Methodology
Every formula on this site is documented below, exactly as it runs. Prices are a mix — some read straight off a retailer's product page, some from a retailer's listing in search results, some estimated — and the price table says which is which, with the counts. This page also covers how often we check them and where we know the numbers are weaker than we'd like. Reviewed quarterly by Sam Redfield.
Most recent price review: (not every row was re-checked; per-row dates are in the table) · Next review
Page last updated
The formula we use — composite decking
- Deck area = length × width.
- Total area with waste = deck area × (1 + waste allowance), where the allowance is fixed per install pattern: 10% straight, 15% diagonal, 20% herringbone, 12% picture-frame. Board length never changes this percentage — only the board count.
- Board coverage = board length × effective board width (0.458 ft for imperial regions, 0.14 m for metric), which already accounts for the gap left between boards for drainage and expansion.
- Boards needed = total area with waste ÷ board coverage, rounded up to a whole board.
- If hidden fasteners are switched on, we add roughly 3.5 clips per sqft of deck area (about 38 per sqm in metric regions — the same physical clip density, just expressed in metric units), plus one starter clip per 2 ft of perimeter (about every 0.6m in metric regions). Until August 2026, UK and Australian results applied the per-sqft clip rates to square metres and undercounted clips roughly tenfold; that's now corrected. See our About page for how to report errors like this one.
The formula we use — retaining wall block
- Blocks per row = wall length ÷ block length, rounded up.
- Rows needed = wall height ÷ block height, rounded up.
- Total blocks = (blocks per row × rows needed) × (1 + waste allowance), rounded up. The waste allowance is fixed per wall shape: 5% straight, 10% curved, 15% terraced — straight walls still carry a small allowance for corner and end cuts.
- Cap blocks, if included, add one full row: wall length ÷ block length, rounded up.
- Base gravel trench = wall length × (block depth × 2) × base gravel depth, converted to cubic yards (imperial) or tonnes at 1.6 t/m³ (metric). The buried base course is included by default — it's the foundation, not an optional extra.
- Backfill gravel, if included, = wall length × backfill zone width (1 ft / 0.3 m) × wall height × 0.5, same unit conversion as above.
- Landscape fabric = wall length × (wall height + trench width) × 2, to wrap the drainage zone.
The formula we use — paint + primer coverage
- Wall area = perimeter × ceiling height, plus floor area if you're also painting the ceiling.
- Paintable area = wall area minus openings (standard door and window sizes, or your own count).
- Coverage rate depends on both surface type and paint tier — bare drywall in our economy tier is rated at 250 sqft/gal, previously-painted walls in our premium tier at 450 sqft/gal. These are deliberately conservative versus manufacturer claims, to protect against underbuying.
- Paint needed = (paintable area ÷ coverage rate) × number of coats.
- Purchase volume rounds to what you can actually buy: US rounds up to the nearest whole gallon can; UK, Australian, and Canadian results solve for the real can-size combination (5L/2.5L/1L in the UK, 10L/4L/1L in Australia, single 3.78L cans in Canada) that covers the need with the least wasted volume.
- Leftover paint = purchased coverage minus paintable area actually needed — shown separately so you know how much is left for touch-ups from the same batch.
- Primer, when recommended, = paintable area ÷ a tier-based primer coverage rate, rounded up to a whole unit.
Price sources
Our calculators use 74 prices. In the October 2026 review, 10 were read directly from the retailer's own product page. 28 were checked against a current retailer listing in search results: 24 matched our figure, and 4 showed a different price we're still reviewing before we change anything. The other 36 could not be checked this cycle. Most sit with retailers that restrict access from outside the regions they serve — Home Depot in the US and Canada, Bunnings and B&Q — which re-checking can't get around; the rest have no named product to look up. Future quarterly reviews will keep leaving lines unchecked for the same reason, so don't expect this number to reach zero. The notes column says which lines are which, row by row. Where we quote a retailer, it's the single-unit list price — not a bulk, kit or sale price. Each row names up to two retailers; where there are two, the notes say which lines come from which.
What economy, mid and premium mean. They're price bands across what a large retailer stocks in each region, chosen so the three ascend and span the range you'll actually see on the shelf. They aren't any one manufacturer's good-better-best ladder, so a brand's entry line can sit in our mid band when a cheaper brand is stocked beside it. Behr Premium Plus, for example, is Behr's entry paint, but Glidden Premium costs less at Home Depot, so here it's mid.
