Deck Calculator
Estimate the materials for a wood deck — decking boards, joists, posts and concrete footings — for any deck size.
A 12×12 ft deck needs about 27 boards of 12 ft 2×6 decking (29 with 10% waste).
How to Use This Deck Calculator
- Enter the deck length and width in feet.
- Choose the decking board length (12 ft is the most common) — the calculator assumes 2×6 boards (5.5 in actual width).
- Set the waste percentage (10% covers cuts and defects).
- Read the totals: boards, joists at 16 in on center, and concrete for footings.
The Formulas
Boards = deck area ÷ (board length × 0.4583 ft), rounded up — 5.5 in of actual width on a 2×6. Joists run across the width every 16 in on center. Each footing post reuses the concrete post-hole calculation from the concrete calculator.
Boards = area ÷ (L × 0.4583)2×6 decking (5.5 in actual width = 0.4583 ft)Joists = ceil(W × 12 ÷ 16) + 116 in on center spacingFootings = posts × concrete bagsReutiliza engine concrete (post-hole)Stain = area ÷ coverageReutiliza engine paintWorked Example: 12×12 Deck
A 12×12 ft deck: area 144 sq ft. Each 12 ft 2×6 board covers 12 × 0.4583 = 5.5 sq ft. 144 ÷ 5.5 = 26.2 → 27 boards (29 with 10% waste). Joists across 12 ft at 16 in OC: 144 in ÷ 16 = 9 + 1 = 10 joists.
Materials by Deck Size
| Deck size | 12 ft 2×6 boards (10% waste) | Joists (16 in OC) |
|---|---|---|
| 10 × 10 ft | 21 | 9 |
| 12 × 12 ft | 29 | 10 |
| 12 × 16 ft | 39 | 13 |
| 16 × 20 ft | 65 | 16 |
Boards rounded up. Joists = width ÷ 16 in + 1.
Decking Materials and Spacing
Pressure-treated pine is the budget standard; cedar and composites cost more but resist moisture better. Joists are spaced 16 in on center for standard residential decks. Railing balusters must be spaced so a 4 in sphere cannot pass through — the classic building-code rule. Stain coverage follows the paint calculator: roughly 350–400 sq ft per gallon per coat.
Quick Tips
- Space deck boards about 1/8 in apart so water drains between them.
- Set posts in concrete below the frost line for your area.
- Order 10% extra decking for cuts and warped boards.
Frequently Asked Questions
How many deck boards do I need for a 12×12 deck?
27 boards of 12 ft 2×6 decking, or 29 with 10% waste (each 12 ft 2×6 covers 5.5 sq ft).
How far apart should deck joists be?
16 inches on center is the standard spacing for residential decks with 2×6 decking.
How far apart should railing balusters be?
No more than 4 inches apart — building codes require that a 4 in sphere cannot pass through the railing.
What size lumber is used for deck boards?
2×6 is the most common decking size — it measures 5.5 in wide (nominal 6 in).