TryBuildCalcRaft / Mat Foundation Calculator(Concrete, Two-Way Mesh Steel & Shuttering)
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 10.00 m × 8.00 m raft (80.0 m² footprint) at 400 mm thick, you need approximately 33.60 m³ of concrete and 1,664.6 kg of reinforcement steel.
Concrete
Concrete volume: 33.60 m³ (1,186.6 cft)
Cement: 271.1 bags
Sand: 14.11 m³ (498.4 cft)
Aggregate: 28.22 m³ (996.7 cft)
Steel Reinforcement
Bottom Mesh — X (along Length): 54 × 12 mm
Bottom Mesh — Y (along Width): 67 × 12 mm
Top Mesh — X (along Length): 41 × 12 mm
Top Mesh — Y (along Width): 51 × 12 mm
Total steel weight: 1,664.6 kg
Edge Shuttering
Perimeter: 36.00 m
Contact area: 14.40 m² (155.0 sqft)
| Mesh Layer | Diameter | Spacing | Bar Count | Cutting Length | Total Length | Weight |
|---|---|---|---|---|---|---|
| Bottom Mesh — X (along Length) | 12 mm | 150 mm c/c | 54 | 9.900 m | 534.60 m | 475.20 kg |
| Bottom Mesh — Y (along Width) | 12 mm | 150 mm c/c | 67 | 7.900 m | 529.30 m | 470.49 kg |
| Top Mesh — X (along Length) | 12 mm | 200 mm c/c | 41 | 9.900 m | 405.90 m | 360.80 kg |
| Top Mesh — Y (along Width) | 12 mm | 200 mm c/c | 51 | 7.900 m | 402.90 m | 358.13 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
What Is a Raft / Mat Foundation Calculator?
A raft (or mat) foundation is one continuous reinforced concrete slab cast under the entire building footprint, rather than separate footings under each column. It's used when the soil's bearing capacity is too low for isolated footings, when column loads are heavy or closely spaced enough that individual footings would overlap, or when the whole structure needs to be tied together to resist differential settlement. This calculator estimates everything needed to cast one — concrete volume and cement bags, a two-way mesh steel schedule (bottom mesh resisting upward soil pressure, optional top mesh resisting hogging moments near columns), and edge shuttering area — from the raft's footprint, thickness, and reinforcement you enter.
It's the natural next step from this site's Concrete Footing / Footing Steel calculators — use those for isolated, strip, or circular footings under individual columns; use this one when the design calls for one continuous slab foundation instead.
What makes this calculator different:
A raft is reinforced with a genuine two-way mesh at both faces, not a single main/distribution bar pair like a suspended slab — bars run in both the X and Y directions at the bottom, and optionally at the top too. This calculator models all four mesh layers independently (different diameter and spacing per layer, per direction, per face), rather than assuming one uniform bar size covers the whole raft.
Applicable standards:
- Raft thickness, mesh diameter/spacing, and whether top mesh is needed at all depend on soil bearing capacity, column loads, and span between columns — always confirm from the geotechnical report and approved structural drawing, not a generic default.
- Concrete cover and mix ratio requirements vary by applicable structural code and exposure class (e.g. IS 456, ACI 318, BS 8500/EN 1992) — confirm the exact figures for your project.
- This calculator estimates material quantity only, not structural design (bearing capacity check, punching shear near columns, or settlement analysis).
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 | 10m × 8m | Sets the footprint area and every mesh bar's run/count direction |
| Thickness | 400 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 | 10.00 × 8.00 × 0.400 | 32.000 m³ |
| With 5% wastage | 32.000 × 1.05 | 33.600 m³ (271.1 bags) |
Step 2 — Steel
| Mesh Layer | Substitution | Weight |
|---|---|---|
| Bottom Mesh — X (along Length) | 54 × 9.900 m × 12²÷162 | 475.20 kg |
| Bottom Mesh — Y (along Width) | 67 × 7.900 m × 12²÷162 | 470.49 kg |
| Top Mesh — X (along Length) | 41 × 9.900 m × 12²÷162 | 360.80 kg |
| Top Mesh — Y (along Width) | 51 × 7.900 m × 12²÷162 | 358.13 kg |
| Total steel | Sum of all mesh layers | 1,664.62 kg |
Therefore, this 10.0 m × 8.0 m raft needs approximately 33.60 m³ of concrete and 1,664.6 kg of mesh steel, plus 14.4 m² of edge shuttering.
