Nail / Screw Quantity Calculator

How the nail and screw calculator works

Enter the area to fix, how many fasteners go into each square foot or square metre, a wastage allowance, the number of fasteners in a box and the price of a box. The calculator multiplies the area by the rate, adds the wastage and rounds up to a whole fastener. It then divides by the box count and rounds up to whole boxes, and shows how many will be left over.

The formulas are N = ⌈A × f × (1 + w)⌉ for the fasteners and B = ⌈N ÷ b⌉ for the boxes, where A is the area, f the fasteners per unit of area, w the wastage as a decimal and b the fasteners per box. On the US units side the rate is per ft²; on the metric side it is per m². One per ft² equals 10.764 per m².

Working out fasteners per area

Most boards and sheets are fixed where they cross a joist, stud or batten, so the rate follows from three numbers: the fasteners at each crossing (n), the spacing of the boards or rows of fixings (p) and the spacing of the supports (s). The rate is f = n ÷ (p × s), with p and s in the same unit as the area.

Decking is the default. The Trex 2023 decking installation guide calls for two screws per board at every joist, lists its standard board as 5.5 in (140 mm) wide, sets a minimum gap of 3/16 in (5 mm) between boards, and spaces joists at 16 in (406 mm) for residential decks. Each board plus its gap takes 5.6875 in, so the rate is 2 × 144 ÷ (5.6875 × 16) = 3.17 screws per ft², or 2 ÷ (0.145 × 0.406) = 34 per m². Its span chart calls for joists at 12 in (305 mm) on commercial decks, which raises the rate to 2 × 144 ÷ (5.6875 × 12) = 4.22 per ft², or about 45 per m². For docks and heavily wooded sites Trex recommends a wider 3/8 in (10 mm) gap, which lowers it slightly to 2 × 144 ÷ (5.875 × 16) = 3.06 per ft², or about 33 per m².

Drywall uses fewer. USG gives screw spacing of 12 in on ceilings and 16 in on walls for non-fire-rated work on wood or steel framing, and nail spacing of 7 in on ceilings and 8 in on walls on wood. On studs or joists at 16 in centres, screws at 16 in on a wall work out to 144 ÷ (16 × 16) = 0.56 per ft², about 6 per m², and screws at 12 in on a ceiling to 144 ÷ (12 × 16) = 0.75 per ft², about 8 per m². These are the rate over the open field; the ends of each panel land on a support that the next panel also needs, so the real count is higher. For fire-rated walls and ceilings, USG points to the tested assembly for fastener type and spacing.

A worked example

A 215 ft² deck at the Trex rate of 3.17 screws per ft² with 10% wastage needs 215 × 3.17 × 1.1 = 749.7, so 750 screws. In boxes of 250 that is exactly 3 boxes, with none spare. The metric version, 20 m² at 34 per m², needs 20 × 34 × 1.1 = 748 screws, so 3 boxes of 250 with 2 left over. Buying a box more than the count gives a margin for dropped and bent screws beyond the wastage allowance.

Choosing the wastage and the box size

The wastage covers screws that are dropped, bent, or stripped and replaced. There is no published standard for it, so 10% is a starting point: a careful fixer with a screw gun needs less, and fixing overhead or from a ladder needs more. Some fasteners are sold by weight rather than by count; enter the count, taken from the label or the maker's data.

Fastener choice matters as much as the count. Trex lists #8 or #10 screws, 2½ or 2¾ in long, for its 1 in boards, and stainless steel near salt water. Structural connections, such as joist hangers, sheathing that braces a wall, and framing nailing, follow the fastener schedule in the local building code or the connector maker's instructions rather than an area rate.

What this tool does not do

It does not set the fixing pattern, check structural capacity, or choose the fastener type, length or coating. It turns a fixing rate you already know into a count and a number of boxes.

Using this calculator with other BuildMetricLab tools

The decking board calculator counts boards and joists for a deck, the drywall calculator counts sheets for walls and ceilings, and the stud wall calculator helps with framing. All BuildMetricLab tools run in your browser with no sign-up, and every formula is shown on the page so the maths can be checked.