1396
Deck screw chart & calculator
Put your deck size in: a 20 x 16 ft deck with 5/4 boards on 16 in joists is 1,120 screws — 3 boxes of 400, 13.49 lb. The table gives the screw length for every board, from 5/4 decking to ledger boards.
★ CONSTRUCTION · DECK · SCREW CHART
Deck Screw Chart: Sizes, Lengths & Screw Calculator
A 20 x 16 ft deck needs 1,120 deck screws — 35 boards, 16 joists, 2 screws at every crossing, in 3 boxes of 400 #9 x 2-1/2 in screws. Deck screw sizes, lengths and counts by board, plus a calculator for your deck. This one is the DECK-SCREW chart — screw length and count for decking — the deck-board-size-chart page is the DECK-BOARD chart — the boards themselves — not the screws; the screw-size-chart page is the SCREW-SIZE chart — screw gauges and diameters — not deck counts; and the deck-cost-calculator page is the DECK-COST tool — the cost of the whole deck — not the fasteners.
How many deck screws does a 20 x 16 ft deck need?
1,120 screws. The deck is 320 sq ft: 35 boards across the 16 ft width (each 5-1/2 in wide with a 1/8 in gap) and 16 joists along the 20 ft length (one every 16 in, plus the end joist). Two screws at every board-joist crossing is 35 × 16 × 2 — 1,120 screws, which fits in 3 boxes of 400 #9 x 2-1/2 in screws and weighs 13.49 lb. That is the whole calculation: boards times joists times two, then divide by the box count and by the screws per pound. The table above gives the screw length for each application, so the same arithmetic works for any of them: a 12 x 12 ft deck is 520 screws, a 20 x 24 ft deck is 1,664, and the same deck in 2x6 boards is still 1,120 screws — only the length and the weight change.
Deck screw length and size by application
| Application | Board | Screw length | Gauge | Screws per board per joist | Penetration into joist | Note |
|---|---|---|---|---|---|---|
| 5/4 deck boards, pressure-treated (actual 1 in) | 5/4 in (1 in actual) | 2-1/2 in | #8–#10 | 2 | 1-1/2 in | The standard deck screw — 2-1/2 in is the most common length. |
| 2x6 deck boards (actual 1-1/2 in) | 2x6 (1-1/2 in actual) | 3 in | #9–#10 | 2 | 1-1/2 in | 3 in keeps the same 1-1/2 in of bite in the joist. |
| Composite deck boards (3/4–1 in) | Composite, 3/4–1 in | 2–2-1/2 in | #8–#10 | 2 | 1–1-1/2 in | Follow the board maker’s spec — most call for 2–2-1/2 in. |
| Hardwood decking, e.g. ipe (3/4–1 in) | Hardwood, 3/4–1 in | 2–2-1/2 in | #9–#10 trim head | 2 | 1–1-1/2 in | Predrill — dense hardwoods split without a pilot hole. |
| Fascia and trim boards (3/4 in) | Fascia, 3/4 in | 1-5/8–2 in | #7–#8 | 2 | 7/8–1-1/4 in | Shorter screws for thin trim — long ones poke through. |
| Stair treads, 2x material | 2x tread (1-1/2 in actual) | 3 in | #10–#12 | 2 | 1-1/2 in | Same 3 in screw as 2x decking, heavier gauge for treads. |
| Ledger boards (structural) | Ledger, 1-1/2 in + sheathing | 3-1/2–6 in | Structural / lag | — | Per code | Structural screws or lags per the building code — not deck screws. |
Lengths and the 1 in penetration rule: a deck screw should be 2 to 3 times the board thickness and bite at least 1 in into the joist — 2-1/2 in screws for 5/4 decking (actual 1 in thick) and 3 in screws for 2x decking (actual 1-1/2 in thick). Screws are #8 to #10 gauge for most decking, #9 to #10 for 2x boards, with coated or stainless steel for pressure-treated lumber. The rule of thumb is 350 screws per 100 sq ft for 6 in boards on 16 in joists — the calculator above counts it exactly from the boards and joists. Box counts: Deck Elite 5-lb boxes hold 400 screws at 2-1/2 in and 350 at 3 in. Screw weights (Deck Mate): 2-1/2 in is 12.048 lb per 1,000 screws, 3 in is 14.286 lb per 1,000. Always check the board maker's spec for composite and hardwood — and your local building code for ledger boards.
