Rafter and joist count calculator
Spacing is centre to centre, and there is always one more member than there are spaces.
Members needed
28
- Number of spaces
- 25
- Actual spacing
- 0.4
- Nominal spacing
- 400.0
- Total linear length
- 112.00
- Blocking or noggin length per row
- 10.80
Show the working
- Length of the run = 10
- Spacing = 0.4
- Length of each member = 4
- Extra members for openings = 2
- Number of spaces = ceil(10 / 0.4) = 25
- Members needed = 25 + 1 + 2 = 28
- Total linear length = 28 * 4 = 112
- Actual spacing = 25 > 0 ? 10 / 25 : 0 = 0.4
- Nominal spacing = 0.4 * 1000 = 400
- Blocking or noggin length per row = 28 > 1 ? (28 - 1) * 0.4 : 0 = 10.8
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
Scenarios & local history
Compare a saved scenario
Recent calculations
The formula
- members = ceil(run / spacing) + 1
The "plus one" catches people out: a 10 m run at 400 mm centres needs 26 members, not 25.