Every design ends in a list of materials, and the arithmetic that gets there is simple enough that it is usually done in someone's head on the way to site. This page writes it down.
Concrete: cement, sand and granite per m³
The materials for a cubic metre of concrete add up to considerably more than a cubic metre loose, because the sand fills the voids between the granite and the cement fills the voids between the sand. The standard allowance for that is a dry volume factor of about 1.54: order 1.54 m³ of loose material for every 1 m³ of finished concrete.
Bags = volume x 1440 kg/m3 / 50 kg
| Nominal mix | Cement 50 kg bags |
Sand m³ |
Granite m³ |
Roughly |
|---|---|---|---|---|
| 1 : 3 : 6 | 4.4 | 0.46 | 0.92 | grade 15 |
| 1 : 2 : 4 | 6.3 | 0.44 | 0.88 | grade 20–25 |
| 1 : 1½ : 3 | 8.1 | 0.42 | 0.84 | grade 25–30 |
| 1 : 1 : 2 | 11.1 | 0.39 | 0.77 | grade 30–40 |
Per cubic metre of finished concrete. Cement taken at 1440 kg/m³ loose.
The Roughly column is doing a great deal of work, and the design does not rest on it — a nominal volumetric mix is not a designed mix, and what you actually get depends on the water added at the mixer, the cleanliness of the sand and whether anything was cured. See cover, grades and mixes, and take cubes if the grade in the calculation matters.
For ordering: a tipper of sharp sand is commonly quoted as 5 or 10 tonnes rather than by volume, so agree the conversion before the first trip rather than after the third.
Blockwork
The standard Nigerian hollow sandcrete block has a face of 450 × 225 mm, in 225 mm (9 inch) and 150 mm (6 inch) thicknesses.
Blocks per m2 1 / 0.1013 = 9.9 (no joints)
With 10 mm joints 0.460 x 0.235 = 0.1081 -> 9.3
Order at 10 per m2 (covers breakage)
Deduct openings over about 1 m² and no smaller — the labour and breakage around a window absorbs the difference on the small ones.
Mortar and render
| Work | Mortar per m² | Roughly |
|---|---|---|
| Laying 225 mm blocks, 10 mm joints | 0.015 – 0.025 m³ | 1 bag per 8–12 m² of wall at 1:6 |
| Render, 12 mm, one face | 0.014 – 0.018 m³ | 1 bag per 8–10 m² at 1:4 |
Ranges, deliberately. Joint thickness on site is not 10 mm, render thickness on a wavy wall is not 12 mm, and both of those move these figures more than the mix does. Measure a completed wall on your own job once and you will have a better number than any table.
Moulding your own blocks
A bag of cement is often said to mould twenty-something 9-inch hollow blocks. The honest answer is that the figure is not a constant — it is inversely proportional to the strength of the block. More blocks per bag means less cement in each, and that is the entire trade-off, not a measure of efficiency.
Reinforcement
Steel is taken off the bar bending schedule, bar by bar. There is no shortcut, and the mass of any bar is:
| Bar | T8 | T10 | T12 | T16 | T20 | T25 |
|---|---|---|---|---|---|---|
| kg/m | 0.395 | 0.617 | 0.888 | 1.579 | 2.466 | 3.854 |
| per 12 m length | 4.74 | 7.40 | 10.7 | 18.9 | 29.6 | 46.2 |
Fuller tables, including steel area per metre width at each spacing, are on the rebar weight chart.
The 12 metre problem
The right method is to pack the bars into lengths, longest first, putting each into the first length it fits. It errs by ordering the occasional extra length rather than one too few, which is the right side to be wrong on: an extra length is stock, and one short stops a pour.
Bars longer than 12 m have to be lapped. That is a detailing decision, not a cutting one — flag them and detail them, do not quietly split them on the schedule.
Rates per m³, as a sanity check only
| Element | kg of steel per m³ of concrete |
|---|---|
| Pad and strip footings | 40 – 80 |
| Solid slabs | 60 – 110 |
| Rafts | 80 – 130 |
| Beams | 90 – 160 |
| Columns | 130 – 250 |
Do not order steel from this table. The ranges are two or three to one wide, which is useless for buying and just right for checking. Take the schedule off properly, divide by the concrete volume, and see where it lands. A slab coming out at 200 kg/m³ is not a slab — something has been double-counted, or the design has a problem worth finding before the steel is bent.
Worked: one suspended floor
A 10.0 × 8.0 m floor, 150 mm solid slab, with 3 m high 225 blockwork walls round a 36 m perimeter, 20% of the wall area in openings, rendered both sides. Grade 25 concrete at 1:2:4.
Concrete 10.0 x 8.0 x 0.150 = 12.0 m3
Cement 12.0 x 6.3 = 76 bags
Sand 12.0 x 0.44 = 5.3 m3
Granite 12.0 x 0.88 = 10.6 m3
Steel from the schedule, say 85 kg/m3 x 12.0 = 1020 kg
WALLS
Gross area 36 x 3.0 = 108 m2
Less openings x 0.80 = 86 m2
Blocks 86 x 10 = 860 no.
Render 86 x 2 faces = 172 m2
Render cement 172 / 9 = 19 bags
Every figure above is net — the quantity in the finished building. It is not what to buy. Add an allowance for wastage, and state on the bill which of the two the bill is showing, because a client comparing two estimates cannot otherwise tell whether one is 8% cheaper or just measured differently. Usual allowances run around 5% on concrete, 5–10% on blocks and 3–5% on steel, and every one of them belongs to your site rather than to a table.
Things this does not cover
- Formwork. Measured by contact area, and on a typical frame it is a larger part of the cost than the concrete it shapes. Reuse is what determines the figure, and that is a programme question.
- Labour, plant and preliminaries.
- Binding wire, spacers, chairs and cover blocks. Small, always forgotten, and the reason cover fails on site. Put them in the bill so they get bought.
- Excavation and disposal, which on a raft or a wide strip is not a small item.
- Blinding under every foundation, usually 50 mm of 1:3:6.
Have the bill come out of the design
Structura produces a bill of quantities from the design itself rather than from a separate take-off, grouped by element the way a client asks about it — foundations, frame, slabs — so the parts always sum to the total. The bar schedule is packed into 12 m commercial lengths by the method above, and bars too long to cut from one are flagged for lapping rather than quietly split.
Quantities are measured net. The wastage allowance is yours to set, per category, and it starts at zero on purpose: installing a new version must never silently restate a figure you have already quoted from. Rates are yours too, in naira, held on your device and stamped with the date you set them.
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