How this is worked out
Floor area is length × width, then a waste allowance is added and the result is divided by the pack coverage:
packs = ⌈ area × (1 + waste) ÷ pack coverage ⌉
The ceiling brackets are the point. Flooring is sold by the pack and a pack is not divisible, so an answer in square metres leaves you doing the division badly at the shop. Rounding up is not optional — being half a pack short stops the job on the last row.
The waste allowance depends on the layout, not just the material. A straight or brick-bond layout carries around 10% for most materials. A diagonal or herringbone layout carries about 5 points more, and the reason is geometric rather than arbitrary: cutting a plank at 45° to meet a wall produces a triangular offcut, and a triangle from one corner rarely fits any other corner. Straight cuts produce rectangular offcuts that often start the next row.
Grout is estimated from tile size, joint width and tile thickness using the standard trade formula, and adhesive at roughly 4 kg per square metre for a 6 mm notched trowel. Both are rules of thumb rather than exact science.
A worked example
- Room
- 4 m × 3 m
- Floor area
- 12 m²
- Laminate, straight layout
- 10% allowance
- Area to buy
- 13.2 m²
- Packs at 2.5 m² each
- 6
- Those packs cover
- 15 m²
- Spare after the job
- 3 m²
- Same room laid diagonally
- 15% — 13.8 m²
Why a diagonal layout costs half a pack more
Laying planks straight, parallel to a wall, means the offcut from the end of one row is a rectangle — and a rectangle of the right width usually starts the next row. Very little is genuinely thrown away, which is why 10% covers it.
Laying at 45 degrees means every plank meeting a wall is cut at an angle, producing a triangular offcut. A triangle cut for one corner does not fit the opposite corner, so most of them are waste. Herringbone is the same problem in both directions at once.
Five extra percentage points sounds small and is usually one or two extra packs. Discovering that shortfall on the last afternoon of a job, with the range discontinued, is considerably more expensive than the packs would have been.
Keep the spare, and label it
The calculator reports how much is left over after the job, and the correct thing to do with it is put it in the loft rather than return it. Boards and tiles from a later production batch rarely match exactly — shade, grain and even thickness drift between runs.
A single plank damaged two years from now is a five-minute repair if you have a spare and a considerable problem if the range has been discontinued, which happens constantly with flooring.
Label the box with the room and the date. In ten years nobody remembers which of three leftover boxes goes with which floor.
Rooms are not rectangles, and doorways count
The calculator takes length × width, which is right for a rectangular room. For an L-shape, split it into rectangles, run each separately and add the results — but add the waste allowance to the total rather than to each piece, or you will over-buy.
Doorways, alcoves, bay windows and chimney breasts all need adding or subtracting by hand. A doorway threshold typically needs an extra 0.2–0.3 m² of material each, which is trivial individually and adds up across an open-plan floor with four openings.
Do not subtract anything for kitchen units or a bath panel unless you are certain the flooring stops there. Running flooring under units is common and makes a future kitchen replacement far easier; stopping at the plinth line saves material and creates a problem later.
What to check before ordering
Pack coverage varies between products and it is printed on the pack — 1.8 m², 2.2 m² and 2.5 m² are all common. Using a generic figure is the most likely source of error in an otherwise correct calculation, so read it from the actual product you intend to buy.
Check the batch number when the delivery arrives, before you start. All packs should carry the same one; a mixed delivery is worth rejecting rather than discovering the mismatch halfway across the room.
Most engineered and laminate floors need acclimatising in the room for 48 hours before laying, because they expand and contract with humidity. Fitting straight from a cold van is a common cause of gaps appearing weeks later, and it is not a fault in the product.
Assumptions and sources
- Waste allowances
- Trade conventions: 10% for most materials in a straight layout, 12% for solid hardwood, plus about 5 points for diagonal, herringbone or a large pattern repeat.
- Packs
- Area including waste divided by the coverage printed on the pack, rounded up. Pack coverage varies by product — read it from the one you intend to buy.
- Grout
- Standard trade formula from tile dimensions, joint width and tile thickness, at approximately 1.6 kg per litre.
- Adhesive
- Approximately 4 kg per square metre for a 6 mm notched trowel. Varies with trowel size and how flat the substrate is.