Concrete Calculator

How to work out how much concrete you need

Every concrete estimate starts with one thing: volume. Once you know the volume of the space you are filling, everything else — bags, ready-mix, weight and cost — follows from it. The three building-block formulas are:

• Rectangular slabs, walls and footings: Volume = Length × Width × Thickness
• Round columns, holes and Sonotubes: Volume = π × (Diameter ÷ 2)² × Height
• Ring / tube shapes: Volume = π × ((Outer ÷ 2)² − (Inner ÷ 2)²) × Height

Keep every measurement in the same unit before multiplying. Because 1 cubic yard equals 27 cubic feet and 1 cubic metre is about 1.31 cubic yards, the calculator converts automatically so you can read the answer in whichever unit your supplier uses.

Turning volume into bags

Pre-mix bags are sold by their dry weight, and each size makes a known amount of finished concrete. To find the bag count, divide your total volume by the yield of one bag and round up. Typical yields are:

Bag size	Approx. yield	Bags per yd³ / m³
40 lb	0.30 ft³	~90 per yd³
50 lb	0.375 ft³	~72 per yd³
60 lb	0.45 ft³	~60 per yd³
80 lb	0.60 ft³	~45 per yd³
20 kg	~0.0094 m³	~106 per m³
25 kg	~0.0117 m³	~85 per m³
30 kg	~0.0141 m³	~71 per m³

Once you are past about twenty bags it is usually cheaper to buy a full pallet rather than loose bags. Counts vary by brand and region, so confirm with your supplier, but the most common US pallet quantities are 42 bags of 80 lb, 56 of 60 lb, 64 of 50 lb and 80 of 40 lb. Those are chosen to keep the pallet near the same weight — roughly 3,200 to 3,400 lb — which means almost any full pallet makes about the same amount of concrete: 0.9 cubic yards, or a little under 25 cubic feet. A useful rule of thumb is that one pallet is slightly less than one cubic yard, so a pour needing 2 yd³ takes about two and a quarter pallets. Check the weight against your vehicle too — a full pallet is well beyond what a half-ton pickup should carry in one trip.

Worked example

Say you are pouring a 10 ft × 10 ft slab, 4 inches thick. In feet that is 10 × 10 × 0.333 = 33.3 cubic feet, which is about 1.23 cubic yards. At 0.6 ft³ per bag that is roughly 56 bags of 80 lb mix. Adding a 10% waste allowance brings it to about 1.36 cubic yards — the point at which ordering ready-mix usually makes more sense than mixing bags by hand.

Estimating tips

• Always round up and add a small safety margin — being short mid-pour is far worse than a little left over.
• Break odd shapes into rectangles and circles, calculate each, then add them together using the Quantity field or separate runs.
• For a set of steps, estimate the flat landing as a slab and the steps themselves with the Stairs option.
• Compact and grade the sub-base well; a dip of even half an inch across a big slab adds real volume.

Estimates only — not professional engineering advice. Site conditions, mix design and over-excavation all affect the real quantity.

What strength of concrete do you need?

Concrete is graded by its compressive strength — how much crushing load it can take — measured in PSI (pounds per square inch) or MPa (1 MPa is about 145 PSI). Choosing the right grade matters as much as getting the amount right:

Rule of thumb: a thicker slab at a lower grade often carries more bending load than a thin slab at a higher grade — for flatwork, depth usually beats PSI. When unsure, ask your supplier for the right mix for the job.

Concrete and its carbon footprint

Concrete is the most widely used building material on Earth, and its binder — cement — carries a real climate cost: producing cement is responsible for roughly 8% of global CO₂ emissions. Cement is only a small part of a mix by volume, yet it accounts for the large majority of concrete's embodied carbon, because making the clinker inside it is so energy-intensive.

The encouraging news is that lower-carbon mixes are now widely available. Replacing part of the cement with supplementary cementitious materials — fly ash (a coal-ash by-product), ground blast-furnace slag or silica fume — can cut a mix's carbon footprint substantially while keeping strength and durability comparable, and newer geopolymer binders go further still. If your project allows it, asking your supplier for a blended, lower-carbon mix is an easy way to build greener at no cost to performance. Costing a larger project around the pour? The home renovation cost calculator budgets the whole job.

Concrete also slowly re-absorbs some of that CO₂ from the air over its lifetime, in a natural process called carbonation.

How to use it & key terms

Pick a shape block, enter its dimensions and choose your units, then press its Calculate button for the volume, number of bags and ready-mix you need.

Planning the pour: preparation, timing and curing

Knowing how much concrete to order is only half the job; the pour itself decides whether a slab lasts for decades or cracks within a season. Good results start below the surface. The ground under a slab should be compacted and level, with a sub-base of gravel that drains water away and gives the concrete a stable bed. Formwork — the timber or metal edging that holds the wet mix in shape — has to be braced firmly, because fresh concrete is heavy and will bow a weak frame. Where a slab will carry real load, reinforcing mesh or steel bars are set on small chairs so they sit inside the pour rather than on the bottom, where they do little good.

Timing matters more than most people expect. Concrete begins to set from the moment water meets cement, so everything should be ready before the mix arrives: tools laid out, helpers briefed, and a plan for moving concrete to the far corners. Mild, still weather is ideal. In hot sun the surface can stiffen too quickly and crust over before it is finished; in near-freezing cold the reaction slows and the set can be spoiled. When you pour a large area, keep working continuously rather than letting one section harden before the next is placed, which avoids a weak cold joint where two pours meet. Once the surface is levelled and floated, resist overworking it, since drawing too much water to the top weakens the finish.

The step most often skipped is curing. Concrete does not simply dry out; it hardens through a slow reaction that needs moisture to continue. Keeping a new slab damp — by covering it or misting it — for the first several days lets it reach far more of its potential strength than a slab left to dry in the wind. Concrete gains the bulk of its strength in the first month but keeps hardening gradually well beyond that. Protecting the fresh surface from rain, foot traffic and heavy loads while it cures is part of the same patience. Building a little patience into the schedule, and adding a small waste allowance when you order so a short delivery never halts the pour, is what turns a good estimate into a slab you never have to think about again.