To find the quantity of an isolated footing, work out five items in order: excavation, PCC bed, footing concrete, column stub up to ground level, and backfill, then add the steel from a small bar schedule. For a 1.5 m x 1.5 m sloped footing at 1.5 m depth, the worked example below gives 6.62 m3 of excavation, 0.22 m3 of PCC, 0.79 m3 of M20 concrete (about 6.4 bags of cement), 5.61 m3 of backfill and about 29.4 kg of 12 mm bars.
Isolated footings carry the columns of almost every RCC framed house in Nepal. Owners want the numbers to check a contractor's bill, and students meet the same problem in estimating and costing. The method is simple once you draw the section and take each part separately. This guide uses one footing as the example; multiply by the number of similar footings for the whole building.
Data for the example
| Item | Value used |
|---|---|
| Footing size at base | 1.5 m x 1.5 m |
| Footing depth | 0.2 m at the edge, 0.45 m at the centre (sloped top) |
| Top of slope | 0.4 m x 0.4 m (column plus 50 mm each side) |
| Column | 0.3 m x 0.3 m |
| Depth of foundation below ground | 1.5 m to the bottom of excavation |
| Working space in excavation | 0.3 m on each side |
| PCC bed | 75 mm thick, 1:3:6, projecting 0.1 m beyond the footing |
| Footing and column concrete | M20 (1:1.5:3) |
| Bottom mesh | 12 mm bars at 150 mm c/c both ways, 50 mm cover |
These are example values only. Use the sizes, depth and reinforcement from your structural drawings; the designer chooses them from the column load and the soil bearing capacity.
Step 1: Excavation
The pit must be wider than the footing so workers can fix formwork and steel. With 0.3 m each side the pit is 1.5 + 0.3 + 0.3 = 2.1 m square.
Excavation = 2.1 x 2.1 x 1.5 = 6.615 m3.
In loose soil or deep pits the sides are cut back to a slope or shored, which adds volume. Your contract or the estimate norms you follow decide how much working space and side slope can be paid. The excavation calculator handles straight and sloped pits.
Step 2: PCC bed
PCC = (1.5 + 0.1 + 0.1) x (1.5 + 0.1 + 0.1) x 0.075 = 1.7 x 1.7 x 0.075 = 0.217 m3.
With a dry volume factor of 1.54 and a 1:3:6 mix (sum 10), cement = 0.217 x 1.54 / 10 = 0.0334 m3, which is 0.0334 x 1,440 / 50 = about 0.96 bag. Sand = 0.10 m3 and aggregate = 0.20 m3.
Step 3: Footing concrete
Split the sloped footing into a rectangular base and a frustum (a pyramid with its top cut off).
- Rectangular base = 1.5 x 1.5 x 0.2 = 0.450 m3.
- Frustum = h / 3 x (A1 + A2 + square root of (A1 x A2)), where h = 0.25 m, A1 = 1.5 x 1.5 = 2.25 m2 and A2 = 0.4 x 0.4 = 0.16 m2. The square root of (2.25 x 0.16) is 0.6, so the frustum = 0.25 / 3 x (2.25 + 0.16 + 0.6) = 0.0833 x 3.01 = 0.251 m3.
- Footing total = 0.450 + 0.251 = 0.701 m3.
If the footing has a flat top (a pad footing of uniform thickness), just multiply length x breadth x thickness.
Step 4: Column stub to ground level
Top of footing below ground = 1.5 - 0.075 - 0.45 = 0.975 m.
Column stub = 0.3 x 0.3 x 0.975 = 0.088 m3. The column above ground level goes in the superstructure estimate.
Total M20 concrete = 0.701 + 0.088 = 0.789 m3. Cement = 0.789 x 1.54 / 5.5 x 1,440 / 50 = 6.36 bags, sand 0.33 m3 and aggregate 0.66 m3. The quick rule of about 8 bags per m3 gives the same answer; see cement bags in 1 m3 of M20 concrete.
Step 5: Backfill
Backfill = excavation - PCC - footing - column stub = 6.615 - 0.217 - 0.701 - 0.088 = 5.610 m3.
Some estimates take backfill only up to the plinth fill level or add a factor for compaction. Follow the method in your BOQ. The surplus soil (6.615 - 5.610 = 1.005 m3) has to be spread on site or carted away.
Step 6: Steel in the footing mesh
- Clear length inside cover = 1.5 - 0.05 - 0.05 = 1.4 m.
- Number of bars in one direction = 1.4 / 0.15 = 9.33, take 9 spaces, so 9 + 1 = 10 bars.
- Length of one bar with a 150 mm leg bent up at each end = 1.4 + 0.15 + 0.15 - 2 x (2 x 0.012) for the two 90 degree bends = 1.652 m.
- Both directions = 2 x 10 = 20 bars, total length = 20 x 1.652 = 33.04 m.
- Weight of 12 mm bar = 12 x 12 / 162 = 0.889 kg/m. Steel = 33.04 x 0.889 = 29.4 kg.
Column starter bars and their L-bends into the footing are added separately, using the bar sizes on the drawing. The steel weight calculator does the kg per metre for any diameter.
Summary for one footing and for twelve
| Item | Unit | One footing | 12 footings |
|---|---|---|---|
| Excavation | m3 | 6.615 | 79.38 |
| PCC 1:3:6 | m3 | 0.217 | 2.60 |
| M20 concrete (footing and stub) | m3 | 0.789 | 9.46 |
| Cement for M20 | bags | 6.36 | 76.3 |
| Backfill | m3 | 5.610 | 67.32 |
| 12 mm steel (mesh only) | kg | 29.4 | 352.4 |
These figures do not include wastage. Many estimators add 2 to 5% for concrete materials and 3 to 5% for steel. Check rates and norms with your local municipality or the district rate in force when you price the work.
Frequently asked questions
Why add working space to the excavation?
Workers need room to place formwork, tie steel and check cover. Without it the pit walls collapse into the concrete or the footing ends up smaller than drawn.
What is the formula for the sloped part of a footing?
Use the frustum formula V = h / 3 x (A1 + A2 + square root of A1 x A2), where A1 is the bottom area, A2 the top area and h the height of the slope.
Is PCC needed under a footing?
Yes, in normal practice. The lean concrete bed gives a clean, level surface, keeps soil out of the reinforcement and helps hold the cover blocks in place.
How do I convert the concrete to cubic feet?
Multiply m3 by 35.31. 0.789 m3 is about 27.9 cft.
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