Joist Count Calculator

Calculator

Unit system

Choose units before entering dimensions.

Spacing entry type

Clear spacing adds joist thickness to get O.C.

Waste factor (%)

Typical framing allowance: 5–10%.

Span length (ft)

Deck width across joists (ft)

Joist spacing entered (in)

Assumed joist thickness (in)

Used only when spacing entry is “clear”.

Edge offset each side (in)

Leave 0 for joists tight to edges.

Span length (m)

Width across joists (m)

Joist spacing entered (mm)

Assumed joist thickness (mm)

Used only when spacing entry is “clear”.

Edge offset each side (mm)

Leave 0 for joists tight to edges.

Openings / concentrated load zones

Examples: stair openings, heavy posts, large cutouts.

Extra joists per zone

Typical: 2 for doubling on both sides.

Cost options

$ / joist

$ / length

Use one pricing method; per-length includes rim length.

Double end joists (adds one each end)

Include rim/band boards (perimeter estimate)

Example data table

Project	Span length	Width across joists	Spacing (O.C.)	Openings	Waste	Estimated main joists
Deck A	12 ft	16 ft	16 in	0	5%	13
Floor B	14 ft	18 ft	12 in	1	8%	20
Patio C	3.6 m	4.8 m	400 mm	0	7%	14

Example counts are illustrative. Field conditions and codes may require additional framing.

The calculator converts dimensions to a consistent internal unit and computes joists across the width:

On-center spacing: distance between joist centerlines.
Clear spacing: clear gap between joist faces; converted to on-center by adding thickness.
Effective width: W_eff = W − 2·offset
Spaces count: N_spaces = ceil(W_eff / S_oc)
Main joists (baseline): N_joists = N_spaces + 1
Extras: doubled ends add 2; openings add openings × extra.
Waste: N_final = ceil(N × (1 + waste%/100))
Total linear length: L = N_final × span

Always verify framing layouts against local requirements and engineered drawings.

Accurate joist counts reduce waste and speed framing decisions.