Methodology
Every number on this site comes from a formula and a price we can point to. This page shows both — the exact math behind each calculator, where the prices come from, how often we check them, and where we know the numbers are weaker than we'd like. Reviewed quarterly by Sam Redfield.
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). Fixed in Session 6.5 — see our About page for how we handle corrections 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
Every price in this site's calculators is sourced from a named retailer, not estimated from a general market average, except where a datapoint is explicitly flagged below as an estimate.
| Calculator | Region | Primary source | Last verified | Confidence |
|---|---|---|---|---|
| Composite decking | US | Home Depot | 2026-07-24 | High |
| Composite decking | UK | Wickes | 2026-07-24 | Medium-high |
| Composite decking | AU | Bunnings | 2026-07-24 | High |
| Composite decking | CA | Home Depot Canada | 2026-07-24 | Medium |
| Retaining wall block | US | Home Depot | 2026-07-27 | High |
| Retaining wall block | UK | Wickes / B&Q | 2026-07-27 | Medium-high |
| Retaining wall block | AU | Bunnings (Eziwall / Versawall) | 2026-07-27 | Medium-high |
| Retaining wall block | CA | Chamberlain Timber Mart | 2026-07-27 | Medium |
| Paint + primer | US | Home Depot | 2026-07-30 | High |
| Paint + primer | UK | Wickes | 2026-07-30 | Medium-high |
| Paint + primer | AU | Bunnings + price trackers | 2026-07-30 | Medium-high |
| Paint + primer | CA | Home Depot Canada + community pricing | 2026-07-30 | Medium |
"Confidence" reflects how directly we sourced each datapoint. High means a direct retailer product page fetch; medium means some figures are interpolated or drawn from a secondary source (price trackers, forums) rather than a live product page. 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. Next scheduled reviews: composite decking on 2026-10-25, retaining wall block on 2026-10-27, and paint + primer on 2026-10-30. If a price moves meaningfully between reviews and we hear about it, we'll fix it sooner — see the About page for how to flag something.
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 direct retailer research on a fixed quarterly schedule, not 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.
- US retaining wall has no named block product yet. Our UK, Australian, and Canadian retaining wall pages reference real named block products. Our US page currently uses generic concrete block pricing instead, while we finish sourcing a stable named US product for the next price refresh.
- 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.
Last updated: August 2026.