Three things to check before you trust the number
The length is set by the board, not the box. A 5/4 board is 1 in thick once milled, so a 2-1/2 in screw bites 1-1/2 in into the joist; a 2x board is 1-1/2 in thick, so it needs a 3 in screw for the same bite. The rule is 2 to 3 times the board thickness with at least 1 in of penetration — a screw that only just reaches the joist pulls out as the boards shrink and swell, and a screw that is too long pokes through the bottom of the joist. Pick the length from the table, then count the screws.
Two screws per crossing, not one. One screw per board per joist lets the board cup and rock; two screws — one each side of the board's centre — hold it flat. Wide boards over 8 in take three. The calculator counts two per crossing, so a deck with 35 boards and 16 joists is 1,120 screws, not 560. If your boards are wider than 6 in, add a third screw per crossing and the count rises by half.
Round the boxes up, and buy a spare. 1,120 screws at 400 to a box is 2.8 boxes, so you buy 3 — and a stripped head, a snapped screw or a mis-drilled hole eats a few more. A 5-lb box of #9 x 2-1/2 in screws holds about 400; a 5-lb box of #9 x 3 in holds about 350, because the longer screws weigh more. Count by the box your screws actually come in: the calculator takes the box count from the length you pick, and the reverses show what happens when the box holds 373.34 screws instead of 400.
Worked example
On the defaults: a 20 x 16 ft deck, 5/4 boards, 16 in joists, 2-1/2 in screws.
- The boards: 16 ft is 192 in; 192 ÷ 5.625 in (a 5-1/2 in board plus a 1/8 in gap) is 34.13, rounded up to 35 boards.
- The joists: 20 ft is 240 in; 240 ÷ 16 in is 15 spaces, plus the end joist, is 16 joists.
- The screws: 35 boards × 16 joists × 2 screws is 1,120 screws.
- The boxes: 1,120 ÷ 400 per box is 2.8, rounded up to 3 boxes.
- The weight: 1,120 × 12.048 lb per 1,000 screws is 13.49 lb.
- The check: 1,120 ÷ 320 sq ft is 3.50 screws per sq ft — the 350-per-100-sq-ft rule of thumb, exactly.
And the other way: 16 in is the widest joist spacing still needing exactly 1,120 screws, 20 ft is the shortest deck length needing exactly 1,120, and 373.34 screws per box is what fills 3 boxes with 1,120 screws.
Every branch, checked
- A 12 x 12 ft deck: A 12 x 12 ft deck: 520 screws — 26 boards, 10 joists, 2 screws at every crossing: 2 boxes of 400, 6.26 lb of screws..
- A 20 x 24 ft deck: A 20 x 24 ft deck: 1,664 screws — 52 boards, 16 joists, 2 screws at every crossing: 5 boxes of 400, 20.05 lb of screws..
- Same deck on 12 in joist spacing: Same deck on 12 in joist spacing: 1,470 screws — 35 boards, 21 joists, 2 screws at every crossing: 4 boxes of 400, 17.71 lb of screws..
- Same deck on 24 in joist spacing: Same deck on 24 in joist spacing: 770 screws — 35 boards, 11 joists, 2 screws at every crossing: 2 boxes of 400, 9.28 lb of screws..
- Same deck in 2x6 boards with 3 in screws: Same deck in 2x6 boards with 3 in screws: 1,120 screws — 35 boards, 16 joists, 2 screws at every crossing: 4 boxes of 350, 16 lb of screws..
- A 10 x 10 ft deck (100 sq ft): A 10 x 10 ft deck (100 sq ft): 352 screws — 22 boards, 8 joists, 2 screws at every crossing: 1 boxes of 400, 4.24 lb of screws..
The reverses, worked backwards
- Widest joist spacing needing exactly 1,120 screws: 16 in — at 16 in the figures are 1,120 screws — 35 boards, 16 joists, 2 screws at every crossing: 3 boxes of 400, 13.49 lb of screws.; at the lower neighbor the figures are 1,190 screws — 35 boards, 17 joists, 2 screws at every crossing: 3 boxes of 400, 14.34 lb of screws.; at the upper neighbor the figures are 1,050 screws — 35 boards, 15 joists, 2 screws at every crossing: 3 boxes of 400, 12.65 lb of screws. — verdict: STEP.
