Why use this calculator
Concrete is the one material on a residential site that cannot be topped up later. Once a pour has started, a short load means a cold joint, which is a permanent line of weakness that will crack and will be visible for the life of the slab.
A second truck ordered mid pour carries a short load fee and usually arrives after the first has begun to stiffen. Over ordering is wasteful but survivable, which is why every experienced concreter orders long. This calculator gives the theoretical volume and the realistic order quantity side by side, converts between the cubic yards used by American suppliers and the cubic metres used everywhere else, counts bags for small pours, and reports the placed weight so you know whether a trailer or a barrow run is even feasible.
How to use it
- Measure the length and width of the pour in feet, taking the dimensions of the finished slab rather than the formwork outside it, and enter the longer side as the length.
- Measure the depth in inches at several points across the excavation and enter the average rather than the nominal figure, because a subgrade that dips by half an inch across a large slab adds real volume.
- Set the waste allowance to suit the job: ten per cent is the trade default, five is enough on a formed slab over a compacted base, and fifteen is sensible over a rough or soft subgrade where the concrete will find every low spot.
- Read the order line rather than the theoretical volume, then round up to the quarter yard or tenth of a metre your supplier sells in.
What each result means
- Cubic yards needed
- The theoretical volume of the excavation. It is the number to check against a quote, not the number to order.
- Order allowing for waste
- The volume to actually order, including your waste allowance. Round it up to the increment your supplier sells in.
- Cubic metres
- The same volume in the unit used by Australian, British and European ready mix suppliers.
- Cubic feet
- The raw volume before conversion, useful for cross checking bag counts and small formwork calculations.
- 60 lb bags needed
- How many 60 lb premix bags the pour would take. Above about forty bags, ready mix is cheaper and far faster.
- 80 lb bags needed
- The same figure for 80 lb bags, which are the common size in North American hardware stores.
- 20 kg bags needed (metric)
- Bag count for the 20 kg premix sold in Australia, New Zealand and the United Kingdom.
- Slab area
- The plan area of the pour, used for ordering mesh, membrane, sand bedding and finishing labour.
- Approximate placed weight
- What the concrete weighs once placed. It decides whether a barrow run, a trailer or a pump is realistic.
The formula
Volume in cubic feet
V = L × W × (D ÷ 12)
Cubic yards are V ÷ 27 because a cubic yard is three feet cubed. Cubic metres are V × 0.0283168. The order quantity is the cubic yard figure multiplied by one plus the waste allowance divided by one hundred. Bag counts divide the cubic feet by the published yield of one bag: 0.45 for 60 lb, 0.60 for 80 lb. Placed weight assumes 2.4 tonnes per cubic metre.
How this number is used in practice
Thickness is the variable that decides whether a slab survives. Four inches, around one hundred millimetres, is the accepted minimum for footpaths, patios and shed floors carrying foot traffic only. A domestic driveway carrying a car wants five to six inches over a properly compacted base, and anything that will see a delivery truck or a caravan needs six inches with reinforcement designed for the load.
Doubling the thickness doubles the concrete cost but roughly quadruples the load the slab can carry without cracking, which is why concreters resist thinning a slab to save a few hundred dollars on a driveway that will be there for forty years.
The bag versus ready mix crossover sits at roughly a third of a cubic yard, which is about twenty 80 lb bags. Below that, bags win because there is no delivery fee and no minimum load charge. Above it, the labour becomes the problem: mixing forty bags by hand is several hours of heavy work, and if the first batch has set before the last is placed you have built the cold joint you were trying to avoid.
Ready mix suppliers usually charge a short load fee below a minimum, often around one cubic metre or one cubic yard, so a small pour can cost the same as a larger one.
Weight catches people out more often than volume. A single cubic metre of concrete weighs about 2.4 tonnes, which is well beyond the rating of a typical box trailer and beyond what most domestic driveways are designed to take from a loaded truck. If a pour is more than a metre or so from where a truck can park, price a pump or a motorised barrow before the day, because a two tonne pour moved by hand in the ninety minute working window is a serious undertaking.
Worked examples
20 ft by 10 ft patio slab at 4 inches, 10% waste
2.47 cubic yards theoretical, order 2.72 cubic yards, 1.89 cubic metres, 112 bags of 80 lb, about 4.53 tonnes placed.
30 ft by 12 ft driveway at 6 inches, 10% waste
6.67 cubic yards theoretical, order 7.33 cubic yards, 5.10 cubic metres and roughly 12.23 tonnes placed.
10 ft by 3 ft path at 4 inches, 5% waste
0.37 cubic yards, which is 17 bags of 80 lb premix and well below any ready mix minimum.
