Example data table
| Use case | Shape | Inputs | Waste | Estimated total volume |
|---|---|---|---|---|
| Garden shed pad | Rectangle | 2.4 m × 2.4 m, thickness 0.10 m | 7% | 0.6168 m³ |
| Round planter base | Circle | Ø 1.5 m, thickness 0.08 m | 5% | 0.1485 m³ |
| Corner stepping slab | Triangle | Base 1.8 m, height 1.2 m, thickness 0.10 m | 8% | 0.1166 m³ |
Examples are illustrative. Real projects vary by formwork and subgrade condition.
Formula used
- Rectangle area: A = L × W
- Circle area: A = π × (D/2)²
- Triangle area: A = ½ × B × H
- Net volume: Vnet = A × T
- Total volume with waste: Vtotal = Vnet × (1 + Waste/100)
- Approx mass: Mass = Vtotal × Density
Conversions: 1 m³ ≈ 35.3147 ft³ and 1 m³ ≈ 1.3080 yd³.
How to use this calculator
- Select a slab shape that matches your garden project.
- Choose the unit you used to measure plan dimensions.
- Enter plan dimensions and slab thickness carefully.
- Set a waste allowance for edges and spillage.
- Optionally add density and cost for planning.
- Press Calculate to see results above the form.
- Download CSV or PDF to share with your supplier.
Typical garden slab applications
Concrete slabs support sheds, grills, benches, compost stations, and paver bases. A stable slab spreads loads, reduces settling, and keeps surfaces level after rainfall. For paths and patio corners, slabs also provide a clean edge for mulch and turf. Measure the finished footprint, not the excavation, then use the calculator to translate plan dimensions into ordering quantities.
Choosing thickness and reinforcement
Thickness depends on use and soil. Light foot traffic often uses 75–100 mm, while heavier garden equipment may need 125–150 mm. Slabs on fill, near tree roots, or spanning soft spots benefit from reinforcement. Add welded mesh, rebar, or fiber, and include control joints to manage shrinkage. Enter thickness carefully because volume scales linearly with every millimeter.
Waste allowance and formwork effects
Waste covers spillage, uneven excavation, and edge thickening at forms. Straight timber forms waste less than curved shapes, and hand mixing typically wastes more than ready mix. A practical allowance is 5–10% for tidy small pours and 10–15% for complex borders. If you plan a thickened perimeter beam, increase waste or model it as an added strip volume.
Unit conversions and bag planning
The calculator outputs cubic meters, cubic yards, cubic feet, and liters so you can match supplier quotes. It also estimates mass using density, commonly near 2400 kg/m³ for normal concrete. Bag counts are mass based, so verify the yield printed on your bag and adjust. For small projects, compare the calculated volume to bag yield to avoid underbuying during placement.
Cost checks and pour readiness
Use the optional rate field to compare delivery versus bag mixing, then add extras such as pumping, short loads, and finishing materials. Before ordering, confirm subgrade compaction, drainage slope, and whether a vapor barrier is needed under covered slabs. Mark forms square, check diagonals, and preplan access routes. Plan curing time and keep the slab moist to limit cracking and dusting. Record results, then download the CSV or PDF for quick contractor communication and approvals.
FAQs
1) Should I order net volume or the total volume?
Order the total volume, because it includes a waste allowance for spillage and uneven base conditions. Net volume is the theoretical slab volume without any practical losses.
2) Which thickness is common for garden walkways?
Many walkways use 75–100 mm, depending on soil and load. Increase thickness for wheelbarrows, small equipment, or weak subgrade. Always compact the base and control water runoff.
3) Why do bag counts differ from supplier yield charts?
This tool estimates bags from mass using density. Bag labels often state volume yield per bag, which varies by mix design and water content. Compare both methods and add a small buffer.
4) What waste percentage should I choose?
Use 5–10% for simple, well-formed pours. Use 10–15% for curved edges, thickened perimeters, or hand mixing. If your forms are irregular, increase waste to stay safe.
5) Can I use feet for plan size and centimeters for thickness?
Yes. Pick a plan unit for length-related fields and a separate thickness unit for thickness. The calculator converts everything to meters internally, then reports results in multiple output units.
6) Does the cost estimate include labor and finishing?
No. The cost estimate multiplies your entered rate by the calculated volume. Add labor, formwork, reinforcement, delivery fees, finishing tools, and curing materials separately for a complete budget.