Concrete slab calculator
Ver el procedimiento — con sus números
The figure above is your slab's three dimensions multiplied together. It does not decide how thick the slab ought to be or whether it will carry what you park on it — those belong to your drawings, your engineer or your building authority. Ring your ready-mix supplier to confirm the order before the truck rolls.
Enter the length, width and thickness of the pad and you get its volume in cubic yards and cubic metres, plus roughly how many bags it would take. Volume is simply length × width × thickness; the waste factor sits on top so an uneven base does not leave you short.
Ejemplo resuelto
A 12 ft × 12 ft shed floor, poured 4 inches thick: four inches is a third of a foot, so the volume is 12 × 12 × 0.333 = 48 cubic feet. Divide by 27 and that is 1.78 yd³. Add 10% for waste and you would order about 2 yd³. Bagging it instead, at 0.6 ft³ per 80 lb bag, is roughly ninety bags with waste — which is exactly when a truck earns its keep.
In metric: a 3.6 m × 3.6 m floor at 10 cm is 3.6 × 3.6 × 0.1 = 1.30 m³. With 10% waste that is 1.43 m³, and the order line rounds it up to 1.5 m³. In 30 kg bags at about 0.014 m³ each, 1.43 ÷ 0.014 ≈ 102 bags.
En la obra
A garage floor with a thickened edge. The floor is 24 ft × 24 ft at 4 in: 576 × 0.333 = 192 ft³ = 7.1 yd³. The perimeter is thickened to 12 in for a foot in from the edge — that is a second box, 96 ft of perimeter × 1 ft wide × the extra 8 in (0.667 ft) = 64 ft³ = 2.4 yd³. Together 9.5 yd³; with 10% that is 10.4, so the ticket says 10.5 yd³. Order the floor alone and you are 2.5 yards short with the truck already gone.
The base was not what the drawing said. The drawing calls for 4 in over a graded base. You check it with a string line and it reads 4, 4½, 5½ and 4¾ at the four corners and 5 in the middle. Pour the deepest — 5.5 in — into the depth field, not 4. On a 20 × 20 pad that is the difference between 4.9 and 6.8 yd³; the waste factor does not cover a base that is an inch and a half low. In metric the same mistake on a 6 m × 6 m pad is 100 mm against 140 mm — 3.6 m³ against 5.04 m³.
Two trucks, one pour. A 30 ft × 40 ft slab at 5 in is 500 ft³ = 18.5 yd³, 20.4 with waste. If the plant's trucks carry 10 yards, that is two full trucks and a splash — book them back-to-back and tell the dispatcher the second truck's exact number after the first is down and you can see the forms. A metric plant counts cubic metres: a 9 m × 12 m slab at 125 mm is 13.5 m³, 14.85 m³ with waste — with 8 m³ trucks, two loads, the second not quite full.
What went wrong?
Quick referencefor a slab
- A cubic yard
- 27 ft³ — about 45 bags at 0.6 ft³
- Thickness as feet
- 4 in = 0.333 · 5 in = 0.417 · 6 in = 0.5 · 8 in = 0.667
- Yards from an area, at 4 in
- ft² ÷ 81
- Yards from an area, at 5 in
- ft² ÷ 64.8
- Yards from an area, at 6 in
- ft² ÷ 54
- Bags at 0.6 ft³
- ft³ ÷ 0.6 — an 80 lb bag; a 60 lb bag is about 0.45 ft³
- Métrico
- 1 m³ = 1.31 yd³ · 100 mm ≈ 4 in · a 30 kg bag ≈ 0.014 m³
- m³ from an area
- m² × thickness in m — at 100 mm m² ÷ 10 · 125 mm ÷ 8 · 150 mm ÷ 6.67
Waste & overagewhat to add, and why
- A 10% overage is the working default on a slab: the gravel base has low spots that swallow mix, and being a wheelbarrow short at the end of a pour is the most expensive shortage there is. Type it in the waste field.
- If the slab pours with the plant's half-yard rounding already on the order line, do not round again — the tool's order figure includes it.
Mistakes to catchbefore you order
- Using the floor plan's room size instead of the pour size: the slab usually runs to the outside of the wall line, so measure the forms.
- Taking the thickness at the edge and forgetting the slab is thicker in the middle where the base dips; measure the base in three or four places and use the deepest.
- Reading the yards figure and phoning it in, when the order line — with waste and rounding — is the number for the plant.
What an inspector looks forthe building inspector, before the pour
On this part of the job
- Base: the thickness and compaction of the granular under the slab, and that it is dry and level. A rutted base is both a fail and the reason the yardage runs over.
- Anything that has to be under the slab and cannot be added later — vapour barrier, under-slab services, sleeves, insulation where the drawing calls for it — in and undamaged.
- Reinforcement height. Mesh or bar has to sit where the drawing puts it in the slab depth, on chairs; pulling it up with a hook as you pour is not accepted anywhere.
- Thickness at the edges and at any thickened beam, measured in the form rather than assumed from the middle.
- Control-joint layout against the drawing — where they go and how soon they are cut is a durability item the inspector will ask about even if it is not measured that day.
Have this ready
- The slab detail showing thickness, reinforcement and any thickened sections.
- Screed pins or a laser set so a depth check takes one minute.
- The delivery tickets, and the number you ordered, written down.
Every figure an inspector measures against comes from the code your jurisdiction has adopted, the stamped drawing, or the manufacturer's printed instructions — so none of them are printed here. Look the number up in your own book, type it into the tool, and the arithmetic is yours.
La fórmula
volume = length × width × thickness
Keep the units in step: drop thickness into feet by dividing inches by 12 before multiplying, then divide the cubic feet by 27 for cubic yards. In metric a depth in centimetres becomes metres over 100, and the answer is already cubic metres. Bags are the volume divided by what one bag yields once mixed.
What this tool assumes
Only that a cubic yard is 27 cubic feet, which is a definition rather than an assumption. Everything that actually varies from slab to slab — the thickness, the waste percentage, the yield per bag — is a field you can edit, so no hidden number is steering your order.
Preguntas
- How much concrete do I need for a slab?
- Multiply the slab length by its width by its thickness, all in the same units, then read it off in cubic yards or cubic metres. A 12 ft by 12 ft pad poured 4 inches thick is 12 × 12 × 0.333 = 48 cubic feet, which is 48 ÷ 27 ≈ 1.8 yd³ before you add any waste. In metric, a 3.6 m by 3.6 m pad at 100 mm is 3.6 × 3.6 × 0.1 ≈ 1.3 m³. Enter your own figures above and the number lands instantly.
- How thick should a concrete slab be?
- That is a design call this tool leaves to you — you pour the thickness your plans or your local authority specify, and you type it in. As a rough guide people pour a shed floor or a patio around 4 inches (100 mm), and a slab meant to take vehicles thicker again, but anything carrying a real load should follow an engineered detail rather than a rule of thumb.
- How many bags does a slab take versus a truck?
- The bag count is shown alongside the volume, using an editable yield per bag. As a feel for it, a single cubic yard is roughly forty-five 80 lb bags — a lot of mixing — and a cubic metre is roughly seventy 30 kg bags. Once a slab climbs past about ten bags, ordering ready-mix by the yard is usually cheaper, and always faster, than bagging it by hand.
- Why does the tool add a waste factor?
- Because a slab is never poured onto a perfectly flat, perfectly sized base. Sub-grade dips, forms bow out under the weight, and screeding leaves a little proud. The waste field defaults to 10% and you can push it higher on a rough hand-dug base — running a slab short halfway through a pour is a far worse problem than a little left in the barrow.
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