TryBuildCalcHardcore Sub-Base Calculator Hardcore/crushed aggregate sub-base estimator
Calculate compacted hardcore sub-base fill quantity for a floor base.
🕒 Last updated: July 30, 2026
Plinth Footprint
ℹ️Internal footprint length to be filled, from the foundation/plinth drawing.
ℹ️Internal footprint width to be filled.
ℹ️Set to more than 1 if several identical rooms/blocks share this same footprint and fill depth.
Fill Layer
ℹ️Height from formation level to the underside of the PCC bed; commonly 150-450 mm for residential plinths.
ℹ️Extra loose volume needed over the compacted volume — pre-filled per material, adjust if your supplier's data differs.
PCC Bed
ℹ️Commonly 50-100 mm for a residential floor base.
DPM (Damp-Proof Membrane)
Cost
For a 300.00 m² (3,229 sqft) plinth footprint filled 300 mm deep with hardcore / crushed aggregate, you need approximately 215.46 tonnes of fill material and 23.625 m³ (834.3 cft) of PCC bed concrete.
Compacted Fill
Material: Hardcore / Crushed Aggregate
Compacted volume: 90.000 m³ (3,178.3 cft)
Loose volume (before wastage): 108.000 m³ (3,814.0 cft)
Loose volume (with wastage): 113.400 m³ (4,004.7 cft)
Material to order: 215.46 tonnes
PCC Bed (1:4:8)
Thickness: 75 mm
Total concrete: 23.625 m³ (834.31 cft)
Cement: 80.7 bags
Sand: 11.195 m³ (395.3 cft)
Aggregate: 22.389 m³ (790.7 cft)
DPM (Damp-Proof Membrane)
DPM not included in this estimate.
Assumptions Used
Fill: loose volume = compacted volume × (1 + compaction%), then wastage on top | PCC: dry volume factor 1.54, wastage applied to cement/sand/aggregate together | DPM: rolls = ceil(area × (1 + wastage%) ÷ (roll area × (1 − overlap%))) | Flooring/screed/tiling above the DPM is outside this calculator's scope — see Limitations.
Approximate results for planning only. Verify with a professional.
Unsure where the DPM should sit relative to the PCC bed? Plinth Filling Guide →
Plinth Filling Cross-Section
Looking for the verification checklist, reference tables, or tips and mistakes?See the complete Plinth Filling Calculator.
Hardcore sub-base estimate
This page is set up for hardcore/crushed aggregate sub-base — the UK-convention equivalent of a granular fill, denser than soil at 1900 kg/m³ and needing a higher 20% compaction allowance because angular crushed particles leave more voids between them than a natural soil fill. The default 20 m x 15 m footprint (300 m²) at a 300 mm compacted depth gives 90 m³ of compacted hardcore, needing 113.4 m³ of loose material once compaction and wastage are applied — about 215.5 tonnes.
Because hardcore sub-base is typically laid thicker than a soil/murram plinth fill (this example uses 300 mm versus the 200 mm general default), it's commonly placed in multiple compacted lifts of roughly 100-150 mm each rather than as one thick layer, even though the total material quantity calculated here already accounts for the full depth.
- Default footprint: 20 m x 15 m (300 m²), 300 mm depth.
- Hardcore density: 1900 kg/m³, 20% compaction allowance (higher than soil's 15%).
- 215.5 tonnes of loose hardcore to order.
How Is Plinth Filling Calculated?
The calculation happens in three parts — compacted fill, PCC bed, and DPM — then an optional cost estimate on top.
Step 1 — Footprint Area
Area = Length × Width
Total Area = Area × Number of Identical Sections
If sections differ in size: Total Area = Sum of (Length × Width × Count) for every section
The footprint is the internal area to be filled, taken from the foundation/plinth drawing. Number of Identical Sections multiplies the whole result for repeated rooms/blocks of the same size; for rooms of different sizes, switch on "Multiple Different-Sized Sections" and the calculator sums every row's area into one combined total automatically.
Step 2 — Compacted Fill
Compacted Volume = Total Area × Fill Depth
Loose Volume = Compacted Volume × (1 + Compaction %)
Loose Volume (with wastage) = Loose Volume × (1 + Wastage %)
Order Quantity (tonnes) = Loose Volume (with wastage) × Material Density ÷ 1000
Delivered fill material is loose and has more air voids than it will once compacted in place, so the compaction percentage inflates the compacted (finished) volume into the loose volume you actually need to order; wastage is then applied on top of that loose volume, and the resulting figure converts directly to a tonnage using the material's density — the same convention this site's Backfill and Aggregate Coverage calculators use. For "Custom Fill Material", the density you enter is used directly in this step instead of a preset figure.