Essential Checklist+−
Complete these critical checks before approving the work or proceeding to the next construction stage.
✓Design & Soil Confirmation+-
- Raft (rather than isolated/strip footings) is confirmed as the correct foundation type from the geotechnical report and structural drawing, not assumed.
- Raft thickness, mesh diameter, and spacing (bottom and top, both directions) are taken directly from the approved structural drawing, not estimated.
- Soil bearing capacity used in design matches the actual geotechnical investigation for this site, not an assumed generic value.
- Column loads and positions used in the raft design match the latest approved structural drawing revision.
- Punching shear at each column location has been checked by the structural engineer, since it commonly governs raft thickness rather than simple bending.
✓Excavation & Blinding+-
- Excavation is taken to the level and extent shown on the drawing, with side slopes/shoring stable for the working depth.
- Formation (excavated base) is firm, level, and free of loose soil, standing water, or soft spots before blinding is poured.
✓Reinforcement Verification+-
- Bottom mesh diameter and spacing (both X and Y directions) match the structural drawing, with bars evenly spaced and properly tied at every intersection.
- Top mesh (where specified) is included at the correct locations and extent, not omitted as a simplification.
- Cover blocks/chairs are placed to maintain the specified bottom, top, and side cover throughout the pour, not just at the initial tying stage.
- Additional/extra top reinforcement around column locations (if specified for punching shear or hogging) is placed exactly where shown, not generalized across the whole raft.
- Column starter bars/dowels projecting up from the raft are positioned accurately and securely braced before the pour, so they don't shift out of position.
✓Casting & Curing+-
- Concrete is poured in the sequence and joint locations specified by the structural engineer, given the raft's typically large single-pour volume.
- Concrete is properly compacted/vibrated throughout the raft's full thickness and around congested reinforcement near columns, without segregating the mix.
- Raft is cured (kept moist) for the applicable minimum period appropriate to its mass/thickness, which is typically longer than a standard slab given the larger pour volume and reduced surface-to-volume ratio.
✓Final Check+-
- Raft top surface level and column starter positions are verified against the drawing before column formwork begins above it.
Full QC Checklist+−
Verification checklist for raft (mat) foundations — covering soil/design confirmation, sizing, reinforcement, casting/curing, and final check. Use the Essential Checklist for critical checks; expand to Full QC Checklist for complete quality assurance.
✓Design & Soil Confirmation+-
- Raft (rather than isolated/strip footings) is confirmed as the correct foundation type from the geotechnical report and structural drawing, not assumed.
- Raft thickness, mesh diameter, and spacing (bottom and top, both directions) are taken directly from the approved structural drawing, not estimated.
- Soil bearing capacity used in design matches the actual geotechnical investigation for this site, not an assumed generic value.
- Column loads and positions used in the raft design match the latest approved structural drawing revision.
- Punching shear at each column location has been checked by the structural engineer, since it commonly governs raft thickness rather than simple bending.
- Construction joint locations (if the raft can't be poured in one continuous operation) are planned and detailed by the structural engineer in advance, not decided ad hoc on site.
✓Excavation & Blinding+-
- Excavation is taken to the level and extent shown on the drawing, with side slopes/shoring stable for the working depth.
- Formation (excavated base) is firm, level, and free of loose soil, standing water, or soft spots before blinding is poured.
- PCC/blinding layer is poured to the specified thickness and allowed to set before reinforcement is placed on it.
- Any waterproofing or damp-proof membrane specified beneath or around the raft is placed and lapped correctly before reinforcement fixing begins.
✓Reinforcement Verification+-
- Bottom mesh diameter and spacing (both X and Y directions) match the structural drawing, with bars evenly spaced and properly tied at every intersection.
- Top mesh (where specified) is included at the correct locations and extent, not omitted as a simplification.
- Cover blocks/chairs are placed to maintain the specified bottom, top, and side cover throughout the pour, not just at the initial tying stage.
- Additional/extra top reinforcement around column locations (if specified for punching shear or hogging) is placed exactly where shown, not generalized across the whole raft.
- Lap splice locations are staggered per the applicable code and structural drawing, not concentrated at the same line across the mesh.
- Column starter bars/dowels projecting up from the raft are positioned accurately and securely braced before the pour, so they don't shift out of position.
- Any service penetrations (plumbing, drainage) through the raft are coordinated with the reinforcement layout before the pour, not cut through placed bars afterward.