- Shortest deck length needing exactly 1,120 screws: 20 ft — at 20 ft the figures are 1,120 screws — 35 boards, 16 joists, 2 screws at every crossing: 3 boxes of 400, 13.49 lb of screws.; at the lower neighbor the figures are 1,050 screws — 35 boards, 15 joists, 2 screws at every crossing: 3 boxes of 400, 12.65 lb of screws.; at the upper neighbor the figures are 1,120 screws — 35 boards, 16 joists, 2 screws at every crossing: 3 boxes of 400, 13.49 lb of screws. — verdict: STEP.
- Screws per box for 1,120 screws to fit in 3 boxes: 373.34 — at 373.34 the figures are 1,120 screws — 35 boards, 16 joists, 2 screws at every crossing: 3 boxes of 373.34, 13.49 lb of screws.; at the lower neighbor the figures are 1,120 screws — 35 boards, 16 joists, 2 screws at every crossing: 4 boxes of 372.34, 13.49 lb of screws.; at the upper neighbor the figures are 1,120 screws — 35 boards, 16 joists, 2 screws at every crossing: 3 boxes of 374.34, 13.49 lb of screws. — verdict: STEP.
What the defaults assume
The defaults are a 20 x 16 ft deck with 5/4 boards (5-1/2 in wide, 1/8 in gaps) on joists 16 in apart, fastened with 2 screws per board per joist. The board count rounds up to whole boards; the joist count is the length divided by the spacing plus one for the end joist. Box counts come from Deck Elite 5-lb boxes (400 screws at 2-1/2 in, 350 at 3 in) and weights from Deck Mate nominal weights (12.048 lb per 1,000 at 2-1/2 in, 14.286 lb at 3 in) — other brands pack slightly differently, so check the label on the box you buy. The model does not count screws for the picture-frame border, the stair treads, the railing or the fascia — those take their own screws, at the lengths in the table. Hidden fastener systems and clips take no face screws at all, so this count does not apply to them. Check your local building code for ledger-board fasteners, which are structural, not deck screws.
These notes repeat in every explainer section because this page is built to be read start to finish: the length is set by the board, not the box; two screws per crossing, not one; and round the boxes up, and buy a spare.
Output · formula · meaning
| Output | Formula | What it means |
|---|---|---|
| Total screws | boards × joists × 2 screws per crossing | 1,120 screws — 35 × 16 × 2. |
| Boards | deck width ÷ (5.5 in board + 1/8 in gap), rounded up | 35 boards — 192 in ÷ 5.625 in. |
| Joists | deck length ÷ spacing + 1 | 16 joists — 240 in ÷ 16 in + 1. |
| Boxes | total screws ÷ screws per box, rounded up | 3 boxes — 1,120 ÷ 400, rounded up. |
| Weight | total screws × lb per 1,000 screws ÷ 1,000 | 13.49 lb — 1,120 × 12.048 ÷ 1,000. |
| Screws per sq ft | total screws ÷ deck area | 3.50 — 1,120 ÷ 320. |
| Reverse: Widest joist spacing needing exactly 1,120 screws | step | 16 in: 1,120 screws — 35 boards, 16 joists, 2 screws at every crossing: 3 boxes of 400, 13.49 lb of screws.. |
| Reverse: Shortest deck length needing exactly 1,120 screws | step | 20 ft: 1,120 screws — 35 boards, 16 joists, 2 screws at every crossing: 3 boxes of 400, 13.49 lb of screws.. |
| Reverse: Screws per box for 1,120 screws to fit in 3 boxes | step | 373.34: 1,120 screws — 35 boards, 16 joists, 2 screws at every crossing: 3 boxes of 373.34, 13.49 lb of screws.. |
| A 12 x 12 ft deck | same model | 520 screws — 26 boards, 10 joists, 2 screws at every crossing: 2 boxes of 400, 6.26 lb of screws.. |
| A 20 x 24 ft deck | same model | 1,664 screws — 52 boards, 16 joists, 2 screws at every crossing: 5 boxes of 400, 20.05 lb of screws.. |
| Same deck on 12 in joist spacing | same model | 1,470 screws — 35 boards, 21 joists, 2 screws at every crossing: 4 boxes of 400, 17.71 lb of screws.. |
| Same deck on 24 in joist spacing | same model | 770 screws — 35 boards, 11 joists, 2 screws at every crossing: 2 boxes of 400, 9.28 lb of screws.. |
| Same deck in 2x6 boards with 3 in screws | same model | 1,120 screws — 35 boards, 16 joists, 2 screws at every crossing: 4 boxes of 350, 16 lb of screws.. |
| A 10 x 10 ft deck (100 sq ft) | same model | 352 screws — 22 boards, 8 joists, 2 screws at every crossing: 1 boxes of 400, 4.24 lb of screws.. |
| 0 sq ft — no deck at all | same model | 0 screws — 0 boards, 0 joists, 2 screws at every crossing: 0 boxes of 400, 0 lb of screws.. |
| A 2 x 5 ft patch (10 sq ft) | same model | 44 screws — 11 boards, 2 joists, 2 screws at every crossing: 1 boxes of 400, 0.53 lb of screws.. |
| A 25 x 40 ft deck (1,000 sq ft) | same model | 3,268 screws — 86 boards, 19 joists, 2 screws at every crossing: 9 boxes of 400, 39.37 lb of screws.. |
| A 50 x 100 ft deck (5,000 sq ft) | same model | 16,264 screws — 214 boards, 38 joists, 2 screws at every crossing: 41 boxes of 400, 195.95 lb of screws.. |
Who uses this
- A DIYer pricing a deck build — a 20 x 16 ft deck needs 1,120 screws in 3 boxes: the shopping list, before the lumber yard.
