Ordering stone short is the one mistake on a project that cannot be quietly fixed. Almost everything else can be adjusted; a floor that runs out halfway needs material from a different block, and the join will be visible for as long as the floor exists. Here is how to arrive at a number you can order against.
Step 1: measure the actual area
Break the space into rectangles, multiply length by width for each, and add them together. Work in feet if your supplier quotes per square foot, and stay in one unit throughout — mixing feet and metres midway is the most common arithmetic error on a site.
Measure the room, not the drawing. Built dimensions drift from plans, and the drift is almost always in the direction that needs more material.
Do not subtract small obstructions. A kitchen island or a column takes material out of the finished floor but generates offcuts that mostly cannot be used elsewhere.
Step 2: add the vertical surfaces
Skirting is easy to forget and adds up fast. Measure the room perimeter and multiply by the skirting height. Stair risers, thresholds, window sills and counter upstands all need to be measured and added the same way.
For a countertop, measure the run length by the depth, then add the edge profile and any upstand as separate items. Cut-outs for a sink or hob do not reduce the material required.
Step 3: add wastage
Wastage is not a supplier's padding — it is cutting loss, breakage and the offcuts a layout unavoidably generates. How much to add depends on the job:
- Simple rectangular room, straight laying: about 10 percent.
- Diagonal laying, or a room with many angles: 15 percent or more.
- Book-matched or pattern-matched work: 20 percent and upwards, because slabs must be used in sequence and non-matching pieces cannot be substituted.
- Countertops with several cut-outs: 15 percent or more.
- Small bathrooms: proportionally higher, because the ratio of cuts to area is worse.
Keep the offcuts on site after the job is done. A spare slab stored flat and dry is the only way to replace a cracked piece years later in material that matches.
Step 4: convert area into slabs
Stone is sold as slabs, not as area, and slabs are irregular — they come from a block, so their sizes vary. Your supplier converts your area requirement into a slab count and shows you which ones. Two things worth confirming at that point: that the slabs are reserved in your name, and that they come from the same block.
That second point is what keeps the colour and the veining continuous. Two lots of the same named marble bought weeks apart will not match, and no amount of care during laying hides that.
A worked example
A living room 20 ft by 16 ft, with 4-inch skirting all round, laid straight in a plain pattern.
- Floor area: 20 × 16 = 320 sq ft.
- Perimeter: 2 × (20 + 16) = 72 ft.
- Skirting: 72 ft × 0.33 ft (4 inches) ≈ 24 sq ft.
- Subtotal: 320 + 24 = 344 sq ft.
- Wastage at 10 percent: about 34 sq ft.
- Order approximately 378 sq ft.
If the same room were laid diagonally, or in a book-matched marble, the wastage line would roughly double and the order would land nearer 410 sq ft. That difference is worth deciding on deliberately rather than discovering during laying.
Before you place the order
- Re-measure the built space rather than trusting the drawing.
- Confirm the thickness — 18 mm and 20 mm are the common options and they are not interchangeable mid-floor.
- Confirm the finish: polished, honed or leather.
- See the actual slabs and mark the ones you are buying.
- Confirm they are from one block and reserved in your name.
- Agree who handles unloading, and make sure there is somewhere flat and shaded to store them on site.
Common questions
- How much extra marble should I order?
- About 10 percent above the measured area for a simple straight layout, 15 percent or more for diagonal laying or many cut-outs, and 20 percent or more for book-matched work where slabs must be used in sequence.
- Do I subtract the area of a kitchen island or column?
- No. Those areas produce offcuts that usually cannot be used elsewhere in the floor, so leaving them in the calculation is the safer arithmetic.
- What happens if I run out of marble partway through laying?
- A second lot of the same named stone will come from a different block and will not match in colour or veining. The join stays visible permanently, which is why ordering slightly long is always cheaper than ordering short.
- Should I keep the leftover pieces?
- Yes. Stored flat, dry and out of the sun, offcuts are the only matching material available if a piece cracks or is damaged years later.