Building and garden · Volumes
How much concrete for a rectangular slab?
The math starts from a rectangular box: volume in m³, then a waste margin. Mass uses 2,400 kg/m³, a typical normal-weight concrete density, not every mix.
Calculate
How the calculator works
You type three dimensions in meters. Geometric volume is L × W × T. A margin (10% default) multiplies that volume to match a real order: leftover in the hopper, pumping, small formwork irregularities. Displayed mass applies 2,400 kg/m³ to the already increased volume. The 25 kg bag equivalent divides that mass by 25: those are bags of bagged concrete mix, not bags of cement for a mixer recipe. US 80-lb bags are a different size; convert from the kg mass rather than treating 25 kg as 80 lb.
Formula and method
V = L × l × e
Geometric volume in m³ if L, W, and T are in meters.
Vcommande = V × (1 + p / 100)
p is the waste percent (0 to 30).
m = Vcommande × 2 400
Mass in kg. 2,400 kg/m³ is an order of magnitude for normal-weight concrete (common aggregates), not an EN 206 requirement for every mix.
nsacs = m / 25
Equivalent number of 25 kg bags of concrete (the mix), not cement alone.
Geometry is a rectangular box. L-shapes, slopes, and isolated footings break into several rectangles, then you add. EN 206 frames specification (strength classes, consistency, exposure); it does not fix “2,400 kg/m³” as a mandatory density. Lightweight, heavy, fiber, or self-consolidating mixes depart from that value. For a ready-mix truck, the cubic meter ordered is fresh concrete agreed with the producer, not a cement volume. US ready-mix is often quoted in cubic yards: 1 m³ ≈ 1.307 yd³.
Worked example
Patio slab 4 m × 2.5 m, 10 cm (about 4 in) thick
L = 4 m, W = 2.5 m, T = 0.10 m, waste = 10%.
- Geometric volume: 4 × 2.5 × 0.10 = 1.00 m³.
- Order volume: 1.00 × 1.10 = 1.10 m³ (about 1.44 yd³).
- Estimated mass: 1.10 × 2,400 = 2,640 kg.
- 25 kg bag equivalent: 2,640 / 25 = 105.6 bags (plan 106 bags, or a truck if the volume warrants it).
1.10 m³ to order, about 2,640 kg at 2,400 kg/m³.
Input notes
- Length
- Inside length of the form, not the overall boards. Tape the subgrade.
- Width
- Width of the rectangle. An L-plan splits into two rectangles; add volumes afterward.
- Thickness
- Target thickness after screeding. 12 cm is 0.12 m. Thickness above 2 m is refused: a thicker wall or mat is another problem.
- Waste allowance
- 10% fits a simple slab. A long pump, dense rebar, or uneven ground can justify more. Perfectly formed work can go toward 5%.
Assumptions and limits
Assumptions
- Volume is a rectangular box (flat faces, right angles).
- The margin applies uniformly to volume, with no rebar takeoff.
- Density 2,400 kg/m³ represents common normal-weight concrete, not lightweight or heavy concrete.
- 25 kg bags are a mass equivalent of the mix, not a cement dosage (kg/m³).
Limits
- No strength class (C25/30, 4,000 psi, etc.) is chosen: this is not structural design.
- Slump, entrained air, and shrinkage are not modeled.
- The producer’s delivered volume may be measured differently (ticket, actual batch density).
- Footings, piers, and odd shapes must be split by hand into elementary volumes.
How to read the result
The figure to take to the supplier is volume with waste. Below about 0.5 m³, bags or a small mixer are often simpler than a truck. Beyond that, compare delivered cubic meters (or yards) with bag cost: 105 bags of 25 kg weigh as much as 1.1 m³ at 2,400 kg/m³, but mixing, water, and place time change the job entirely. Check density and mix type on the order ticket.
Common mistakes
Typing thickness in centimeters (10 instead of 0.10) or in inches without converting.
All dimensions are meters. 10 cm = 0.10 m; 4 inches ≈ 0.102 m.
Confusing cement bags with concrete bags.
A 25 kg cement bag is not 25 kg of concrete. A typical cement content (for example 350 kg of cement per m³) is another calculation, not done here.
Using 2,400 kg/m³ for lightweight aggregate or heavy concrete.
EN 206 distinguishes lightweight, normal-weight, and heavy concretes. Ask for the density of the mix actually delivered.
Forgetting that waste also covers makeup and pumping.
Geometric volume alone often understates the order. Leave a margin, then round to the producer’s delivery increment.
Comparison
| Quantity | Value here | Where to check |
|---|---|---|
| Geometry | Box L × W × T | Formwork plan |
| Waste | 10% default (adjustable) | Jobsite practice, not a physical constant |
| Density | 2,400 kg/m³ (typical normal-weight) | Mix sheet / EN 206 density classes |
| Strength class | Not calculated | Engineer or project specification |
Related calculations
- Gravel calculatorFor a base course, a path, or drainage: volume comes from the box, mass applies 1,600 kg/m³. That is an order of magnitude; delivered crushed stone can be heavier or lighter.
- Flooring calculatorNet area is the room rectangle. A cutting margin is added (10% default, 25% at most) to match an order in boxes, planks, or tiles.
- Mulch calculatorA bed is not formwork: mulch settles in a few weeks. The math gives spread volume, a small margin, then the equivalent in 50 L bags.
Frequently asked questions
Does 2,400 kg/m³ come from EN 206 as a single requirement?
No. It is a typical normal-weight value; the standard classes several concrete families.
EN 206 specifies concrete (composition, performance, conformity). It recognizes lightweight, normal-weight, and heavy concretes, with different density ranges. 2,400 kg/m³ is a common order of magnitude for ordinary siliceous or calcareous aggregates. Lightweight aggregate, heavy concrete, or a highly aerated mix should not use this figure. The producer states the delivered product’s density.
Must I always add 10%?
No. 10% is a reasonable default for a simple slab, not a universal jobsite rule.
A well-formed rectangular slab may need only 5%. Pumping, uneven ground, makeup, or tight rebar push toward 10% or more. The calculator caps waste at 30%. Then round to the multiple the supplier accepts (often 0.5 m³, or 0.5 yd³ in some US plants).
How do I handle a shape that is not a rectangle?
Split it into rectangles (or prisms), compute each volume, then add before applying waste.
An L-plan, a bent walk, or a pool shell decomposes. For a disk, area πr² times thickness gives volume, but this tool stays on the box. Slopes (a mortar bed under tile) are treated as a prism: average thickness × area.
Do 25 kg bags replace a truck?
Only for small volumes. The bag equivalent is a mass, not a mixing schedule.
1.10 m³ at 2,400 kg/m³ weighs 2,640 kg, or 105.6 bags of 25 kg. Mixing more than a hundred bags by hand is generally not realistic. The display helps compare a small job (pier, curb, grout) with bulk. Mix water, open time, and consistency class stay those of the bag sheet. US 60-lb or 80-lb bags need a separate count from the kg mass.
Author and update
Written by Rédaction HexaCalc (editorial team). Content last updated: August 25, 2026. No third-party medical or financial review is claimed.
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Category: Building and garden