Which calculators do concreters need for slabs, falls and bases?
Published · By Sai Nath
A concreting job needs five sums: the set-out diagonal, the dig and base in tonnes, the concrete volume plus about 10%, the bags from the maker's yield, and the falls. A 3 × 2.4 m shed slab, 100 mm thick, with a 300 mm wide edge beam 200 mm below it, is 1.296 m³: order about 1.43 m³.
Which calculations does a concreting job need?
Volume is only one of them. Before the truck comes you set out the forms square, dig, order and compact a base, and set the falls and levels the finished surface will drain on. Each is a short sum, and each has its own calculator.
| Job | The sum | In MeisterCalc | Free on this site |
|---|---|---|---|
| Set out the forms square | diagonal = √(length² + width²) | Check Square (free) | Square-up calculator |
| Excavation | length × width × depth, then × density for tonnes | Soil | — |
| Base course | length × width × compacted depth × density | Gravel | — |
| Slab, strip footing, edge beam | length × width × depth, + 10% to order | Concrete ▸ Slab (free) | Concrete calculator |
| Piers and post holes | π × radius² × depth × holes, + 10% | Concrete ▸ Holes (free) | Concrete calculator |
| Falls | drop = run ÷ N for a fall of 1 in N | Pitch | Slope calculator |
| Levels | reduced level = benchmark + backsight − foresight | Dumpy Level | — |
| Asphalt driveway | length × width × thickness × density | Asphalt | — |
The rest of this page works one shed slab from set-out to order, then piers and a patio fall. The detailed guides are linked as they come up, so this page doesn't repeat them.
How much concrete does a shed slab need, and how much do I order?
Work the slab and any thickened edge as separate boxes, add them, and order about 10% more. The example: a 3000 × 2400 mm slab, 100 mm thick, with an edge beam 300 mm wide that goes 200 mm below the underside of the slab all round. These sizes are an example; yours come from the drawings.
- Set out: a square 3000 × 2400 mm form measures √(3000² + 2400²) = 3841.9 mm on both diagonals. The method is in how to set out a building or slab square.
- Slab: 3.0 × 2.4 × 0.1 = 0.720 m³.
- Edge beam: measure it along its centreline, 150 mm in from each edge, so 2 × (2700 + 2100) = 9600 mm. Then 9.6 × 0.3 × 0.2 = 0.576 m³. The centreline rule for corners is in strip footing concrete.
- Total: 0.720 + 0.576 = 1.296 m³. With 10%: 0.792 + 0.634 = 1.426 m³ to order (1.86 yd³).
Bags or a truck? Boral's data sheet says 108 of its 20 kg Concrete Mix bags make 1 m³, so 1.4256 × 108 = 153.96, or 154 bags: about 3.1 tonnes to lift and mix. Boral also says on its website that many agitator trucks carry around 6 to 8 m³ and that minimum loads vary by plant, so ring the plant and ask what a short load costs. The allowance itself is covered in how much extra concrete to order.
How many tonnes of base, and how much soil comes out?
Multiply the area by the compacted depth for cubic metres, then by the supplier's density for tonnes. Say the base runs 300 mm past the slab on every side, so 3600 × 3000 mm, 100 mm thick after compacting, and steps down under the edge beam at the same thickness.
- Volume: 3.6 × 3.0 × 0.1 = 1.080 m³.
- Tonnes at a compacted density of 2,000 kg/m³: 1.080 × 2,000 ÷ 1,000 = 2.160 t. With 10%, order 2.376 t. The 2,000 is an example figure; ask your supplier for the compacted tonnes per m³ of the product they quote.
- The dig: 150 mm over the same area leaves the slab top 50 mm proud: 3.6 × 3.0 × 0.15 = 1.620 m³. The beam trench goes 200 mm deeper along its 9600 mm centreline, 300 mm wide: 9.6 × 0.3 × 0.2 = 0.576 m³. So 2.196 m³ comes out of the ground; at the Soil calculator's starting 1,200 kg/m³ the first part alone weighs 1.944 t.
Dug soil takes up more room loose than it did in the ground, so ask the skip or truck hire how they measure a load. In the app, use Gravel for the base (type your supplier's density over the starting 1,500 kg/m³) and Soil for the dig. More in how much road base you need and cubic metres to tonnes. Asphalt works the same way at a starting 2,400 kg/m³: see how much asphalt for a driveway.
How much concrete do piers or post holes take?
Each hole is a cylinder: π × radius² × depth, times the number of holes, plus 10%. Six piers for a carport, 450 mm across and 900 mm deep (example sizes from the drawings):
- Radius 0.225 m, so one pier is π × 0.225² × 0.9 = 0.1431 m³.
