To find the quantity of an RCC slab, multiply length by width by thickness to get the concrete volume in cubic metres, then work out cement, sand and aggregate from the mix, and count every bar from the drawing to get the steel in kilograms. For a 125 mm thick M20 slab, each square metre needs 0.125 m³ of concrete, which is about one bag of cement (1.008 bags). In the worked example below, a single 4.23 m by 3.73 m room slab needs 1.97 m³ of concrete, about 16 bags of cement, 29 cft of sand, 59 cft of aggregate and about 167 kg of 8 mm bars before wastage.
This guide is written for site engineers, overseers and house owners in Nepal who need to check a contractor's quantity or order material for a slab casting day. It uses the slab detail that most small houses here follow, explains the code limits you should check, and shows every step so you can repeat it for your own drawing.
What you need before you start
Collect these from the structural drawing or the site before doing any calculation:
- Slab outline: the outer size of the slab, normally measured to the outer face of the edge beams. Note any openings such as the staircase opening or a skylight.
- Slab thickness: NBC 205:2024, the ready-to-use detailing guideline for small RC buildings, gives 125 mm for roof and floor slabs. Your designer may use a different value.
- Concrete grade and mix: NBC 205:2024 uses M20, written as 1:1.5:3 by volume for nominal mix.
- Bar size and spacing: NBC 205:2024 uses 8 mm Fe500 bars in slabs. Its slab figures show spacing of 100 to 150 mm depending on location, and top bars near the supports that reach about a quarter of the span (0.25 L) into the slab.
- Cover: IS 456 asks for 20 mm nominal cover for slabs in mild exposure.
NBC 205 applies only to small buildings: up to 3 storeys or 12 m height, bays of up to 4.5 m, and slab panels no larger than 13.5 m² with each side between 2.1 m and 4.5 m. If your building is outside these limits, the slab must come from a structural design, and you should take bar sizes and spacing from that design, not from this example.
Step-by-step method
- Measure the slab area. Use the outer dimensions of the slab. Deduct openings like the staircase hole. Small pipe sleeves are normally ignored.
- Find the concrete volume. Volume = area x thickness. Keep thickness in metres (125 mm = 0.125 m).
- Convert wet volume to dry volume. Multiply by 1.54. This factor covers voids in sand and aggregate and the bulking of sand, and it is the same factor used in the M20 cement bag calculation.
- Split the dry volume by the mix ratio. For 1:1.5:3 the parts add to 5.5. Cement = dry volume x 1/5.5, sand = x 1.5/5.5, aggregate = x 3/5.5.
- Turn cement volume into bags. Cement weighs about 1,440 kg/m³ and one bag is 50 kg, so bags = cement volume x 1,440 / 50.
- Count the bars in each direction. Number of bars = (length across which the bars are spread minus covers) / spacing + 1, rounded up.
- Find the length of each bar. Straight bar length = slab dimension minus two end covers. Add bends, hooks or legs shown on the drawing.
- Add top bars and their distribution bars along each supported edge.
- Convert length to weight. Weight per metre = D²/162 kg, where D is in mm. For 8 mm, 64/162 = 0.395 kg/m. See the steel weight calculator for other diameters.
- Add an allowance for chairs, laps and cutting waste, using the figure your office or contract follows.
Worked example 1: one room slab
Take a single room with beams at 4.0 m and 3.5 m centre to centre, beams 230 mm wide, and a 125 mm slab cast over the beams. The slab outline is 4.0 + 0.23 = 4.23 m by 3.5 + 0.23 = 3.73 m. The detail assumed here: 8 mm Fe500 bottom bars at 150 mm both ways, 8 mm top bars at 150 mm over all four beams reaching 0.25 of the short span (0.875 m) beyond the inside face of the beam with a 100 mm leg at the slab edge, five 8 mm distribution bars under each set of top bars, and 25 mm side cover at the slab edge. Replace these with the values on your drawing.
