- 8results checked against published UK figures
- 6match the published figure
- 2deliberately more cautious, and why
- 13problems found and fixed in the October 2026 review
I'm Valerii Zugravu, and I've worked in UK construction for 10 years (about me). A calculator is only useful if the number it gives is the number you'd get from the people who make and sell the material. So for the calculators where a manufacturer, merchant or the Approved Documents publish a figure, I entered the same measurements and compared. The tests below were run on 3 November 2026 and anyone can repeat them: every row says what to enter.
How a result is made
The results
Wastage was set to 0% (or the lowest option, then taken off) so the calculator's figure can be compared directly with the published one.
| Calculator and what to enter | Published figure | Calculator gives | Result |
|---|---|---|---|
| Concrete: 2.5m × 1m × 1000mm, mix 1:5, 0% wastage | 35 × 25kg bags of cement per 2.5m³ of general-purpose concrete (Tarmac) | 35 bags | Match |
| Concrete: the same, mix 1:6 | 30 bags per 2.5m³ of foundation concrete (Tarmac) | 30 bags | Match |
| Brick: wall 10m × 1m, single skin, 5% wastage | 60 bricks per m² in a half-brick wall (Travis Perkins) | 630 bricks = 600 + 5%, so 60 per m² | Match |
| Plaster: area 10m², Multi-Finish, 2mm, 0% wastage | About 10m² per 25kg bag at 2mm (Jewson listing for Thistle Multi-Finish) | 1 × 25kg bag, 10.0m² per bag | Match |
| Roof tiles: one slope 10m × 10m, plain tile, 5% cuts | About 60 plain tiles per m² at a 100mm gauge (Marley) | 6,300 = 6,000 + 5%, so 60 per m² | Match |
| Roof tiles: the same, large interlocking concrete tile | 9.9 tiles per m² at 75mm headlap to 10.7 at 100mm (Marley Modern) | 1,050 = 1,000 + 5%, so 10 per m² | Within the published range. At a 100mm headlap, choose 10% cuts. |
| Paint: area 30m², standard emulsion, 1 coat | "Up to" 18m² per litre on smooth, primed walls (Dulux Trade Supermatt) | 12m² per litre, plus 10% spare | More cautious on purpose: real, patched walls take more than a test panel |
| Gutter: drainage check, 112mm half round | 75mm per hour design rainfall (Approved Document H); FloPlast flow figures for a level gutter | 0.82 l/s, about 39m² of effective roof per outlet | More cautious on purpose: one stockist quotes 0.92 l/s, the site uses the lower figure |
Where the calculators round up on purpose
- Wastage you can see5–15% for cuts and breakages, shown in the result and changeable.
- Whole packsYou can't buy 0.4 of a bag, so every pack count is rounded up, and the spare is shown.
- Generous cementThe 1:5 mix gives Tarmac's 14 bags per m³. Measured by level buckets it's nearer 12, so you won't run short.
- Soil grows when dugSoil measured in the ground is increased by 25% for the skip, because it loosens once it's out.
What the calculators don't do
- No structural designFoundations, lintels and retaining walls that hold anything up need building control or an engineer. The calculators give quantities.
- No ground surveyClay, trees and soft spots change how deep you dig. See Approved Document A and the foundation depth guide.
- Not your exact productCoverage varies between brands. The bag or tin you buy is the final word.
Fixes so far
When I find a mistake, or someone reports one, I fix it and update the page's "last reviewed" date. These are the fixes to calculator results so far. Every dated change, including new tools, is in the changelog.
| Date | What was wrong, and what changed |
|---|---|
| 24 Oct 2026 | A full review found 13 problems, all fixed the same day. They included the mixing water for plaster, how many boxes of tiles to buy, the saw cuts when cutting sleepers, the haunching for block paving, primer tub sizes, swapped hip measurements on the roof calculator, quantity boxes that accepted half a door, openings bigger than the wall, and results that showed when the measurements were zero or impossibly large. |
| 24 Oct 2026 | Eight calculators that mix sand, cement and ballast moved to one shared volume-ratio method, so the same mix gives the same answer everywhere. Radiator room factors were corrected. |
| 27 Oct 2026 | The foundation advice replaced a rule of thumb with the Approved Document A rule (concrete at least as thick as it sticks out past the wall, minimum 150mm). Sources lists were added to the main calculators. |
| 2 Nov 2026 | The concrete calculator now says clearly that mix ratios are buckets by volume and that 1:6 is a nominal mix, not a strength class. Optional steel reinforcement mesh was added, with laps and cover from supplier guidance. |
Found something wrong?
If a result doesn't match what you see on site or at the merchant's counter, use Report a problem under any result. It sends your measurements with the message, so I can check exactly what you entered. You can also report a problem here.