Calculators › Groundworker
Free groundworker calculator
Concrete calculator: volume, ready-mix loads, bags and mix designation
Work out the volume of concrete for a shed base, slab, strip footing or fence post holes, see what to ask the plant for (GEN1, GEN3, RC28/35, PAV1) and get it as ready-mix wagons, as bags of cement and ballast for a site mix, or as pre-mixed bags. A C20-type site mix is 12 bags of cement and 1.75 tonnes of ballast per cubic metre; 7 bags is a blinding mix, not a slab.
What you need
Concrete
- Concrete to order
- 1.98 m³4 × 3m × 150mm: 1.80m³ plus 10% over-orderCemex: allow a small contingency
- Ask for
- GEN3 (C16/20) or ST4 External slabs, garage floors, oversite (unreinforced)Cemex BS 8500 designated mixes
- Weight
- 4.8 tonnes2.4 tonnes per m³
Ready-mix
- Wagon loads
- 0.33 × 6m³ loadswagons carry 6 to 8m³; mini-mix 1 to 4m³; part loads carry a surchargeconcretecalculate truck loads
Reinforcement
- A142 mesh
- 2 × 4.8m × 2.4m sheets12.0m² plus 10% for 300mm laps
- Mesh weight
- 29.3 kg2.22kg per m²
- Mesh chairs or spacers
- 48 pcs4 per m² for 40 to 50mm cover, mesh in the middle of the slab not on the ground
Shuttering
- Edge shuttering
- 15.4 mperimeter plus 10%
- Boards 150 × 25mm
- 5 × 3.6m boards
- Pegs
- 24 pcsevery 600mm
- Area
- 12.0 m²for the DPM and Type 1 under a slab, see the hardcore calculator
Round up to whole packs and bags. Figures include the wastage you chose and match TradeCalc in the app.
For groundworkers: turn this into a quote
In the app, TradeCalc hands these quantities straight to ProQuote with your regional material prices, and the customer gets a branded PDF. Free to download.
Or claim your free profile link nowHow the maths works
- Volume is length × width × thickness for a slab, length × trench width × concrete depth for a footing, or holes × π × (diameter ÷ 2)² × depth for posts. 10% is added because a sub-base is never level and running short mid-pour is worse than a bit left over (Cemex says allow a contingency).
- The designation is what you ask the plant for (Cemex BS 8500 designated mixes): GEN0 (C6/8) blinding; GEN1 (C8/10) or ST2 for trench fill, strip footings and kerb backing; GEN1 to GEN2 for domestic foundations and oversite; GEN3 (C16/20) or ST4 for external slabs and garage floors; RC28/35 for designed reinforced slabs; PAV1 for drives, which needs air entrainment against frost.
- A ready-mix wagon carries 6 to 8m³ and a mini-mix 1 to 4m³. Under about 1m³ is a part load with a surcharge (concretecalculate), so bags or a site mix usually win. Concrete weighs about 2.4 tonnes per m³.
- A site mix for a C20-type 1:2:4 is 12 × 25kg bags of cement and 1.75 tonnes of all-in ballast per m³ (tradecalculator, concretemath, diydata). A stronger 1:4 all-in (C25-type) is 17 bags and 2 tonnes; a lean 1:8 GEN1-type for blinding and haunching is 8 bags and 1.9 tonnes. Ballast is in bulk bags of about 800kg (Selco; BMF).
- Pre-mixed 20kg bags (postmix, multipurpose concrete) yield about 0.01m³ each, so 100 bags to the cubic metre. Fine for a few post holes, silly for a slab.
- Mesh sheets are 4.8 × 2.4m (11.52m²) with 10% added for 300mm laps: A142 is 2.22kg per m², A193 3.02, A252 3.95. Sit it on chairs for 40 to 50mm cover. Loose rebar is a placeholder 100kg per m³ with tie wire at 1.5%; the engineer's drawing decides.
- Shuttering is the slab perimeter plus 10% in 3.6m lengths of 150 × 25mm board, pegged every 600mm.
Worked example
A 4m × 3m slab 150mm thick is 1.8m³ of concrete, or 1.98m³ with the 10% over-order: ask for GEN3 or ST4, a third of a 6m³ wagon. Mixing it yourself at 1:2:4 is 24 bags of cement and 3.46 tonnes of ballast (5 bulk bags). Add 2 sheets of A142 mesh, 48 chairs, 5 shuttering boards and 24 pegs.
Questions people ask
How many bags of cement do I need per cubic metre of concrete?
12 to 14 × 25kg bags of cement and about 1.75 to 2 tonnes of all-in ballast per m³ for a C20-type 1:2:4 or 1:5 mix, which is what a slab, base or footing wants. 7 bags per m³ is a lean blinding mix (about 1:10), not a slab: it is roughly half what you need.
What concrete do I ask for?
Tell the plant the job and they will give you a BS 8500 designated mix: GEN1 or ST2 for footings and trench fill, GEN3 or ST4 for a slab, garage floor or shed base, RC28/35 if the drawings show a designed reinforced slab, PAV1 for a driveway. GEN mixes are not designed for reinforced structural elements, so anything with an engineer's drawing follows the drawing.
Should I order ready-mix or mix it myself?
Anything over about 1m³ is hard work by hand and a 150 litre mixer only does about 0.09m³ a go. Ready-mix arrives in one pour, which is better for a slab, but a part load under a cubic metre carries a surcharge. For small pours, or where a wagon cannot get near, bags and a mixer are fine, or ask about a mini-mix or a barrow mix service.
What thickness should a concrete slab be?
100mm is enough for a light shed or path on firm ground. 150mm is the usual for a garage floor, extension oversite or driveway, on 150mm of compacted Type 1 with a DPM. Footings are usually deeper; Building Control will say.
Do I need mesh in a concrete base?
For a shed base, not always. For a garage floor, extension slab or anywhere a car goes, A142 mesh helps control cracking and is cheap compared to the concrete. Sit it on chairs so it is in the middle of the slab, not on the ground, and lap sheets by 300mm.
How much does a cubic metre of concrete cost?
It varies by area, mix and how much you order, and part loads carry a surcharge. Get a couple of quotes from local plants with your volume and designation, and ask whether the price includes waiting time and a pump if the wagon cannot reach.
Related calculators
How much soil you will dig out, what it weighs, how many skips or grab loads once it bulks up, and the tonnes of MOT Type 1, blinding and DPM to fill the hole back.
Brick calculator: bricks, blocks and mortar for a wallHow many bricks, blocks, bags of cement, lime and sand you need for a wall, with the right mortar designation, wastage, openings and packs.
Block paving calculator: blocks, sand, sub-base, edging and haunchingHow many paving blocks a driveway or patio needs by block size and pattern, with the Type 1 (or Type 3 for permeable), sharp sand, kiln-dried sand, edging, haunching concrete, drainage channel and the dig-out.
Extension materials calculator: footings, bricks, blocks and roofA full materials take-off for a single or double storey extension from its footprint: concrete by designation, Type 1, bricks, blocks, mortar, ties, trays, insulation, roof deck and plasterboard.