Step 3 — PCC Bed
Wet Volume = Total Footprint Area × PCC Thickness
Dry Volume = Wet Volume × 1.54
Cement / Sand / Aggregate = Dry Volume split by mix ratio parts
Wastage % applied to cement bags, sand, and aggregate together
This uses the same footprint area as Step 1 (so it stays correct whether you're using a single footprint or several differently-sized sections) and the same PCC formula as this site's dedicated PCC Calculator — a thin, lean-mix concrete bed that gives a firm, level surface for the DPM and floor slab above, not a structural element in its own right.
Step 4 — DPM (Damp-Proof Membrane)
Final Area = Total Area × (1 + Wastage %)
Roll Coverage = Roll Length × Roll Width × (1 − Overlap %)
Rolls Needed = ROUND UP(Final Area ÷ Roll Coverage)
Wastage accounts for the edge upturn and offcuts, while the overlap percentage accounts for the minimum lap required at every joint between adjoining sheets so ground moisture can't find a path through.
Step 5 — Add Contingency (Optional)
Base Cost = Fill Cost + PCC Cost + DPM Cost
Contingency Amount = Base Cost × (Contingency % ÷ 100)
Total Cost = Base Cost + Contingency Amount
Contingency is an optional buffer on top of the combined base cost to cover unexpected variance — supplier rate changes, delivery charges, or material substitution. It defaults to 0%; a commonly used range is 5-15%.
Worked Example
This example walks through your current inputs above, using the same steps as the Formula section. Each table shows the calculation, the values substituted in, and the result it produces.
Input Values Used
| Input | Value | Why it is used |
|---|---|---|
| Plinth footprint | 20 m × 15 m, 1 section(s) | Sets the area every layer's volume is built on |
| Fill material / depth | Hardcore / Crushed Aggregate (MOT Type 1 / GSB), 300 mm | Sets fill density and compacted volume |
| Compaction / wastage | 20% compaction, 5% wastage | Converts compacted volume into the loose quantity to order |
| PCC bed | 75 mm, 1:4:8, 5% wastage | Sets PCC wet volume and cement/sand/aggregate split |
Step 1 — Footprint Area
The footprint area is the base every layer above is built on — fill volume, PCC volume, and DPM area all scale directly from it.
| Calculation | Substitution | Result |
|---|---|---|
| Area | 20 m × 15 m | 300.000 m² |
| Total area | 300.00 × 1 | 300.000 m² (3,229.17 sqft) |
Step 2 — Compacted Fill
The compacted volume is what the finished, settled fill will occupy. Compaction inflates that into the loose volume, wastage inflates it once more into the loose volume actually ordered, and that final volume converts directly into a tonnage using the material's density — the same figure the cost step below uses.
| Calculation | Substitution | Result |
|---|---|---|
| Compacted volume | 300.00 m² × 300mm | 90.000 m³ |
| Loose volume | 90.000 m³ × 1.20 | 108.000 m³ |
| Loose volume (with wastage) | 108.000 m³ × 1.05 | 113.400 m³ (4,004.7 cft) |
| Order quantity | 113.400 m³ × 1900 kg/m³ ÷ 1000 | 215.46 tonnes |
Step 3 — PCC Bed
The PCC bed volume comes from the same total footprint area as Step 1, not the fill's compacted volume — it's a separate, thin layer cast on top of the finished fill.
| Calculation | Substitution | Result |
|---|---|---|
| Wet volume | 300.00 m² × 75mm | 22.500 m³ |
| With 5% wastage | 22.500 × 1.05 | 23.625 m³ (834.3 cft, 80.7 bags) |
Step 5 — Cost & Contingency
| Calculation | Substitution | Result |
|---|---|---|
| Enable cost estimation above to see fill, PCC, and DPM cost | — | |
Therefore, this 300.0 m² plinth needs approximately 215.46 tonnes of fill material, 23.625 m³ of PCC bed concrete.
This page keeps things focused on the calculation above. For the full construction guide — verification checklist, reference tables, usage steps, tips, common mistakes, and limitations.See the complete Plinth Filling Calculator.