How this calculator works
Siding clads the outside walls, so the job starts like a paint or drywall estimate: a gross wall area (each wall’s length times height), with the doors and windows taken off to leave the net area you actually cover. From there the two common products are bought differently - vinyl by the square and the box, lap boards by their exposure:
gross wall area = sum of (wall length × wall height)
net area = gross − (door & window areas)
squares = net area (sq ft) ÷ 100
vinyl boxes = ceil( net area × wastage ÷ box coverage ) (carton ≈ 2 squares)
lap board run = net area ÷ exposure → boards = ceil( run × wastage ÷ board length )
J-channel = perimeter of every opening + eave / soffit run
corner posts = outside corners × wall height
A square is 100 sq ft of finished wall - the same trade unit roofers use - and vinyl is normally boxed two squares to the carton. Lap boards (vinyl, fibre-cement or timber) overlap, so a board only covers its exposure, the visible reveal of each course; the lineal run you need is the net area divided by that exposure. The trim is sized off the geometry, not the field area: J-channel wraps every opening and runs the eaves, and corner posts run the wall height at each outside corner.
Worked example
This example follows the unit system you pick in the calculator above.
Four 10 m × 2.7 m walls clad in vinyl, less two 1.2 m × 1.2 m windows, 10% wastage:
- Gross wall area: 4 × (10 × 2.7) = 108 m².
- Deduct the openings: 2 × (1.2 × 1.2) = 2.88 m², so net = 105.12 m².
- Add 10% wastage: 105.12 × 1.10 = 115.6 m².
- At 0.93 m² per board (the coverage printed on each board): ceil(115.6 ÷ 0.93) = 125 boards of vinyl.
Common vinyl siding profiles and their exposure
The profile sets the exposure - the height each course covers - which is what turns a wall area into a panel count. These are the everyday horizontal profiles:
A box of vinyl covers about 2 squares (200 sq ft) whatever the profile - the profile changes the panel count in the box, not the wall area it covers - so the calculator works in net area and squares and lets you set the coverage per box.
Tips for measuring and ordering siding
- Measure each wall at its widest and tallest points. For a gable end, switch the wall’s shape to “Gable / triangle” and enter the gable width as the base and the height to the peak - or enter the rectangle below the eave and the triangle above as two walls.
- Take off the doors and windows so you are not paying to clad over them, then let the waste percentage cover the offcuts around each opening.
- Buy the J-channel, corner posts, starter strip and finish trim as their own lines - the calculator gives the J-channel and corner-post runs, and trim is sold in fixed sticks, so round each up to whole lengths.
- Order a little long and keep the batch / lot number; a later run of vinyl or fibre-cement can vary slightly in shade.
- For the full method by house and shed size, see how much siding do I need; weighing materials, read vinyl vs fibre-cement siding; and before you order, how to measure for siding.
- If you are also re-roofing, the roof shingle calculator works in the same squares, and the paint calculator uses the same wall-minus-openings area if you are painting instead of cladding.
What siding is called, and how it's sold, where you are
In the United Kingdom, exterior wall cladding is called cladding or weatherboarding, not "siding" - uPVC, fibre-cement or timber boards laid with a lap, and it is bought by the board or by the square metre.
Boards are quoted per length or per m² rather than by the American "square", but the lap still means each board only covers its exposed face, so size the run off the net wall area. Set the siding type and the box coverage or board exposure in the
calculator above to match what your supplier stocks, so the count comes out right.
For the recognised specification and fixing detail, see BS 8605, the British Standard for specifying and installing external timber cladding.
