How We Calculate

Last reviewed: 30 September 2026

Most online calculators show you a number with no way to check where it came from. We'd rather show our working - what's derived, what's a sourced industry figure, and what we've had wrong before and fixed. This page is the general approach; each calculator's own FAQ and reference tables cite the specific figures that page uses.

Derived, not hardcoded, wherever a formula exists

If a figure can be calculated from the inputs you actually give us, we calculate it - we don't fall back to a single flat number and hope it's close enough.

Featheredge boards per metre is the clearest example. A single "10 boards per metre" figure is genuinely correct - for 125mm boards at a 25mm overlap. The same figure quoted for a 100mm board is wrong by 33%. Our close board fence calculator derives boards per metre from whatever board width and overlap you actually enter (1,000 ÷ (board width − overlap)), so the answer is correct for your boards, not for whichever width a competitor happened to assume.

Postcrete is the same idea applied to posts: bags per post vary by post type (a 100mm timber post needs less than a 100mm concrete post, since the concrete section itself displaces more of the hole), so our fence and close board calculators use a rate per post type with a stated hole-size basis, not one flat "2 bags a post" rule that happens to be right for exactly one post type.

Sourced figures, stated as sourced

Where a real formula doesn't exist - a material's bulk density, a settling allowance, a bag size - we use a sourced industry figure and say so on the page, rather than presenting an assumption as if it were a calculated fact.

MOT Type 1's compacted density (1,920 kg/m³) is a good example of why the distinction matters: compacted and loose density genuinely differ by around 7% for the same material, and conflating them is a real, common way to under-order. Our calculators state which basis a figure uses, and use the compacted figure consistently for tonnage (since mass doesn't change with compaction, only volume does) rather than mixing the two. See our MOT Type 1 standards section for the full detail.

Topsoil density is stated by moisture state for the same reason: screened topsoil ranges from around 1,200 kg/m³ dry to 1,800 kg/m³ soaked, with 1,500 kg/m³ ("damp, as delivered") as the realistic default - a single density figure with no stated moisture basis is a common source of the wide, seemingly-contradictory density figures you'll find quoted elsewhere.

We check our own numbers, including after they ship

Sourcing a figure correctly once isn't the end of it - implementation bugs happen after the fact, and we'd rather find and fix them ourselves than leave them for a visitor to discover. Two examples from this month:

• A floating-point rounding defect meant Math.ceil() could silently over-order by one unit whenever a waste-adjusted quantity landed exactly on a whole number - found on one calculator, then confirmed present on ten, all fixed with the same shared rounding function.
• Two "quick reference" tables (hand-typed summaries of common scenarios) had drifted out of step with the calculators they were supposed to summarise. We now generate those tables at build time from the same code the live calculator runs, rather than hand-typing a number that can silently go stale.

We're not claiming to have never been wrong - we're claiming to check, and to fix what we find.

Where our calculators simplify

Every calculator simplifies something. We would rather tell you what than leave you to find out on site. The skirting board calculator plans every run and packs the pieces into boards so offcuts get reused, but it does not track which end angle an offcut has. A mitred offcut can only serve a piece that needs the same angle, so a real cutting plan can need slightly more boards than ours does. It also ignores scarf joint overlap, which costs a few millimetres per join. The waste allowance is there to cover both, and it is yours to raise.

The sleeper raised bed calculator works the same way, staggering joints between courses and packing every piece into sleepers together. It has the same limits, plus one deliberate allowance: pieces can come out up to one saw cut (3mm) short, so a 2.4m sleeper makes two halves, which is well within normal sleeper tolerance. It is for free-standing beds and edging only: anything retaining soil over about 600mm needs proper design, which no calculator can do for you.

The loft boarding calculator counts loft legs from the layout, using the maker's rule of a leg wherever a joist meets a board edge, not a per-square-metre guess, and we checked that rule against Loft Leg's own layout drawings (80 legs for their 4800 x 2440mm example, and the calculator agrees). To make every board end land on a joist it trims boards to whole joist gaps, and it takes the far edge to the next joist. Its hatch check looks at board width only and does not model swinging a long board round above the hatch. It is for storage boarding on ceiling joists, and we give no load or Building Regulations advice.

The stud wall calculator lays out a single non-load-bearing partition. It checks every 1200mm plasterboard joint against the studs and adds a stud where one is missing, frames each door with king and trimmer studs, a header and cripples, and packs every piece into timber lengths. It does not model services, fire or acoustic walls, or fixings counts, and it makes no load or Building Regulations claims.

The turf calculator lays lawns as rows of rolls with staggered joints, tries several stagger patterns and keeps the one that needs the fewest rolls, and packs every cut piece into rolls so offcuts are reused. It only lays rectangles, so curved edges need the bounding rectangle and an extra allowance, and it does not allow for turf shrinking. It adds no percentage to the cutting plan count, because the plan already reuses offcuts and turf cannot be kept, and shows a spare roll on its own line.

