# Deck Calculator: Boards, Joists, Screws & Footings

> Estimate the decking boards, joists, screws and footings for a deck from its dimensions, board size and joist spacing.

**Quick answer:** A 12 × 16 ft deck with 5.5-inch boards and a 1/8-inch gap has 26 rows of decking; with 16-ft boards that is about 29 boards including 10% waste. Joists at 16 inches on center need 13 joists across a 16-ft length.

_Last updated: October 3, 2026. Estimates only; confirm quantities with your supplier._

## How to calculate deck boards

Decking runs perpendicular to the joists. Count the rows of boards across the width (board width plus gap), then how many stock-length boards each row needs along the length.

```
rows = deck width × 12 ÷ (board width + gap)
boards per row = deck length ÷ board length (round up)
boards = rows × boards per row × (1 + waste %)
```

### Decking boards by deck size (5.5 in boards, 16-ft lengths, 10% waste)

| Deck size | Area | Rows | Boards |
| --- | --- | --- | --- |
| 10 × 10 ft | 100 sq ft | 22 | 25 |
| 10 × 12 ft | 120 sq ft | 22 | 25 |
| 12 × 12 ft | 144 sq ft | 26 | 29 |
| 12 × 16 ft | 192 sq ft | 26 | 29 |
| 12 × 20 ft | 240 sq ft | 26 | 58 |
| 16 × 16 ft | 256 sq ft | 35 | 39 |
| 16 × 20 ft | 320 sq ft | 35 | 77 |
| 20 × 24 ft | 480 sq ft | 43 | 95 |

### Joists, beams and footings

Joists (usually 2×8 or 2×10) run from the house or beam to the outer beam and are spaced 16 inches on center for most decking, 12 inches for diagonal patterns or some composites. Beams rest on posts set in concrete footings, spaced up to about 8 feet apart depending on lumber size and loads. Footing depth must reach below the frost line. Use the [post hole concrete calculator](../post-hole-concrete-calculator/) for exact concrete.

### Screws and fasteners

With joists 16 inches on center and 5½-inch boards, you need roughly 3.3 screws per square foot (two per board per joist). Hidden-fastener systems use clips instead, counted separately.

### Permits and code

Most decks over about 30 inches high or attached to a house need a permit and inspections. Railings are normally required above 30 inches. Confirm local rules before buying lumber.

## Frequently asked questions

### How many deck boards do I need for a 12×16 deck?

With 5.5-inch boards and a 1/8-inch gap, a 12×16 deck has 26 rows. Using 16-foot boards that is 26 boards plus 10% waste, roughly 29.

### How wide should the gap between deck boards be?

About 1/8 inch for pressure-treated lumber that dries and shrinks, and 3/16 to 1/4 inch for composite and some hardwoods. Follow the manufacturer's spacing.

### How far apart should deck joists be?

Sixteen inches on center is standard for most decking; 12 inches for diagonal patterns or composite that calls for it.

### Which direction should deck boards run?

Perpendicular to the joists, and typically along the house's long dimension for appearance.

### How many joists do I need for a deck?

Divide the deck length in inches by the joist spacing and add one. For a 16-ft deck at 16 inches on center, that's 13 joists.

### How many screws do I need for decking?

About 3.3 screws per square foot with 16-inch joist spacing and 5.5-inch boards — roughly 350 screws per 100 sq ft.

### How much does it cost to build a deck?

Material cost depends on lumber type. Pressure-treated decking is the least expensive, with cedar, composite and tropical hardwoods costing progressively more. Frame, footings, railings and hardware are separate.

### How many footings do I need?

A typical 12×16 deck needs 6–9 posts, depending on beam span and local code.

## Sources and further reading

- [Prescriptive Residential Wood Deck Construction Guide (DCA 6)](https://web-media.awc.org/wp-content/uploads/2022/02/17210514/AWC-DCA62015-DeckGuide-1804.pdf) — American Wood Council
- [Call 811 before you dig](https://call811.com/) — Common Ground Alliance