Deck Calculator
Estimate the number of decking boards, fasteners, and total cost for your deck project. Accounts for board size, gap spacing, wastage, and fastener type with instant results.
Size of Deck
Deck Board Size
1/8" (0.125) is standard for most decks.
10% for straight lay, 15% for diagonal.
Fasteners
Cost Estimation
(Optional)Deck Estimate
To build a 20 ft × 12 ft deck (240.0 sq ft) with 3.5 in wide boards, you will need approximately 40 boards. Including 10% wastage, you should order 44 boards.
How many decking boards do you actually need?
Whether you’re building a new deck or replacing old boards, running out of materials mid-project is one of the most frustrating mistakes you can make. Our deck calculator takes the guesswork out — enter your deck dimensions, board size, and gap spacing, and get an instant count of exactly how many boards and fasteners to order, including wastage.
The math is straightforward, but the details matter. A gap of even 1/8 inch changes your board count across a large area. And without accounting for waste from cuts, defects, and edge trimming, you’ll find yourself ordering more boards at the last minute — often from a different batch that doesn’t quite match in color or grain.
How to calculate decking boards by hand
Here’s the step-by-step process for figuring out your board count:
- Measure your deck area. Multiply length by width to get total square footage (or square meters).
Deck Area = Length × Width
- Calculate effective board width. Add the gap spacing to your board width.
Effective Width = Board Width + Gap
- Determine boards across the width. Divide deck width by effective board width and round up.
Boards Across = Deck Width ÷ Effective Width
- Determine boards per row. Divide deck length by board length and round up.
Boards per Row = Deck Length ÷ Board Length
- Calculate total boards. Multiply boards across by boards per row.
Total Boards = Boards Across × Boards per Row
- Add wastage. Multiply by 1 + (waste percentage / 100).
Boards to Order = Total Boards × (1 + Waste%)
Example: A 20 ft × 12 ft deck with 5.5-inch boards, 12 ft long, 1/8 inch gap, and 10% waste:
- Deck area = 20 × 12 = 240 sq ft
- Effective board width = 5.5 + 0.125 = 5.625 in = 0.469 ft
- Boards across = (12 × 12) ÷ 5.625 = 25.6 → 26 boards
- Boards per row = 20 ÷ 12 = 1.67 → 2 boards
- Total boards = 26 × 2 = 52 boards
- With 10% waste = 52 × 1.10 = 57.2 → 58 boards
Common deck board widths
Deck boards come in a range of widths. Here’s a reference table:
| Width (in) | Width (mm) | Effective with 1/8” gap | Boards per 100 sq ft |
|---|---|---|---|
| 3.5 | 90 | 3.625 in | ~29 |
| 4.0 | 100 | 4.125 in | ~26 |
| 5.0 | 125 | 5.125 in | ~21 |
| 5.5 | 140 | 5.625 in | ~19 |
| 6.0 | 150 | 6.125 in | ~18 |
Wider boards cover more area per piece, so you’ll need fewer of them. However, they can be harder to handle and may require more structural support underneath.
Understanding board gaps
The gap between deck boards isn’t just aesthetic — it serves a critical purpose:
- Water drainage: Rain and meltwater need a path to drain off the deck surface. Without gaps, water pools and accelerates rot.
- Expansion space: Wood and composite boards expand and contract with temperature and humidity changes. Gaps prevent buckling.
- Air circulation: Proper airflow beneath the deck helps prevent mold and mildew growth.
Standard gap recommendations:
- 1/8 inch (3mm): The most common default. Works well for most residential decks.
- 3/16 inch (5mm): Better drainage in rainy climates or for composite boards.
- 1/4 inch (6mm): Maximum recommended gap; wider than this and small objects can fall through.
How much waste should you plan for?
Waste comes from cuts at edges and obstacles, board defects, and pattern alignment. How much you need depends on your deck layout:
| Deck Layout | Waste % | Why |
|---|---|---|
| Simple rectangle, straight lay | 5–10% | Minimal cuts, predictable pattern |
| Rectangle with obstacles | 10–15% | Cuts around posts, planters |
| Diagonal or herringbone | 15–20% | Every edge requires angled cuts |
| Multi-level or curved | 20%+ | Many complex cuts |
Pro tip: Order all your boards at once. Even the same board from the same manufacturer can vary between production runs. Matching boards from a different batch months later is nearly impossible.
Tips for buying deck boards
- Order 10% extra as a baseline. It’s cheaper to have extras than to reorder and risk a color mismatch.
- Buy all boards at once. Batches can vary in color, grain, and shade.