The cladding calculator works from each board's stated cover width, not its full width, cuts each course round windows and doors, staggers the joints, and packs every piece into boards so offcuts are reused. It lays rectangular walls only, so a gable is entered as a rectangle with a higher waste allowance, and it does not count corner trims, membrane or fixings.

The wall panelling calculator spaces frames or battens equally along one flat wall, then packs every strip into stock lengths so offcuts are reused. It plans a single row on a single wall, so corners, doors, windows, sockets, top rails, capping and adhesive are for you to allow.

The laminate flooring calculator takes the expansion gap off every side, lays rows of planks with end joints at least 300mm apart and no piece under 200mm, and cuts the first row narrower if the last would be under 60mm. It shares a cut plank between one row start and one row end only, which is our reading of how click ends fit, so a floor with different profiles may allow more. Rooms are separate rectangles. It does not model door frames, pipes, threshold bars or underlay.

The block paving calculator lays the pattern over the whole area and clips it to the edges, so every block is counted as whole or cut. It lays blocks on a 200 x 100mm grid with the joint inside the block size, because the packs are exactly 50 to the m2. Where the cuts are straight it shares a block between pieces, turning a piece round where that fits, with no allowance for the cut. At 45 degrees the cuts are diagonal and it does not match offcuts, so it counts every edge block as one block and slightly over-counts. Basket weave, curves, borders and the sub-base are not modelled.

The shower panel calculator covers Hydrolock tongue and groove panels only. It takes 18mm off each panel width for the joint, as Multipanel's guide says, and lays each wall as a start piece, whole panels and an end piece. It shares a cut panel between one start piece and one end piece, or a single piece, which is our reading of how the tongue and groove edges fit, and it uses a 100mm shortest end piece of our own. You enter the space between the trims, because the guide gives no deduction for the profiles. Walls taller than the panel, cut-outs, base profiles and adhesive or silicone are not modelled.

The artificial grass calculator lays a lawn as strips with the pile all one way, adds the 50mm trim and 50mm seam overlap from Wickes' guide, and packs the strips from every lawn onto the roll in rows across its width. It keeps the layout that needs the least roll, choosing between full-width and equal strips and both directions, and works out the order the way each product is sold: whole metres for cut-to-length grass (as the B&Q and Grass Warehouse size selectors offer), whole rolls for a fixed-size roll. Where a lawn nearly fits the roll width it also shows the no-seam option laid tight to the two side edges, which is our own option, not Wickes' method. The pile direction is yours to set by choosing which way the strips run. Curves, the base, weed membrane and edging are not modelled, and pins along the seams are not counted because we have no sourced spacing.

The slat wall panel calculator lays each wall as a run of 600mm panels with the last one cut to width, as Cheshire Mouldings' fitting instructions say. It shares a cut panel between the end of one wall and the start of another on the basis that the projecting edge is on the left and the set-back edge on the right, which is our reading of those instructions, and it uses a 100mm shortest end piece of our own. Walls taller than the panel, sockets, battens and two courses from one panel on a low wall are not modelled.

The shed roof felt calculator follows IKO's Shed Felt instructions: strips along the eaves that overhang the edges by 50mm, a 75mm lap between strips, and a 300mm ridge strip on an apex roof. It lets the side strips of an apex roof stop up to 100mm short of the ridge line, which is our own choice because the ridge strip covers 150mm each side. No strip is narrower than 150mm (our rule): on an apex a sliver is left out and the ridge strip widened past IKO's 300mm to cover the gap with at least 50mm over each side strip, and on a pent the last strip is cut to 150mm and laps further. Strips are packed across the roll width and into whole rolls. It counts nails at the IKO spacings but gives the adhesive as a length of lap and ridge, because IKO gives no coverage figure. Hip roofs, dormers, skylights and timber edge strips are not modelled.

The kitchen worktop calculator tries each run going through each corner, with the other butting against its front edge and shorter by the worktop depth, as Howdens describes. It keeps joints along a run at least 100mm from the sink and hob cut-outs (B&Q) and warns if a cut-out is within 60mm of a worktop end (Howdens). Treating the corner as a joint for that clearance, the 3mm saw cut and the packing of pieces into 3000 or 4100mm boards are our own working. It adds no length for a mason's mitre, because Howdens gives none, and it does not know where your cabinet joints are.

The dado and picture rail calculator uses the skirting calculator's run and packing engine. A door or window breaks the rail only if it and its architrave reach the rail's height, and the rail then stops at the outside edge of the architrave, which is our reading since no merchant guide covers it. Default heights are from Skirting World (dado: wall height divided by 5 then doubled; picture rail: wall height minus 400mm), taken as the top of the rail. It covers rectangular rooms only, with no external corners or stairs.

The hardboard calculator follows B&Q's floor levelling guide: long side across the floorboards, joints staggered with at least a quarter of a sheet between rows, and nails no more than 150mm apart and 15mm from the edges. The half-sheet stagger, shifting the pattern to avoid slivers under 100mm, and packing offcuts into sheets with straight cuts, turning a piece 90 degrees where that fits, are our own working. It covers one rectangular room, not hearths, pipes or door frames.

