How do I calculate how many screws I need?

Divide the board length by your desired spacing, round up to the next whole number, and multiply by the number of rows of screws. The calculator does this automatically once you enter length, spacing and rows.

What is a safe screw spacing for decking?

A common rule of thumb is 12–16 inches (300–400 mm) along the board for each joist, with at least two screws per joist. Always follow the decking and fastener manufacturer’s instructions and local building code.

Can this fastener calculator be used for structural design?

No. The calculator is for preliminary estimating and education only. For structural connections you must follow the relevant design standard and manufacturer data, and have critical designs checked by a qualified engineer.

Fastener Calculator

Estimate screws, nails or bolts for decking, sheathing and light framing. Get quantity, spacing, edge distances and approximate pull‑out & shear capacity.

This tool is for preliminary estimating only. For structural design, always follow building codes, manufacturer data and consult a qualified engineer.

Units

Imperial (in, ft) Metric (mm, m)

Fastener type

Member geometry

Board length (ft)

Board width (in)

Support spacing (joists/studs) (in)

Typical: 12, 16 or 24 in (300, 400 or 600 mm).

Rows of fasteners across width

e.g. 2 screws per joist for decking.

Spacing & edge distances

Spacing along board (in)

End distance (from board end) (in)

Edge distance (from board edge) (in)

Number of supports under board

For a single board span, supports ≈ 1 + length / spacing.

Units

Imperial (lb, in) Metric (kN, mm)

Wood density class

Fastener geometry

Diameter (in)

e.g. #8 screw ≈ 0.164 in (4.2 mm).

Embedment depth (in)

Number of fasteners in connection

How this fastener calculator works

This fastener calculator combines two common tasks:

Quantity & spacing: how many screws, nails or bolts you need for a board, panel or connection.
Capacity: a rough estimate of pull‑out and shear resistance based on fastener size, embedment and wood density.

1. Quantity & spacing

The calculator treats your board as a rectangle supported by joists or studs. You specify the board length and width, support spacing, number of rows of fasteners, and spacing along the board.

Fasteners per support line

Let:

L = board length
s = spacing along the board
e = end distance
r = number of rows across the width

Number of fastener positions along one support line:

\( n_{\text{along}} = \left\lceil \dfrac{L - 2e}{s} \right\rceil + 1 \)

Fasteners per support:

\( n_{\text{support}} = n_{\text{along}} \times r \)

If there are \( N_{\text{sup}} \) supports under the board:

\( n_{\text{total}} = n_{\text{support}} \times N_{\text{sup}} \)

2. Basic spacing and edge distance rules (rule‑of‑thumb)

Exact requirements depend on your building code (e.g. NDS, Eurocode 5, local timber codes) and the fastener manufacturer. As a rough guide for wood screws and nails:

End distance: at least 7–10 × diameter along the grain.
Edge distance: at least 4–5 × diameter across the grain.
Spacing along grain: at least 7–10 × diameter.
Spacing between rows: at least 3–4 × diameter.

The calculator checks your inputs against simple multiples of the diameter and shows warnings if values are unusually small.

3. Pull‑out and shear capacity (simplified)

The capacity tab uses simplified empirical relationships that scale with:

Fastener diameter
Embedment depth in the main member
Wood density class (soft, dense softwood, hardwood)

Approximate pull‑out (imperial units)

\( R_{\text{pull}} \approx k_{\rho} \cdot d \cdot l_{\text{ef}} \)

\( d \) = diameter (in)
\( l_{\text{ef}} \) = embedment depth (in)
\( k_{\rho} \) = density factor (soft < dense < hardwood)

Approximate shear

\( R_{\text{shear}} \approx k_{\rho} \cdot d^{2} \)

These formulas are deliberately conservative and smoothed. They are not a substitute for code‑based design values or manufacturer tables.

Limitations and good practice

This tool does not check bearing, splitting, combined loading or group effects.
It does not apply load duration, moisture, temperature or service class factors.
Always follow the fastener and decking/sheathing manufacturer’s installation instructions.
For structural connections (beams, ledgers, hangers, shear walls), have a qualified engineer review the design.

FAQ

How many screws per joist for decking?

A common practice is two screws per joist per board, placed about 1"–1¼" (25–30 mm) from each edge. Spacing of joists is typically 12", 16" or 24" on center, depending on decking thickness and local code. Always confirm with the decking manufacturer.

What screw length should I use?

For wood‑to‑wood connections, a typical rule is that the screw should penetrate at least 2–3 times its diameter into the main member. For example, a #8 deck screw (≈4.2 mm) often has 1½" (38 mm) or more embedment into the joist.

Can I use this calculator for bolts and lag screws?

Yes, for quantity and spacing it works the same way. For capacity, the simple formulas are more appropriate for screws and nails; bolts and lag screws usually require code‑based design checks (bearing, tear‑out, combined shear and tension).

Why are my spacing values flagged as “tight”?

If your end distance, edge distance or spacing is less than a multiple of the fastener diameter, the risk of splitting and reduced capacity increases. The calculator highlights these cases so you can adjust the layout or confirm with design tables.

Does this replace manufacturer or code connection calculators?

No. Tools from fastener manufacturers and standards bodies (e.g. AWC, Simpson Strong‑Tie, Eurocode‑based tools) implement specific design provisions. This calculator is meant as a fast, visual estimator to help you plan layouts and get ballpark capacities.