- Check board lengths. 12 ft and 16 ft are standard; longer boards reduce seams but are harder to handle.
- Keep extras stored after installation for future repairs.
Planning a larger outdoor project?
Your deck is just one piece of the puzzle. These calculators can help you plan the full scope:
- Paver Calculator — Estimate pavers for patios and walkways
- Flooring Calculator — Calculate flooring for indoor spaces
Frequently Asked Questions
Common questions about this calculator.
How many deck boards do I need?
To find the number of deck boards, multiply your deck length by width to get the total square footage, then divide by the coverage of a single board. For standard 5.5-inch wide boards, you need roughly 19 boards per 100 sq ft with a 1/8-inch gap. Always add 10–15% for wastage from cuts and defects. Our deck calculator does this automatically — just enter your dimensions and board size for an accurate count.
How much wood do I need for a deck?
The amount of lumber depends on your deck's square footage, board dimensions, and framing layout. Beyond decking boards, you'll need joists (typically spaced 16 inches on center), beams, posts, and rim boards. A 200 sq ft deck with standard framing can require 1,000–1,500 board feet of lumber total. Use our calculator for the decking boards, and plan your deck framing separately based on your local building codes.
How many deck screws do I need?
A good rule of thumb is 350 deck screws per 100 square feet of decking, or about 2 screws per board joist intersection. For a 200 sq ft deck, that's roughly 700 screws. If you're using hidden fasteners or clips, you'll need about half as many — around 175 per 100 sq ft. Always buy a box extra; screws are inexpensive insurance against stripped heads and misplaced fasteners.
How many deck posts do I need?
Deck posts support your beams and are typically spaced 6–8 feet apart, depending on beam span and load requirements. For a simple 12 ft × 16 ft deck, you'll usually need 6 posts — three per beam line. Larger or multi-level decks require more. Check your local building codes for maximum post spacing, as requirements vary by region and soil conditions. Posts are part of the deck framing, not the surface boards.
How many joists do I need for my deck?
Joists are the framing members that run perpendicular to your deck boards and support them. Most residential decks use joists spaced 16 inches on center, though 12-inch spacing is needed for diagonal or 5/4-inch boards. To calculate, divide your deck width (in inches) by the joist spacing, then add one for the end joist. For a 12 ft wide deck with 16-inch spacing, that's (144 ÷ 16) + 1 = 10 joists.
How many 5/4 deck boards do I need?
5/4-inch deck boards are thinner than standard 2-by lumber and are commonly used for residential decking. They're typically 5.5 inches wide, so the board count is the same as standard widths — roughly 19 boards per 100 sq ft with a 1/8-inch gap. However, 5/4 boards require joists spaced no more than 16 inches on center for proper support. They're lighter and easier to work with, making them a popular choice for DIY deck construction.
How many deck blocks do I need?
Deck blocks (precast concrete pier blocks) support your deck posts without pouring footings. You need one block per post, typically spaced 6–8 feet apart along each beam line. For a basic 12 ft × 16 ft deck, that's usually 6 blocks. Place blocks on level, compacted gravel for stability. Deck blocks work well for low-level decks under 24 inches, but taller structures may require buried footings per local building codes.
How do I square a deck?
The most reliable method is the 3-4-5 triangle technique. From one corner, measure 3 feet along one edge and mark it. Measure 4 feet along the adjacent edge and mark it. The diagonal between those marks should be exactly 5 feet. If it's not, adjust the framing until it is. For larger decks, multiply by a common factor — use 6-8-10 or 9-12-15 for more precision. Always check both diagonals; they should be equal when the deck frame is square.
How much deck stain do I need?
Most deck stains cover 200–400 square feet per gallon, depending on the product and wood porosity. For a 200 sq ft deck, plan on 1–2 gallons for a single coat, or 2–4 gallons for two coats. New, unsealed wood absorbs more and may require an extra coat. Check the manufacturer's coverage rate on the can and factor in your deck's lumber type — rough-sawn boards soak up significantly more stain than smooth, factory-finished surfaces.
How much does it cost to build a deck?
Deck costs vary by material, size, and complexity. Pressure-treated lumber runs $3–6 per sq ft for materials alone. Composite decking costs $8–15 per sq ft, and exotic hardwoods can reach $15–30 per sq ft. A 200 sq ft deck in pressure-treated wood typically costs $1,000–3,000 for materials, while composite can run $2,000–5,000. This doesn't include labor, railings, stairs, or permits. Our calculator estimates material costs for boards and fasteners to help you budget.