About this calculator
This calculator takes off the materials for a timber joist floor over a rectangular opening: how many joists and how long, rim joists or joist hangers, blocking (noggins) between the joists, subfloor sheets in a staggered layout, and the screws to fix them.
It does not size anything. The joist section, spacing, span and bearing and the subfloor thickness are structural decisions that must come from an EN 1995 (Eurocode 5) design or the timber supplier's span tables. Enter what that design gives you; the calculator counts it.
Describe the joist direction by the two dimensions: the clear span is the distance between the two supports the joists run between, the floor width the distance across them, along which they are spaced. Joists either bear on the walls (and are longer than the span by the bearing at each end) or sit in hangers on ledgers.
Formula
joist centres = edge joist at each side + one every spacing from the first side
joists = number of centres
joist length = clear span + 2 × bearing (0 bearing in hangers)
rim joists = 2 × floor width (bearing on walls)
hangers = 2 × joists (in hangers)
blocking rows = ceil(span / interval) − 1
blocks = rows × gaps between joists (length = clear gap)
sheet length used = floor(sheet length / spacing) × spacing (ends on joists)
sheet rows = ceil(span / sheet width), every other row offset by half a sheet
sheets = sheet pieces nested into whole sheets (+ waste %)
screws = per joist under each piece: sheet ends at the edge spacing,
intermediate joists at the field spacing,
ceil(strip width / spacing) + 1 per line
Worked example
A 4 m clear span, 5 m wide, joists 50 × 200 mm at 625 mm centres bearing 100 mm on the walls, rim joists, 2500 × 1250 mm OSB, screws every 150 mm at sheet ends and 300 mm on intermediate joists, 5 % sheet waste:
joists = 25, 625, 1250 … 4375, 4975 = 9
joist length = 4000 + 2 × 100 = 4200 mm
rim joists = 2 × 5000 mm
timber = (9 × 4200 + 2 × 5000) × 50 × 200 = 0.478 m³
sheet used at = 4 × 625 = 2500 mm
rows = ceil(4000 / 1250) = 4 (last one 250 mm wide)
pieces = 2500 + 2500 | 1250 + 2500 + 1250 | 2500 + 2500 | 3 strips 250 wide
sheets = 5 + 1 (two 1250 halves) + 1 (the strips) = 7, + 5 % → 8
screws = 38 per full sheet, 270 in total
FAQ
Will the calculator tell me what joist size I need? No, deliberately. Joist sizing depends on the span, the load, the timber strength class, deflection limits and vibration — an EN 1995 design or a span table from the timber supplier answers it. The calculator only counts the size you enter.
Why is the sheet used shorter than the sheet? Every sheet end has to sit on a joist, so the usable length is a whole number of joist bays. At 625 mm centres a 2500 mm sheet fits exactly; at 600 mm it would be used at 2400 mm.
Why are the rows staggered? So that the end joints of neighbouring rows do not line up on one joist — one OSB manufacturer requires the rows to be staggered by at least one joist bay. The calculator starts every other row with a half sheet.
Do I need blocking? That is a design decision (for lateral restraint, to reduce vibration, to support a partition or sheet edges). Switch it on and give the largest distance between rows.
Are the joist hangers' nails or screws included? No — they depend on the hanger and are given by its manufacturer.
Assumptions and limits
No standard is cited, and no structural check is made. Citing EN 1995 would imply a check the calculator does not do.
Defaults to replace: 4 × 5 m floor, 50 × 200 mm joists at 625 mm, 100 mm bearing, rim joists, 2500 × 1250 × 18 mm sheets, screws at 150/300 mm, 5 % sheet waste. 625 mm spacing and the 150/300 mm screw centres are one OSB manufacturer's figures, not a rule.
Geometry: a rectangular floor with no openings (stairwell, chimney trimmers); trimmer joists, double joists under partitions, joist laps and stock-length nesting of the joists are not modelled. The subfloor covers the clear span by the floor width; perimeter expansion gaps and tongue-and-groove lengths are ignored. Rim joists are counted at the floor width; blocking pieces use the joist section.