How this concrete base calculator works
This calculator estimates the amount of concrete needed for a rectangular slab or base. It starts with your length, width, and thickness, then adds a waste allowance so you can order with confidence. You also get quick estimates for ready-mix cost and bagged concrete needs.
It is useful for shed bases, patio slabs, workshop floors, hot tub pads, and general garden construction. If your project has unusual geometry (steps, curves, slopes, or thickened edges), split it into smaller rectangles and calculate each section separately.
Formula used
1) Raw slab volume
Volume (m³) = Length (m) × Width (m) × Thickness (m)
Since thickness is entered in millimeters, the calculator converts it first:
Thickness (m) = Thickness (mm) ÷ 1000.
2) Waste-adjusted volume
Adjusted Volume = Raw Volume × (1 + Waste % ÷ 100). A 5–15% allowance is common depending on site conditions, formwork accuracy, and spill risk.
3) Optional cost and bag estimates
- Ready-mix cost = Adjusted Volume × Price per m³
- Total liters = Adjusted Volume × 1000
- Bags needed = Total liters ÷ liters yield per bag (rounded up)
- Bagged cost = Bags needed × bag price
Typical thickness guide for concrete bases
Always follow local building codes and engineering requirements, but these ranges are common:
- Garden path / light-use slab: 75–100 mm
- Shed or small outbuilding base: 100–125 mm
- Driveway or vehicle area: 125–150 mm (or more)
- Heavy load areas: engineer-specified thickness and reinforcement
Example calculation
Suppose your base is 5 m × 3 m × 100 mm with 10% waste.
- Thickness in meters = 100 ÷ 1000 = 0.1 m
- Raw volume = 5 × 3 × 0.1 = 1.5 m³
- Adjusted volume = 1.5 × 1.10 = 1.65 m³
You should plan for about 1.65 m³ of concrete. If your local supplier sells by cubic meter, ordering close to this value (and confirming minimum order quantities) usually works well.
Practical tips before you pour
Prepare the sub-base well
Concrete performs best on a stable, compacted base (often MOT Type 1 or similar crushed aggregate). Poor compaction can lead to settlement and cracking.
Use reinforcement where appropriate
Mesh, fiber, or rebar can improve crack control and structural performance depending on loading conditions. Match reinforcement choice to project requirements.
Plan placement and finishing time
Concrete work is time-sensitive. Confirm labor, weather window, tools, and access ahead of the delivery day. For larger pours, arrange enough help so finishing is completed on time.
Frequently asked questions
Should I include footings in this calculator?
Not directly. Calculate footing trenches as separate rectangular volumes and add them to your slab total.
Is bagged concrete cheaper than ready-mix?
Small jobs can favor bags, but larger slabs are often faster and more economical with ready-mix. This page gives both estimates so you can compare quickly.
How much extra concrete should I order?
Most people use 5–15% extra. Use the higher end for uneven ground, difficult access, or first-time pours.