The tile backer board calculator follows NoMorePly's installation guide: 12mm boards on stud walls with studs at most 600mm centres and noggins on the board joints, 6mm boards on floors, joints staggered, and about 9 screws a board on walls and 8 on timber floors. It puts vertical joints only on studs and counts a row of noggins at every horizontal joint. Laying wall boards long side horizontal, the stagger distances, the 3mm saw cut (with edge pieces allowed to come out up to 3mm short against a wall end, floor or ceiling), turning offcuts 90 degrees and packing them into boards are our own working. It does not cover solid walls, windows, niches, pipes, tanking or adhesive.

The kitchen plinth calculator gives each run of base cabinets one front plinth and each open end a return, and uses the skirting calculator's splitting and packing: runs longer than a length are split with no piece under 300mm, and pieces are packed into lengths longest first with a 3mm saw cut. Clips follow Howdens' installation guide: a clip on every adjustable leg, 2 plinth clips in each base cabinet's leg pack, and a spring clip at each inside corner. Counting 2 extra clips per return is our reading of Howdens' drawings. The default return of 480mm is our estimate from Howdens' planning drawing (575mm from the wall to the front of a base cabinet) less its front legs sitting 92.5mm back. It compares the plan with DIY Kitchens' sum (cabinets plus a nominal 600mm a return, over the length), which is right for most kitchens but short when no two runs fit in one length. It does not cover curved plinths, vents or sealer strip.

The scotia beading calculator uses the skirting calculator's runs, splitting and packing for a rectangular room: the scotia stops at each door at the outside edge of the architrave (our reading, the same as skirting), no piece is under 300mm, no join is within 300mm of a door, and pieces are packed into lengths longest first with a 3mm saw cut. Mitred corners, a return piece at each door end, and nails every 30cm into the skirting are from Richard Burbidge's fitting guide, as is the perimeter plus 10-20% it is compared with. Counting a nail at each end of every piece is our reading. It does not cover chimney breasts or other external corners.

The tile trim calculator follows DIYData's estimating guide: runs are allocated to strips longest first, a cutting chart rather than the total divided by the strip length, and runs are not joined unless they are longer than a strip. The 45 degree cut at corners is from Homelux's fitting guide. Adding the trim's depth to each mitred end, the 300mm shortest piece and the 3mm saw cut are our own working, and it uses the skirting calculator's splitting and packing. It does not cover curved edges, corner pieces or end caps.

The window board calculator sizes each board from Skirting 4 U's measuring guide: the reveal width plus a horn of about 50mm at each end, and the reveal depth plus a front overhang equal to the board's thickness. It compares the plan with Skirting World's buying method of one length per window. Cutting the boards from lengths longest first with a 3mm saw cut, and trying every mix of long and short lengths to buy the least, is our own working. It does not cover angled bay boards or uPVC window boards.

The vinyl flooring calculator treats sheet vinyl as sold: a roll 2m, 3m or 4m wide cut to any length and paid for across the full width, as on Flooring King's order form. From Online Carpets' guide it adds 10cm to the length and width, keeps the pattern running along the roll, and plans for the fewest seams. It tries every width both ways, keeps the fewest seams and then the least vinyl, and compares that with the area plus 10%. Adding one pattern repeat per extra drop is our own rule. It does not reuse offcuts between rooms.

The kitchen cornice and pelmet calculator follows DIY Kitchens: cornice along the wall units, tall units and dressers, pelmet along the wall units only, and it compares the plan with DIY Kitchens' separate sums (unit widths plus a nominal 400mm a return, over the length). Returns default to 290mm, the wall cabinet depth on Howdens' planning drawing, and corners are mitred as Howdens' guide says. Adding the profile depth to each mitred end (25mm by default, the depth of the Wickes cornice), packing cornice and pelmet into the same lengths, the 300mm shortest piece and the 3mm saw cut are our own working.

The coping stone calculator follows Primethorpe's installation guide: 600mm stones with a 10mm joint between each, an overhang of about 20-30mm each side, and a 20-30mm mortar bed (worked out at 25mm). It compares the plan with Primethorpe's sum of the wall length over 600mm plus about 10%. Sharing the length between the last two stones rather than leaving a piece under 150mm, and cutting the short pieces from as few stones as possible, are our own working. It does not cover pier caps or curved walls.

The alcove shelving calculator builds each shelf as Wickes' alcove shelving guide does: a back and front piece of timber the width of the alcove, two side pieces the depth less the back and front, and a solid MDF top. Taking the timber as fixed flat to the wall, laying the tops out on the sheet with straight cuts (turned round where that fits), cutting the frame from lengths longest first, and the 3mm saw cut are our own working. It compares the plan with the shelf area plus 10% over the sheet, and the frame length plus 10% over the timber. It does not cover shelf spacing, loads or wall fixings.

Standards we've actually verified

Where a calculator cites a British Standard or a Specification for Highway Works clause, we've checked what that document actually says - what it covers and, just as importantly, what it doesn't - rather than repeating a citation we found somewhere else. See the standards reference for the ones we've verified so far.