- Six piers: 0.8588 m³. With 10%: 0.9447 m³ to order (1.24 yd³).
- Bags, from each maker's sheet: 0.9447 × 108 = 102.03, so 103 bags of Boral Concrete Mix 20 kg; 0.9447 ÷ 0.016 = 59.04, so 60 bags of Dricon HandiCrete 40 kg; or, in the US, 33.36 ft³ ÷ 0.60 = 55.6, so 56 bags of 80 lb QUIKRETE Concrete Mix.
In the app: Concrete ▸ Holes, Diameter 450 mm, Depth 900 mm, Number of Holes 6 shows Per Hole 0.1431 m³, Total 0.8588 m³ and Order (+10%) 0.9447 m³. It doesn't subtract posts or steel, and hand-dug holes usually come out bigger than the sum. Concrete for post holes covers belled bottoms and whether to subtract the post.
How much fall to a drain across a patio, and how do I set it?
For a fall of 1 in N, the drop is the run ÷ N. A patio running 4500 mm from the house to a strip drain at its outer edge, at 1 in 60, drops 4500 ÷ 60 = 75 mm (0.2 in per foot). In England, Approved Document H recommends at least 1 in 60 for paved areas.
In Australia, check the first metre too. Next to a slab-on-ground house, NCC Housing Provisions 3.3.3 asks for not less than 25 mm over the first 1 m for concrete or clay paving in low rainfall intensity areas, and 50 mm in any other case. A straight 1 in 60 gives only 16.7 mm there. Take 50 mm over the first metre and 1 in 60 over the other 3500 mm: 50 + 3500 ÷ 60 = 50 + 58.3 = 108.3 mm in all.
Set it with a dumpy level. Hold the staff on the finished level at the door edge, taken as RL 100.000 m on an assumed datum, and read 1.420 m: the height of instrument is 101.420 m. One metre out the level is 99.950 m, so the staff should read 101.420 − 99.950 = 1.470 m. At the drain it is 100.000 − 0.108 = 99.892 m, a staff reading of 1.528 m. In the app's Dumpy Level, Benchmark RL 100.000, Staff Reading at BM 1.420 and Staff Reading at Measure 1.528 show Height of Instrument (HI) 101.420 m and Measured RL 99.892 m. The set-up is in how to use a dumpy level, and the figures by run are in how much fall a patio needs.
Which mistakes cost concreters the most?
- Ordering the exact volume. A base 20 mm low across the shed slab adds 3.0 × 2.4 × 0.02 = 0.144 m³, more than the 0.130 m³ that 10% adds to 1.296 m³. Check the depth in several spots before you order.
- Counting bags with a rule of thumb. The shed's 1.426 m³ is 154 bags of Boral Concrete Mix 20 kg (108 per m³) but about 170 of Cement Australia's standard 20 kg Concrete Mix, which works out at about 119 per m³ from the coverage on its product sheet. That's 16 bags, 320 kg, apart. Use the yield on your bag: bags of concrete per m³ lists them.
- A loose density on a compacted depth. Gravel starts at 1,500 kg/m³. For the shed base that gives 1.620 t, against 2.160 t at the supplier's compacted 2,000 in the example: 0.54 t short. Use the compacted depth with the compacted density.
- Millimetres as metres. 100 mm is 0.1 m. Typed as 0.01 m, the shed slab comes out at 0.072 m³ instead of 0.720 m³, a tenfold error. Entering millimetres, as the app asks, avoids it.
- Reading 1 in 60 as 60 mm per metre. 1 in 60 is 16.7 mm per metre: 75 mm over the 4.5 m patio, not 270 mm.
Which codes set slab sizes, falls and levels?
Not the calculator. Slab thickness, reinforcement, concrete strength, footing sizes and finished levels come from the drawings and the engineer, under these documents:
- Australia: NCC Housing Provisions Part 4.2, footings, slabs and associated elements, and Part 3.3, drainage.
- New Zealand: the Building Code through building.govt.nz, including E1 Surface Water.
- England: Approved Document A (structure) and Approved Document H (drainage).
Your council, certifier or building control decides what applies to a job. MeisterCalc does the arithmetic; it doesn't make work compliant.
Which concreting calculators are free in MeisterCalc?
A concreter uses Concrete (Slab and Holes), Check Square, Gravel, Soil, Pitch, Dumpy Level and Asphalt. MeisterCalc has 16 calculators. Concrete, Stairs and Check Square are free; the other 13 unlock with a weekly subscription (3-day free trial) or a one-time lifetime unlock, and Google Play shows the price in your currency. Calculations run on the phone, so they work without signal, on Android 7.0 or later.