How we calculate: carpentry

Skirting board calculator

Open the skirting board calculator

1. Each wall's piece = wall length − door widths + allowance per piece (for squaring ends and mitres)
2. A wall longer than a stock length is made from full lengths plus one shorter piece; each extra piece is a join
3. Pieces are packed into stock lengths longest first (first-fit decreasing), trying each stock length you can buy and keeping the one that buys the fewest metres
4. Mitre or scribe cuts = 2 per corner (each corner joins two pieces)
5. Adhesive tubes = metres needed ÷ metres per tube, rounded up

Internal corners are often scribed rather than mitred; either way each corner needs two cuts.

Spindle spacing calculator

Open the spindle spacing calculator

1. Level: n = (L − g) ÷ (w + g), rounded up; gap = (L − n × w) ÷ (n + 1), where L = length between newels, w = spindle width, g = largest gap allowed
2. Raked: the horizontal length is L × cos(rake angle); n and the horizontal gap use that length; centres along the rail = (gap + w) ÷ cos(rake angle)
3. First centre from the newel = gap + w ÷ 2 (along the rail when raked)

The 100 mm sphere rule applies to the gap between spindles and under the bottom rail; measure the finished gap after fitting.

Stair calculator

Open the stair calculator

1. Rise = total rise ÷ number of risers (n); treads = n − 1
2. Going = the larger of the Table 1.1 minimum going and rise ÷ tan(maximum pitch), rounded up to the next mm (or your going, if you enter one)
3. 2R + G = 2 × rise + going; pitch = arctan(rise ÷ going); total going = treads × going
4. Riser counts compared: from total rise ÷ maximum rise (rounded up) to total rise ÷ minimum rise (rounded down)
5. Suggested option: the fewest risers with nothing outside the guidance, 2R + G in the normal range and a flight that fits the space

2R + G between 550 and 700 mm is described as the normal relationship in Approved Document K, so it's shown as Advisory, not a pass or fail.

Measure the total rise between finished floor levels (after flooring and screed), not between joists.

Stud wall calculator

Open the stud wall calculator

1. Studs = wall length ÷ centres, rounded up, + 1, + 2 per opening (king and jack studs)
2. Plates = 2 × wall length (the sole plate is cut out at doors after fixing)
3. Noggins = rows × (studs − 1), each about one stud centre long
4. Boards = (length × height − openings) × sides × (1 + waste) ÷ board area, rounded up
5. Timber lengths: studs, plates, noggins and door heads packed into stock lengths (first-fit decreasing)

A stud wall that carries a load, or replaces one that did, needs a structural engineer or building control: this calculator is for non-load-bearing partitions.

Joist calculator

Open the joist calculator

1. Joists = room length ÷ centres, rounded up, + 1
2. Length of each joist = clear span + 2 × bearing
3. Hangers = 2 per joist; strutting = rows × (joists − 1)
4. Check: the clear span against the largest span you entered for your joists

Choosing a joist size is out of scope here: use TRADA's span tables, an I-joist maker's tables, or ask a structural engineer or building control.

Ramp gradient calculator

Open the ramp gradient calculator

1. Gradient 1:n means 1 unit of rise for every n units of going (on plan)
2. Going = rise × n; or, from the length available, n = going ÷ rise
3. Percentage = 100 ÷ n; degrees = arctan(1 ÷ n); length along the slope = √(rise² + going²)
4. Approved Document M allows steeper gradients only on shorter flights: the longest flight for your gradient is read from the table (between rows, interpolated)
5. Flights = going ÷ longest flight, rounded up; landings = flights + 1

Approved Document M also sets ramp widths, landing lengths and handrails: check the full guidance or ask building control.

This calculator has no built-in defaults: every figure comes from you or your product's data.

Architrave calculator

Open the architrave calculator

1. Per door face: 2 legs (opening height + architrave width) and 1 head (opening width + 2 × architrave width)
2. Each piece gets the allowance per piece for squaring and mitring
3. Pieces are packed into stock lengths longest first (first-fit decreasing), keeping the stock length that buys the fewest metres
4. Mitre cuts = 4 per face (2 at the top of the legs, 2 on the head)

Heads are short, so two often come out of one length: the cut list shows which.