- A contractor quoting a job — 3.50 screws per sq ft turns any deck area into a screw count and a weight, and the branches show what 12 in spacing does to it.
- A homeowner re-screwing a popping deck — the table says 2-1/2 in for 5/4 boards: the screws that are too short are usually the ones that popped.
- A student learning deck construction — one worked example from deck size to box count, the length table, and the reverses that solve for spacing, length and box size.
Accuracy & limitations
Figures are the model's own totals on the numbers you type, computed from unrounded values: the board count is the deck width divided by the board pitch (5-1/2 in board plus 1/8 in gap) rounded up to whole boards, the joist count is the deck length divided by the spacing plus one, the screw count is boards times joists times two, the boxes are the screw count divided by the box count rounded up, and the weight is the screw count times the nominal weight per 1,000 screws. The displayed figures are rounded to 2dp. The model assumes 6 in (5-1/2 in actual) boards with 1/8 in gaps and 2 screws per crossing — wider boards, wider gaps, 12 in or 24 in spacing, and hidden fasteners all change the count, and the branches show the spacing effect. Box counts and weights vary by brand: the page uses Deck Elite box counts and Deck Mate weights, and the label on the box you buy is the final word. This page is a calculation tool, not a building-code document: check your local code before you build.
Cite this calculator
A 20 x 16 ft deck (320 sq ft) with 5/4 boards on 16 in joists needs 1,120 screws — 3 boxes of 400 #9 x 2-1/2 in screws, 13.49 lb. Screw length is 2 to 3 times the board thickness with at least 1 in of bite in the joist. Source: Bright Side Kit, brightoolbox.com.
Frequently asked questions
What size screws do I need for a deck?
2-1/2 in screws for 5/4 decking and 3 in screws for 2x decking — #8 to #10 gauge, 2 screws per board per joist. The screw should be 2 to 3 times the board thickness with at least 1 in of bite in the joist.
How many screws do I need for a 20 x 16 ft deck?
1,120 screws — 35 boards and 16 joists at 2 screws per crossing, in 3 boxes of 400. That is 3.50 screws per sq ft.
How many screws per 100 sq ft of deck?
About 350 — the rule of thumb for 6 in boards on 16 in joists, and the calculator gives 1,120 on 320 sq ft, which is 3.50 per sq ft. Tighter joist spacing raises it; wider spacing lowers it.
How many deck screws are in a 5 lb box?
About 400 for #9 x 2-1/2 in screws and about 350 for #9 x 3 in screws (Deck Elite 5-lb boxes). Longer screws weigh more, so fewer fit in the same weight.
How many screws per deck board per joist?
Two — one each side of the board's centre, so the board cannot cup or rock. Boards wider than 8 in take three.
What screws do I use for composite decking?
2 in to 2-1/2 in composite screws, #8 to #10 gauge — and follow the board maker's spec, which is the final word. Some composite boards need specific screws or hidden clips.
Do deck screws need to be stainless steel?
Not always — coated or stainless steel both work in pressure-treated lumber, and the coating matters more than the metal. Stainless costs more and is weaker, so it usually needs predrilling; coated screws are the common choice.
What screws hold a ledger board?
Structural screws or lag screws, 3-1/2 in to 6 in, sized per the building code — not deck screws. A ledger carries the deck, so its fasteners are structural.