When people use it
- Ordering ready mix for a driveway, shed floor or patio without risking a short load
- Deciding whether a small pour is a bag job or a delivery job
- Checking a concreter's quoted volume against the dimensions on the plan
- Estimating the placed weight before committing to barrowing rather than pumping
- Converting a slab specified in cubic metres into the cubic yards a US supplier quotes
Tips
- Order the waste inclusive figure, never the theoretical volume. Running short mid pour creates a cold joint that cannot be repaired, only disguised.
- Measure depth at several points and average it. A subgrade that dips half an inch across a 200 sq ft slab adds nearly a tenth of a cubic yard.
- Above about twenty 80 lb bags, ready mix is cheaper once you count your own labour and the risk of the first batch setting early.
- Ask the supplier about short load fees before ordering. Many charge a minimum of around one cubic metre regardless of what you take.
- Compact and dampen the subgrade before the pour. Dry ground pulls water out of the mix and weakens the bottom of the slab.
- Have somewhere planned for the surplus, a spare form or a post hole, so the last quarter yard is not a panic decision.
- Concrete gains most of its strength in the first week but keeps curing for a month. Keep it damp for at least seven days in warm weather.
What it assumes, and where it stops being accurate
- The excavation is a rectangular prism of uniform depth. Thickened edges, footings, steps and rebates are extra volume you must add separately.
- Bag yields are the manufacturer's published figures for standard premix. Wetter mixes, high slump mixes and fibre mixes yield slightly differently.
- Placed weight assumes normal weight concrete at about 2.4 tonnes per cubic metre. Lightweight and aerated mixes are substantially lighter.
- It is a volume tool, not a structural design tool. It does not size reinforcement, calculate load capacity or set control joint spacing.
- It does not account for the concrete lost in the truck, the chute or the barrow, which is part of what the waste allowance covers.
- Formwork, membrane, mesh, sand bedding and finishing are not priced. Only the concrete volume is calculated.
Terms used on this page
- Cold joint
- The weak plane formed when fresh concrete is placed against concrete that has already begun to set.
- Subgrade
- The compacted ground surface a slab is poured onto. Its accuracy determines how much extra concrete a pour absorbs.
- Ready mix
- Concrete batched at a plant and delivered by truck, sold by the cubic yard or cubic metre.
- Slump
- A measure of how wet and workable a concrete mix is. Higher slump is easier to place and generally weaker.
- Short load fee
- A surcharge applied when a delivery is below the supplier's minimum volume.
Questions people ask
- How thick should a concrete slab be?
- Four inches, about one hundred millimetres, is the accepted minimum for paths, patios and shed floors carrying foot traffic. A domestic driveway wants five to six inches over a compacted base, and anything carrying a delivery truck or a caravan needs six inches with reinforcement designed for the load. Local building codes set minimums for structural slabs, so check before pouring.
- How much extra concrete should I order?
- Ten per cent is the trade default and covers spillage, an uneven subgrade and concrete lost in the chute and barrow. Drop to five per cent on a formed slab over a well compacted base where you can measure the depth accurately. Increase to fifteen over a rough or soft subgrade, where the mix will find every low spot you did not measure.
- Is it cheaper to use bags or order ready mix?
- Bags win below roughly a third of a cubic yard, which is about twenty 80 lb bags, because there is no delivery or minimum load charge. Above that, ready mix is cheaper once you count your own labour, and it avoids the cold joints that form when the first hand mixed batch sets before the last is placed.
- How many 20 kg bags make a cubic metre?
- About one hundred, because a 20 kg bag of standard premix yields roughly ten litres of set concrete and a cubic metre is a thousand litres. That is two tonnes of bags to move and mix, which is why a cubic metre is almost always a ready mix job rather than a bag job in any country.
- What does a cubic yard of concrete weigh?
- About 4,050 pounds, a little over two short tons, which is 2.4 tonnes per cubic metre for normal weight concrete. That weight decides your logistics: it is beyond the rating of most box trailers, and moving it by barrow inside the ninety minute working window is heavy work for at least two people.
- Why does my concreter's quoted volume differ from this?
- Usually because they have included thickened edges, footings or a step that this calculator treats as a plain rectangular prism, or because they have allowed a larger waste margin for a subgrade they have seen and you have not. Ask them to break the volume into slab, edge beams and waste so the two figures can be compared directly.
Use this with
Concrete Volume Calculator answers one part of the question. These pick up where it stops: Concrete Slab Calculator (cubic metres and bags), Deck Materials Calculator, Mulch, Soil and Gravel Calculator, Area Calculator and Fence Post and Rail Calculator.
Where these figures come from
Figures and rules on this page follow United States federal rules.
- American Concrete Institute. Slab thickness, curing and mix design standards
- Cement Concrete & Aggregates Australia. Australian residential slab and pavement guidance
- Portland Cement Association. Concrete density, yield and placement reference material
Concrete Volume Calculator produces an estimate for general information. It is not financial, tax, medical, legal or engineering advice. read the full disclaimer. Spotted a wrong number? Report it and it gets fixed. The form picks up this calculator and the inputs you used.