First: neither number is real
Nominal lumber names lie. A “5/4×6” board is actually 1 inch thick × 5.5 inches wide, and a “2×6” is 1.5 inches × 5.5 inches. Same width, half-again the thickness — that half inch is the entire debate. With the standard 1/8″ gap, each board run covers 5.625 inches of deck width, which is exactly what the deck calculator uses for its row count.
The joist spacing verdict
This is usually the deciding factor, and it's not close:
- 5/4×6: 16″ on-center maximum for wood, 12″ for most composites and any diagonal installation.
- 2×6: comfortable at 16″, and rated for 24″ on-center in many species/grades — the standard fix for older frames framed at 24″.
If your frame is already built at 16″ on center (most are), the strength difference stops mattering — 5/4 boards at 16″ feel fine under normal residential loads. That's why 5/4×6 dominates the market: it's the thinnest board that passes code on a standard frame, so it takes the least material per square foot of deck.
Why thickness wins: the math
Deck boards fail by bending, and a board's resistance to bending scales with the cube of its thickness — so a half inch of gauge multiplies fast.
| Profile | Actual size | Coverage per row | Joist spacing | Stiffness | Linear ft / 100 sq ft |
|---|---|---|---|---|---|
| 5/4 × 6 | 1.0 × 5.5 in | 5.625 in | 16 in | 1.0× | 214 ft |
| 2 × 6 | 1.5 × 5.5 in | 5.625 in | 16–24 in | 3.4× | 214 ft |
| 2 × 4 | 1.5 × 3.5 in | 3.625 in | 16–24 in | 3.4× | 332 ft |
1.53 ÷ 1.03 = 3.375. Both boards are 5.5″ wide, so width contributes nothing to the difference — the entire gap is that half inch. It is also why 2×6 holds up at 24″ joist spacing that would bounce under a 5/4 board.
Notice that 2×4 carries the same 3.4× stiffness as 2×6, because bending resistance only cares about thickness — both are 1.5″. What it gives up is width: at 3.5″ of face it needs 332 linear feet per 100 square feet instead of 214, a 56% increase, which is why it is rare outside stairs, railings, and small landings.
What 100 square feet actually takes
Here is the part almost nobody mentions: 5/4×6 and 2×6 consume identical linear footage. Same 5.5″ face, same 1/8″ gap, same 5.625″ of coverage per row — so 100 square feet needs about 213 linear feet either way. Choosing a profile does not change how much decking you buy. Only the board length you buy it in does.
| Board length | No waste | +10% straight | +15% diagonal |
|---|---|---|---|
| 8 ft | 27 | 30 | 31 |
| 10 ft | 22 | 24 | 25 |
| 12 ft | 18 | 20 | 21 |
| 16 ft | 14 | 15 | 16 |
| 20 ft | 11 | 12 | 13 |
Longer boards cut the piece count, but they only pay off when the length divides your span cleanly. On a 12 ft wide deck, 16 ft boards get trimmed on every single row — the offcut is material you bought and cannot use. Run your dimensions through the deck calculator and it applies this row math to your exact size instead of a rule of thumb.
Screws and labor
Both profiles take 2 fasteners per joist crossing, so a 16 ft board on 16″ joists crosses 13 joists — 26 screws. The profile decides how many rows you fasten; the joist spacing decides how many times per row. A 10×10 deck needs about 436 screws at 16″ spacing but only about 291 at 24″ — which is where the real labor difference lives.
The 30-second decision
Standard 16″ frame, ordinary patio use → 5/4×6. Frame already at 24″, a low-to-ground deck, or you want maximum solid feel → 2×6. Stairs, railings, and small landings → 2×4. Either way, run your exact dimensions through the deck calculator — that link opens on 5/4×6 at 16″ on center, and switching the profile to 2×6 returns the same linear footage, which is the point of this page. It outputs board counts, linear footage, and screws for all three profiles at 8, 10, 12, 16, or 20 ft board lengths.