| Calculator | Region | Retailers | Last verified | Confidence | Notes |
|---|---|---|---|---|---|
| Composite decking | US | Home Depot | 2026-10-11 | Medium-high | Boards and fastener clips checked October 2026 from Home Depot search listings. Economy and mid-tier boards and clips matched; economy prices are the single-board list price (not the bulk rate or a sale price). The 12 ft and 16 ft premium boards listed at different prices and are under review. 20 ft premium board not confirmed. Starter clips are an assumption, priced the same as fastener clips. |
| Composite decking | UK | Wickes | 2026-10-11 | Medium-high | 4.8 m mid and premium boards and both clip prices read directly from Wickes, October 2026. Economy boards and the 3.6 m premium board have no matching Wickes product; July prices kept. |
| Composite decking | AU | Bunnings | 2026-10-12 | Medium-high | Ekodeck boards and fastener clips matched Bunnings search listings, October 2026. The clip price is the per-clip cost of a 15-pack of Quickfix clips. Starter clips are an assumption, priced the same as fastener clips. |
| Composite decking | CA | Home Depot Canada | 2026-10-11 | Low | Only the fastener clip price could be checked in October 2026. Board prices are from July 2026, with 12 ft and 20 ft interpolated from 16 ft. Starter clips are an assumption, priced the same as fastener clips. |
| Retaining wall block | US | Home Depot (generic concrete block, no named product) | 2026-07-27 | Low | No named product recorded, so nothing could be re-checked in October 2026. Natural stone has no recorded source. |
| Retaining wall block | UK | Wickes / B&Q | 2026-07-27 | Medium | Not re-checked in October 2026: no named products recorded for blocks, cap or gravel. Natural stone is an estimate (4–5× economy). Fabric is B&Q's Heavy Duty 100gsm membrane. |
| Retaining wall block | AU | Bunnings (Eziwall / Versawall) | 2026-10-11 | Medium-high | Blocks, cap, gravel and fabric checked October 2026 from Bunnings search listings. Natural stone is an estimate (4–5× economy). Cap price is Versawall's, used for both block tiers. |
| Retaining wall block | CA | Chamberlain Timber Mart (economy); RONA (premium) | 2026-10-11 | Medium-low | Economy blocks read directly from Chamberlain, October 2026. Premium (Basalite Valley Stone) not re-checked. Natural stone is an estimate. Cap, gravel and fabric: source not recorded. |
| Paint + primer | US | Home Depot | 2026-10-11 | Medium | Economy (Glidden Premium) checked October 2026 from Home Depot search listings. The mid-tier listing showed a different price, under review. Premium and primer gallon prices could not be found. |
| Paint + primer | UK | Wickes | 2026-10-12 | High | All four lines read directly from Wickes, October 2026: Wickes Matt Emulsion (economy), Dulux Trade Supermatt (mid), Dulux Trade Diamond Matt (premium), and Zinsser Bulls Eye 1-2-3 Primer Sealer. |
| Paint + primer | AU | Bunnings | 2026-10-12 | Medium | All four lines checked October 2026 from Bunnings search listings. Economy is Taubmans Easycoat Low Sheen 4L. |
| Paint + primer | CA | Home Depot Canada | 2026-10-11 | Medium | Two tiers only. Mid (Behr Premium Plus) and primer confirmed October 2026 from Home Depot Canada search listings; premium (Behr Marquee) now lists higher and is awaiting a recheck. No economy tier: we have no Canadian economy price we can stand behind, so we don't show one. |
"Confidence" reflects how directly we sourced each row. High means we read the prices off the retailer's own product pages. Medium means some lines came from the retailer's search listings rather than the page itself (Home Depot, Home Depot Canada and Bunnings restrict access from outside the regions they serve), or are interpolated, estimated, or drawn from a secondary source. Low means little of the row could be checked against a named product. Medium-high and Medium-low sit between those. We'd rather tell you a number is soft than present it as more certain than it is.
How often we update
We re-check prices quarterly. The October 2026 review ran on 11 October. Next scheduled reviews: composite decking on 2027-01-25, retaining wall block on 2027-01-27, and paint + primer on 2027-01-30. If a price moves meaningfully between reviews and we hear about it, we'll fix it sooner. Email corrections@thebuildcost.com to flag something, or see the About page for more on how corrections work.
What we don't do
- No hidden formulas. Every calculation above is exactly what runs when you use the tool — nothing simplified for this page and nothing more complex running behind it.
- No inflated waste factors. The waste percentages shown above are the same ones the calculators use, not padded numbers meant to make the site look more conservative than it is.
- Affiliate links don't affect outputs. Prices come from our quarterly retailer research, or are flagged in the table as estimates — never from whichever product happens to pay the best commission.
Known limitations
We'd rather list what we know is imperfect than let you find it the hard way.
- 13 of our 74 prices aren't retailer prices. They're estimates or assumptions, and each one is named in the table above: natural stone in all four regions (estimated, not priced from a product — the US figure has no recorded source at all); starter clips in the US, Australia and Canada (assumed to cost the same as fastener clips); and Canadian 12 ft and 20 ft decking boards (interpolated from the 16 ft price).
- Some rows have no named product recorded. Our US retaining wall prices are generic concrete block pricing, with no specific product behind them. Our UK block, cap and gravel prices, and our Canadian cap, gravel and fabric prices, have no recorded product either, which is why those lines couldn't be re-checked in October. We're sourcing named products for the next review.
- Paint can combinations can look impractical. Our paint calculator picks the can combination that wastes the least paint, which occasionally means several smaller cans instead of one larger one. It's the mathematically leanest answer, and we know it doesn't always look like the tidiest shopping list — a "prefer fewer cans" option is on our list.