✓Casting & Curing+-
- Concrete is poured in the sequence and joint locations specified by the structural engineer, given the raft's typically large single-pour volume.
- Concrete is properly compacted/vibrated throughout the raft's full thickness and around congested reinforcement near columns, without segregating the mix.
- Raft is cured (kept moist) for the applicable minimum period appropriate to its mass/thickness, which is typically longer than a standard slab given the larger pour volume and reduced surface-to-volume ratio.
- Edge shuttering (where used) is not removed, and no load is placed on the raft edge, before adequate strength has developed.
- Ambient/curing temperature is monitored for mass-concrete effects (heat of hydration) on a thick raft, with a cooling/curing plan if the pour is large enough to be a concern.
✓Final Check+-
- Total concrete and steel used is reconciled against this calculator's estimate (or the project BBS) before closing out the item in records.
- Raft top surface level and column starter positions are verified against the drawing before column formwork begins above it.
- No visible honeycombing, cracking, or exposed reinforcement on the finished raft surface or edges.
- Completed raft is inspected and signed off by the site engineer, with a photographic record kept, before backfilling or covering begins.
Reference Tables
Typical raft thickness by load condition
| Building Type / Load | Commonly Seen Thickness Range |
|---|---|
| Light residential (2-3 storey) | 300-450 mm |
| Medium residential/commercial (4-8 storey) | 450-700 mm |
| Heavy load or poor soil | 700-1000+ mm, confirm with structural/geotechnical design |
Standard bar diameters and unit weight
| Diameter (mm) | Unit Weight (kg/m) |
|---|---|
| 10 mm | 0.617 |
| 12 mm | 0.889 |
| 16 mm | 1.580 |
| 20 mm | 2.469 |
| 25 mm | 3.858 |
These are commonly referenced conventions, not a universal standard — always confirm raft thickness, mesh reinforcement, and cover against your project's applicable structural code and geotechnical report before finalizing.
Usage Guide
- Use once the geotechnical report and structural drawing confirm a raft (rather than isolated footings) is the right foundation type for your soil and loads.
- Enter the raft's overall footprint dimensions and thickness exactly as shown on the structural drawing, not a rounded assumption.
- Set bottom and top mesh diameter/spacing separately for each direction if your drawing specifies different reinforcement in X vs Y.
- Cross-check the reinforcement against the structural drawing before ordering steel or pouring concrete.
- Download the checklist PDF alongside the estimate for a site-ready verification record.
Practical Raft Foundation Tips
- Confirm whether top mesh is actually required near column locations before skipping it — hogging moments under column loads are a common reason a raft needs top reinforcement even where a simple slab wouldn't.
- Cast a raft in one continuous pour wherever possible; if construction joints are unavoidable, plan their location and detailing with the structural engineer in advance.
- Keep the blinding/PCC layer under the raft level and clean before placing reinforcement — an uneven base makes it hard to maintain consistent bottom cover.
- Stagger lap splice locations across the mesh rather than lapping every bar at the same line, per your applicable code's staggering requirement.
- Reconcile the schedule's total steel weight against delivered tonnage before accepting a consignment as complete.
Common Mistakes
- Treating a raft like a simple slab and skipping top mesh entirely, missing the hogging reinforcement needed near column locations.
- Using one uniform bar size/spacing across the whole raft when the drawing specifies different reinforcement by zone or direction.
- Not confirming raft thickness and reinforcement against an actual geotechnical/structural design, defaulting to a guessed value instead.
- Forgetting cover reduces every bar's straight length — cutting bars to the full raft dimension overestimates bar length.
- Ordering a single combined steel total instead of a breakdown by diameter and direction, causing wrong proportions to be delivered.
Limitations
- Estimates material quantity for a single rectangular raft only — does not model L-shaped or multi-section raft footprints, punching shear zones near columns, or thickened column strips.
- Does not perform structural design (bearing capacity check, punching shear, settlement analysis) — raft thickness and reinforcement must come from an approved geotechnical report and structural drawing.
- Assumes uniform mesh spacing across the whole raft — many real designs use closer spacing or extra bars near column locations that this calculator does not model.
- Does not include a separate PCC/blinding layer quantity — use a concrete or PCC calculator separately for that layer if needed.
- Cost excludes labour, transport, and wastage/offcuts beyond the calculated quantities.
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.