Kumar IronUpdated
In this guide
What you need from the drawing
If you do not have a drawing yet, use a thumb rule for planning only, and get the slab designed before you buy. Bar size and spacing are a structural decision, not a shop decision.
- Slab size: the clear span in both directions, in metres.
- Main bars: diameter and spacing, for example 10 mm at 150 mm centre to centre. Main bars run along the short span.
- Distribution bars: diameter and spacing, for example 8 mm at 200 mm. These run along the long span, on top of the main bars.
- Extra top bars or cranked bars at supports, if the drawing shows them.
- Clear cover, usually 15 to 20 mm for a slab.
The four steps
- Step 1, number of bars: (length across which bars are spaced ÷ spacing) + 1. Round up.
- Step 2, cut length of one bar: span + anchorage into the beam or wall at both ends, minus cover. Add bends or cranks if the drawing shows them.
- Step 3, total length: number of bars × cut length.
- Step 4, weight: total length × weight per metre, where weight per metre in kg = D² ÷ 162 and D is the diameter in mm.
Do this once for main bars and once for distribution bars, add any extra top bars, then add 3 to 5 percent for laps and cutting waste.
Worked example: a 4 m × 3.5 m room slab
Assume the drawing shows 10 mm main bars at 150 mm along the 3.5 m short span, 8 mm distribution bars at 200 mm along the 4 m long span, and roughly 0.3 m total anchorage per bar. These numbers are for illustration only. Use your own drawing.
| Item | Main bars (10 mm) | Distribution bars (8 mm) |
|---|---|---|
| Spaced across | 4.0 m at 150 mm | 3.5 m at 200 mm |
| Number of bars | 4000 ÷ 150 + 1 = 28 | 3500 ÷ 200 + 1 = 19 |
| Cut length | 3.5 + 0.3 = 3.8 m | 4.0 + 0.3 = 4.3 m |
| Total length | 28 × 3.8 = 106.4 m | 19 × 4.3 = 81.7 m |
| Weight per metre | 10² ÷ 162 = 0.617 kg | 8² ÷ 162 = 0.395 kg |
| Weight | about 65.7 kg | about 32.3 kg |
Total is about 98 kg. Add 5 percent for laps and waste and order about 103 kg, which is roughly 7.3 kg per square metre of slab. Extra top bars over beams and cranked bars will push this up, so count them separately if your drawing has them.
Turning kg into bars to order
Try it: Steel calculator
Estimated steel: 4,000 kg (4.00 tonnes)
| Bar size | Approx kg | 12 m bars |
|---|---|---|
| 8 mm | 1,000 | 211 |
| 10 mm | 1,000 | 136 |
| 12 mm | 1,000 | 94 |
| 16 mm | 600 | 32 |
| 20 mm | 400 | 14 |
Dealers sell full-length bars, usually about 12 m. Divide the total length of each size by 12 and round up, then check cut lengths fit without too much offcut. In the example, 106.4 m of 10 mm is 9 bars and 81.7 m of 8 mm is 7 bars. Our weight chart converts bars to kg for the bill.
Common mistakes
- Forgetting the +1 bar when counting from spacing. On a small slab that is several percent short.
- Using the overall slab size including walls for the spacing count, then adding anchorage again on top.
- Leaving out extra top bars at supports and around openings such as stair cut-outs.
- Ordering exactly the calculated kg with no allowance for laps.
- Mixing up main and distribution directions. Main bars go along the short span.
Frequently asked questions
- How much steel per square metre does a house slab need?
- For a typical 4 to 5 inch residential slab it often falls between 6 and 10 kg per square metre, depending on span, bar size, spacing and extra top bars. Use your drawing for the real figure.
- Why is the weight formula D² ÷ 162?
- Steel weighs about 7850 kg per cubic metre. Multiply that by the area of a circle of diameter D mm over one metre and the result simplifies to D² ÷ 162 kg per metre.
- Can I use 8 mm for main bars to save money?
- Only if your engineer designs it that way. Changing bar size or spacing changes the slab's strength.