TryBuildCalcRaft Foundation Calculator for Residential BuildingsResidential raft foundation estimator
Calculate concrete, two-way mesh steel, and shuttering quantity for an RCC raft (mat) foundation.
🕒 Last updated: August 12, 2026
Raft Dimensions
ℹ️Overall raft footprint length, usually the building's outer plan dimension.
ℹ️Rafts are typically 300-1000+ mm thick depending on soil bearing capacity and building load — confirm from the structural drawing.
Concrete
ℹ️Foundation cover is typically higher than a suspended slab's — commonly 50 mm or more, confirm against your applicable code and exposure class.
Bottom Mesh (resists upward soil pressure)
Top Mesh (resists hogging near columns)
Lap Splices
Shuttering
Cost
For a 12.00 m × 9.00 m raft (108.0 m² footprint) at 350 mm thick, you need approximately 39.69 m³ of concrete and 2,255.2 kg of reinforcement steel.
Concrete
Concrete volume: 39.69 m³ (1,401.6 cft)
Cement: 320.3 bags
Sand: 16.67 m³ (588.7 cft)
Aggregate: 33.34 m³ (1,177.4 cft)
Steel Reinforcement
Bottom Mesh — X (along Length): 61 × 12 mm
Bottom Mesh — Y (along Width): 81 × 12 mm
Top Mesh — X (along Length): 46 × 12 mm
Top Mesh — Y (along Width): 61 × 12 mm
Total steel weight: 2,255.2 kg
Edge Shuttering
Perimeter: 42.00 m
Contact area: 14.70 m² (158.2 sqft)
| Mesh Layer | Diameter | Spacing | Bar Count | Cutting Length | Total Length | Weight |
|---|---|---|---|---|---|---|
| Bottom Mesh — X (along Length) | 12 mm | 150 mm c/c | 61 | 11.900 m | 725.90 m | 645.24 kg |
| Bottom Mesh — Y (along Width) | 12 mm | 150 mm c/c | 81 | 8.900 m | 720.90 m | 640.80 kg |
| Top Mesh — X (along Length) | 12 mm | 200 mm c/c | 46 | 11.900 m | 547.40 m | 486.58 kg |
| Top Mesh — Y (along Width) | 12 mm | 200 mm c/c | 61 | 8.900 m | 542.90 m | 482.58 kg |
Assumptions Used
Steel weight: d² ÷ 162 (kg/m) | Mesh bar count: ROUND UP((span − 2 × cover) ÷ spacing) + 1 | Concrete dry volume factor: 1.54 | Shuttering (if included) is edge-only, since the raft bears on PCC/blinding below
Looking for the verification checklist, reference tables, tips, or common mistakes?See the complete Raft / Mat Foundation Calculator.
Raft foundation for a typical residential building
This page is pre-filled with a 12 m × 9 m footprint and 350 mm thickness — a scale typical of a small-to-medium residential building on moderate soil, where a raft may be chosen over isolated footings due to soil conditions or closely spaced columns.
Residential rafts are often on the lighter/thinner end of the typical range compared to commercial or high-rise construction — still, always confirm thickness and reinforcement against your project's own geotechnical report and structural drawing rather than this page's starting example.
- Residential raft thickness commonly falls in the 300-450 mm range.
- Bottom mesh is typically uniform across a residential raft; top mesh near columns depends on the specific structural design.
- Edit the footprint dimensions above to match your actual building plan.
How Is the Raft Foundation Quantity Calculated?
The calculation happens in three parts — concrete volume, two-way mesh steel, and shuttering area — then an optional cost estimate on top.
Step 1 — Concrete Volume
Wet Volume = Length × Width × Thickness
Dry Volume = Wet Volume × 1.54
Cement Bags = Cement Volume ÷ 0.0347 m³/bag (50 kg bags)
Dry volume accounts for the voids between aggregate particles that disappear once the concrete is mixed and compacted. The dry volume is split into cement, sand, and aggregate using the selected mix ratio, and wastage is applied once to the final quantities.
Step 2 — Two-Way Mesh Steel
X-direction bar length = Length − (2 × Cover)
X-direction bar count = ROUND UP((Width − 2×Cover) ÷ Spacing) + 1
Y-direction bar length = Width − (2 × Cover)
Y-direction bar count = ROUND UP((Length − 2×Cover) ÷ Spacing) + 1
Unit Weight (kg/m) = Diameter² ÷ 162
Bars running along the length are spaced out and counted across the width, and vice versa — the same convention used on this site's Slab Steel calculator, applied once for the bottom mesh and again for the optional top mesh.
Step 3 — Edge Shuttering (Optional)
Perimeter = 2 × (Length + Width)
Contact Area = Perimeter × Thickness
Only the vertical edge strip around the raft's perimeter needs formwork — the underside rests on PCC/blinding and the top is finished open, the same logic used for isolated/strip footings.
Worked Example
This example walks through your current inputs above, using the same steps as the Formula section.
Input Values Used
| Input | Value | Why it is used |
|---|---|---|
| Raft length × width | 12m × 9m | Sets the footprint area and every mesh bar's run/count direction |
| Thickness | 350 mm | Sets concrete volume and the edge shuttering height |
| Mix ratio / wastage | 1:1.5:3, 5% wastage | Converts wet volume to cement bags and adds a buffer for site losses |
| Cover | 50 mm | Reduces every bar's straight length from the raw span |
| Bottom mesh (X / Y) | 12 mm @ 150 mm / 12 mm @ 150 mm | Resists upward soil bearing pressure on the raft underside |
| Top mesh (X / Y) | 12 mm @ 200 mm / 12 mm @ 200 mm | Resists hogging moments near column locations |
Step 1 — Concrete
| Calculation | Substitution | Result |
|---|---|---|
| Wet volume | 12.00 × 9.00 × 0.350 | 37.800 m³ |
| With 5% wastage | 37.800 × 1.05 | 39.690 m³ (320.3 bags) |
Step 2 — Steel
| Mesh Layer | Substitution | Weight |
|---|---|---|
| Bottom Mesh — X (along Length) | 61 × 11.900 m × 12²÷162 | 645.24 kg |
| Bottom Mesh — Y (along Width) | 81 × 8.900 m × 12²÷162 | 640.80 kg |
| Top Mesh — X (along Length) | 46 × 11.900 m × 12²÷162 | 486.58 kg |
| Top Mesh — Y (along Width) | 61 × 8.900 m × 12²÷162 | 482.58 kg |
| Total steel | Sum of all mesh layers | 2,255.20 kg |
Therefore, this 12.0 m × 9.0 m raft needs approximately 39.69 m³ of concrete and 2,255.2 kg of mesh steel, plus 14.7 m² of edge shuttering.
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Disclaimer: This calculator provides approximate results for planning and estimation purposes only. Actual requirements may vary based on site conditions, materials, workmanship, and local building regulations. Always consult a qualified engineer, architect, or construction professional before making final decisions.