 10 × 12 = 120 sq ft

Boards per row = ⌈(144″ + 0.125″) ÷ 5.625″⌉ = 26

Rows = ⌈10′ ÷ 12′⌉ = 1 · Total = ⌈26 × 1.10⌉ = 29 boards

12′ × 16′ Deck = 39 boards

Area = 12 × 16 = 192 sq ft

Boards per row = ⌈(192″ + 0.125″) ÷ 5.625″⌉ = 35

Rows = ⌈12′ ÷ 12′⌉ = 1 · Total = ⌈35 × 1.10⌉ = 39 boards

16′ × 20′ Deck = 95 boards

Area = 16 × 20 = 320 sq ft

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

Rows = ⌈16′ ÷ 12′⌉ = 2 · Total = ⌈86 × 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/boardLifespan:Varies by climateMaintenance:Needs periodic staining/sealingBest for:Budget builds, traditional look

Capped Composite

Price:~$25–45/boardLifespan:See manufacturer warrantyMaintenance:Occasional cleaningBest for:Low maintenance, hidden fasteners

PVC / Cellular PVC

Price:~$35–55/boardLifespan:See manufacturer warrantyMaintenance:Minimal; 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′.

Prices are approximate US retail ranges as of mid-2026 and vary by region, supplier, and quantity. Lifespan depends on climate, installation quality, and maintenance. Always check manufacturer specifications and get local quotes. Board-count formulas follow standard construction math (area ÷ effective width); joist guidance aligns with common IRC practice. Consult a licensed contractor for structural requirements and your local building department for code compliance.

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. Some calculators omit board gap entirely, which can overestimate material by 2–5%. This calculator includes an adjustable gap slider so the count reflects your actual spacing.

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.

Expecting structural estimates from a decking calculator

This calculator estimates deck boards and fasteners only — it does not size joists, beams, posts, or footings, and does not estimate load capacity, labor, or permit requirements.

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

Composite decking requires the same board count calculation as wood, but with a few key differences: use a 3⁄16" gap (not ⅛") to allow for thermal expansion, and always use 12" or 16" OC joist spacing. For a 16'×20' deck with 5½" composite boards, 3⁄16" gap, and 10% waste you'd need about 95 boards — the same as wood for this particular size, though the wider gap can reduce the count for smaller decks. Change the material dropdown and board gap slider in our calculator for composite-specific results.

Pressure-treated wood is generally the most affordable upfront. Capped composite costs more per board but requires less maintenance. PVC decking is typically the most expensive but is fully waterproof. Over the lifetime of the deck, composite and PVC can cost less overall than wood when ongoing maintenance like staining and sealing is factored in. Use the cost toggle in the calculator to compare based on your local prices.

Estimates disclaimer: This calculator provides material estimates based on the dimensions and values you enter. Board counts assume rectangular deck areas, straight/field rows, and the gap/waste percentages you select. Actual requirements vary with board condition, supplier tolerances, on-site cuts, and installation method. Always consult a qualified contractor for structural elements (joists, beams, posts, footings) and check local building codes before starting any deck project.

Embed Decking Calculator

Add this calculator to your website or blog for free.

Explore more calculators

Last updated Jul 4, 2026