If you are building a house, the roof slab is one of the biggest single material purchases you will make. Order too little and the pour stops halfway, leaving a cold joint. Order too much and you are stuck with hardening cement bags. Here is how to work out the quantities yourself, the way a site engineer does it.
For a 1000 sq ft slab that is 5 inches thick in M20 concrete, you need about 94 bags of cement, 172 cft of sand and 344 cft of 20 mm aggregate, plus about 5% extra for wastage. The rest of this article shows where those numbers come from, so you can adjust them for your own house.
Step 1: find the wet volume
Convert the area to square metres: 1000 sq ft × 0.0929 = 92.9 m². Convert thickness to metres: 5 inches is 0.127 m, and most drawings specify 125 mm (0.125 m). Multiply: 92.9 × 0.125 = 11.61 m³ of concrete. That is the volume the slab will occupy once it has set.
Step 2: convert to dry volume
Loose cement, sand and aggregate have air gaps between particles. When they are mixed with water and compacted, the smaller particles slip into the gaps, and the mix shrinks by about a third. To end up with 11.61 m³ of wet concrete, you need more dry material: 11.61 × 1.54 = 17.88 m³.
Step 3: split by the mix ratio
M20 nominal mix is 1 : 1.5 : 3 (cement : sand : aggregate), adding up to 5.5 parts.
| Material | Share | Volume | In site units |
|---|---|---|---|
| Cement | 1 / 5.5 | 3.25 m³ | 4,682 kg or about 94 bags |
| Sand | 1.5 / 5.5 | 4.88 m³ | about 172 cft (1.7 brass) |
| Aggregate (20 mm) | 3 / 5.5 | 9.75 m³ | about 344 cft (3.4 brass) |
| Water (w/c 0.5) | 0.5 × cement weight | 2.34 m³ | about 2,340 litres |
Cement is converted using its bulk density of 1440 kg/m³, and each bag holds 50 kg. Sand and aggregate are converted to cft by multiplying by 35.31.
Step 4: add wastage
Some material is always lost to spillage, uneven formwork and the concrete left inside the mixer drum. Add 3 to 5% on a well-run site. For our slab, that brings cement to about 98 bags. If you are buying from a ready-mix plant instead, order the wet volume plus about 2%.
Try it for your own slab
Change the dimensions or mix grade below and the quantities update instantly.
Five mistakes that cost house owners money
- Using thickness from memory. Always take slab thickness from the structural drawing. The difference between 4.5 and 5 inches is more than 10% of your materials.
- Forgetting beams. Beams below the slab add concrete. Calculate them separately and add them to the order.
- Adding water on site. A little extra water makes concrete flow easily and quietly cuts its strength. If workers want a softer mix, a plasticiser does the job without the damage.
- Stockpiling cement too early. Cement loses strength once it absorbs moisture from the air. Buy it within a few weeks of use and store it off the ground.
- Skipping curing. Concrete keeps gaining strength for weeks, but only if it stays wet. Pond or wet-cover the slab for at least 7 days, and ideally 14.
What about steel?
Reinforcement steel is not calculated from concrete volume. It comes from the bar bending schedule prepared from the structural drawings. As a very rough check, residential slabs use somewhere around 80 to 100 kg of steel per cubic metre of concrete, but never order steel on that thumb rule alone.
Frequently asked questions
How many cement bags per sq ft of slab?
For a 5 inch M20 slab, about 0.094 bags per sq ft before wastage, or roughly one bag for every 10 to 11 sq ft.
Is M20 enough for a house roof?
M20 is the usual minimum for reinforced concrete in residential buildings under IS 456. Your structural engineer may specify M25 for coastal or severe exposure areas.
Ready-mix or site-mix concrete?
Ready-mix gives more consistent quality and faster pours, and is often similar in cost once you count labour and wastage. Site-mix makes sense where transit mixers cannot reach.