Concrete and materials
- Area = 4.23 x 3.73 = 15.78 m²
- Wet volume = 15.78 x 0.125 = 1.97 m³
- Dry volume = 1.97 x 1.54 = 3.04 m³
- Cement = 3.04 / 5.5 = 0.552 m³, x 1,440 / 50 = 15.9 bags, so order 16 bags
- Sand = 3.04 x 1.5 / 5.5 = 0.83 m³ = 29.3 cft
- Aggregate = 3.04 x 3 / 5.5 = 1.66 m³ = 58.5 cft
The concrete calculator gives the same figures if you enter the slab size and pick M20.
Steel
| Bar mark | Description | Number | Length each (m) | Total length (m) |
|---|---|---|---|---|
| A | Bottom bars along the 3.73 m direction | (4.23 - 0.05) / 0.15 + 1 = 28.9, say 29 | 3.73 - 0.05 = 3.68 | 106.72 |
| B | Bottom bars along the 4.23 m direction | (3.73 - 0.05) / 0.15 + 1 = 25.5, say 26 | 4.23 - 0.05 = 4.18 | 108.68 |
| C | Top bars over the two long beams | 2 x 29 = 58 | 0.205 + 0.875 + 0.10 = 1.18 | 68.44 |
| D | Top bars over the two short beams | 2 x 26 = 52 | 1.18 | 61.36 |
| E | Distribution bars under top bars, long edges | 2 x 5 = 10 | 4.18 | 41.80 |
| F | Distribution bars under top bars, short edges | 2 x 5 = 10 | 3.68 | 36.80 |
| Total length of 8 mm bars | 423.80 | |||
Weight = 423.80 x 0.395 = 167.4 kg. With a 5 percent allowance for chairs, laps and cutting (an assumption for this example), order about 176 kg of 8 mm bars. That works out to about 10.6 kg of steel per m² of slab, or about 85 kg per m³ of slab concrete. Use these ratios only to check a contractor's figure, not to order steel for a different detail.
Quick code checks on this slab
- Minimum steel (IS 456 clause 26.5.2.1): 0.12 percent of the gross section for high strength bars, so 0.0012 x 1000 x 125 = 150 mm² per metre width. 8 mm at 150 mm gives 50.3 x 1000 / 150 = 335 mm² per metre, so it passes.
- Maximum bar size (clause 26.5.2.2): one eighth of slab thickness, 125 / 8 = 15.6 mm. 8 mm, 10 mm and 12 mm bars are allowed.
- Maximum spacing of main bars (clause 26.3.3): 3 times effective depth or 300 mm, whichever is less. With 20 mm cover and 8 mm bars, effective depth is about 125 - 20 - 4 = 101 mm, so the limit is 300 mm. Distribution bars may go up to 5d or 450 mm.
- One-way or two-way: the long span divided by the short span is 4.0 / 3.5 = 1.14, which is less than 2, so this is a two-way slab and bars in both directions carry load.
Photo: Building a house nearby the street between Kathmandu and Nagarkot by Rainer Haessner, CC BY-SA 3.0, via Wikimedia Commons
Worked example 2: a full floor of a house
A common two or three storey house in the Kathmandu Valley has a floor of about 1,000 sq ft. Take a slab outline of 1,000 sq ft = 92.90 m², with a staircase opening of 3.0 m x 1.2 m = 3.60 m².
- Net slab area = 92.90 - 3.60 = 89.30 m²
- Concrete = 89.30 x 0.125 = 11.16 m³
- Cement = 11.16 x 8.064 = 90.0 bags
- Sand = 11.16 x 0.42 = 4.69 m³ = 165.6 cft
- Aggregate = 11.16 x 0.84 = 9.38 m³ = 331.1 cft
For steel, a rough check with the 10.6 kg/m² from Example 1 gives 89.30 x 10.6 = about 950 kg. A real floor has continuous panels, where one set of top bars serves two panels, cantilever balconies and different panel sizes, so the true figure can be lower or higher. Prepare a bar list panel by panel, the same way as a bar bending schedule for a beam, before ordering.
Material for slabs of different thickness
The table below is for 100 m² of slab in M20 (1:1.5:3) with the 1.54 dry volume factor. For any other area, multiply by area / 100.
| Thickness | Concrete (m³) | Cement (bags) | Sand (m³ / cft) | Aggregate (m³ / cft) |
|---|---|---|---|---|
| 100 mm | 10.0 | 80.6 | 4.20 / 148 | 8.40 / 297 |
| 125 mm | 12.5 | 100.8 | 5.25 / 185 | 10.50 / 371 |
| 150 mm | 15.0 | 121.0 | 6.30 / 222 | 12.60 / 445 |
Water is not included. Ready-mix concrete is ordered by the cubic metre, so for a ready-mix pour you only need the wet volume plus a small extra for spillage and uneven formwork.
How slab and beam quantities fit together
When the slab and beams are cast together, the concrete where they meet must be counted only once. The usual practice in Nepali estimates is to take the slab over its full outline and take the beam depth below the slab only. For a 230 x 355 mm beam under a 125 mm slab, the beam part is 230 x 230 mm. If you take the full beam depth and the full slab, you count the shared strip twice. On a 1,000 sq ft floor with about 60 m of beams, that error is 60 x 0.23 x 0.125 = 1.7 m³, or about 14 bags of cement.
Common mistakes seen on Nepali sites
- Mixing feet and metres. Many drawings give room sizes in feet while the bar spacing is in mm. Convert everything to metres first. 1 m³ = 35.31 cft.
- Forgetting the +1 when counting bars. A 4.18 m spread at 150 mm needs 29 bars, not 28.
- Leaving out top bars. Owners often count only the bottom mesh and are surprised when the steel runs short on casting day. Top bars over beams and their distribution bars were 49 percent of the total length in Example 1.
- Counting slab and beam overlap twice, as explained above.
- Not deducting the staircase opening, or deducting it but forgetting the extra trimmer bars shown around it.
- Using a cement figure for a different mix. 8.064 bags per m³ is for 1:1.5:3. A 1:2:4 mix (M15) uses less cement and is not what NBC 205 asks for.
- Too little cover. Bars lying directly on the plywood rust and the slab cracks underneath. Use cover blocks so the bottom bars sit 20 mm above the shuttering.
- Thin slabs to save money. Cutting a slab from 125 mm to 100 mm saves about 20 bags of cement per 100 m², but a thin slab sags and cracks. Keep the thickness on the approved drawing.
Frequently asked questions
How many cement bags are needed for a 1,000 sq ft slab?
For a 125 mm M20 slab with no openings, 92.90 m² x 0.125 = 11.61 m³, which needs about 94 bags. With a 3.6 m² staircase opening it drops to about 90 bags.
How much steel goes into a house slab per square foot?
In Example 1 it was about 10.6 kg/m², which is about 0.99 kg per sq ft. Treat this as a check figure only. The real number depends on panel size, bar spacing and how many top bars your design has.
Can I use 10 mm bars instead of 8 mm in the slab?
Only if the designer changes the spacing. A 10 mm bar has 1.56 times the area of an 8 mm bar, so the spacing is usually increased. Do not swap bar sizes on site without the engineer's approval.
How long should I keep the slab props?
IS 456 lists 7 days for props of slabs up to 4.5 m span and 14 days for longer spans, when ordinary Portland cement is used. Keep the slab wet during that time; see the guide on curing of concrete.
Where can I practise slab design questions for exams?
The RCC and steel design MCQ set has questions on slab limits, cover and reinforcement that come up in NEC licence and Lok Sewa exams.
Sources and further reading
- NBC 205:2024, Ready-to-use detailing guideline for low rise RC buildings (copy hosted by Birendranagar Municipality)
- IS 456:2000, Plain and Reinforced Concrete, Code of Practice
- Department of Urban Development and Building Construction (DUDBC), which publishes the Nepal National Building Code
Material rates and the exact detail your municipality accepts change from place to place, so confirm both with your designer and the municipality's building permit section before you buy material.
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