Concrete Calculator: Slabs, Footings, Columns, Steps, Bags and Cost

How to use this tool

Pick the shape you are pouring, type its dimensions in the boxes that appear, and the answer is in the dark panel straight away: the volume in cubic yards, with cubic metres, cubic feet, the bag count, the weight and the waste allowance underneath it. The quick buttons under the dimensions load the sizes people ask about most, a 10x10 slab at 4 inches, a 12 inch sonotube, a 16 by 8 footing, so you can start from something close and edit.

The six cards below the panel break the same pour down: cubic yards, cubic metres, bags at the size you chose, weight in tons, the cost of ready mix at your price per yard, and the cost in bags at your price per bag. Set how many identical pours there are, for instance ten deck footings, and every figure scales. Set the waste percentage to what your base and formwork deserve; 10 percent is the default and the section on waste below explains when to use more.

Below the cards the tool tells you how it worked the volume out, whether the pour is a bag job or a truck job, and how to batch it on site at the mix ratio you pick, with a table of bags at every bag size and a table of cement, sand and gravel. Switch to metric with the button beside the shape list and everything moves to metres, centimetres, cubic metres and kilogram bags.

Every export carries the pour you entered and nothing else. Copy result grabs the headline, Copy link gives an address that reopens the calculator on exactly this shape and size, and the CSV, image and print buttons produce a quantity sheet you can hand to a supplier or keep with the job. The Project bar at the top saves named pours in this browser so an estimate with several pours can be built up and restored.

How the volume is worked out for each shape

Every shape on this page reduces to cubic feet, and cubic feet divided by 27 is cubic yards. For a slab or a pad that is simply length times width times thickness, with the thickness converted from inches to feet first. A 10 by 10 foot slab at 4 inches is 100 square feet times 0.333 feet, 33.3 cubic feet, 1.23 cubic yards. A strip footing is the same calculation with the width and depth in inches and the length in feet, so a 16 by 8 inch footing running 50 feet is 1.33 times 0.667 times 50, 44.4 cubic feet or 1.65 cubic yards.

A round column or sonotube is the area of the circle times the height: pi times the radius squared times the height, with the diameter in inches converted to feet. A 12 inch tube is 1 foot across, so its area is pi times 0.5 squared, 0.785 square feet, and 4 feet of it holds 3.14 cubic feet. A square column is width times breadth times height. A round pad is the same circle calculation with the diameter in feet and the thickness in inches.

Solid steps are treated as a stack of blocks. Step one is a block one rise tall and one run deep across the full width; step two behind it is two rises tall and one run deep; the top step is as tall as all the rises together and as deep as the platform. Adding the blocks gives the poured volume, which is exactly what you get with formwork on a compacted base. Three steps 4 feet wide at a 7 inch rise and 11 inch run with an 11 inch top come to 12.8 cubic feet. If you pour steps against an existing slab or fill part of the volume with rubble the real figure is smaller, so treat the result as the upper bound and subtract the fill.

A curb is its cross section times its length: the curb height times its width, plus the gutter width times the gutter thickness if there is one, times the run in feet. Fifty feet of 6 by 6 inch curb with an 18 inch gutter 6 inches thick is 50 cubic feet, 1.85 cubic yards, which is why curb and gutter is nearly always a truck pour.

Bags or a truck

An 80 lb bag of premixed concrete yields 0.60 cubic feet, so a cubic yard is 45 bags, 3,600 lb of dry mix to carry, lift into a mixer and pour before it goes off. Sixty pound bags yield 0.45 cubic feet and it takes 60 of them; 50 lb bags yield 0.375 and take 72; 40 lb bags yield 0.30 and take 90. The bag table on the tool shows all four sizes for your pour, with the total weight and the bill, because the right size depends as much on who is lifting as on the price per bag.

Ready mix is priced per cubic yard delivered, and the plant sets a minimum load. Below that minimum, typically 4 or 5 yards, a short load fee is added, and it can double the price of a single yard. That fee is what makes the decision. For anything under about half a yard, a few bags and a wheelbarrow are cheaper and simpler. Above about two yards, 90 bags or more, the truck wins on cost and on the quality of the pour, because everything arrives at the same consistency and goes in before the first part sets. In between, the calculator compares the two bills at your prices and points out the fee.

There are two things a bag count hides. The first is water: a yard of bagged mix weighs 3,600 lb in the store and about 4,050 lb once it is placed, and you will need roughly 35 to 45 gallons of clean water to get it there. The second is time. One person with a mixer places about a bag every three minutes, so 62 bags is three hours of steady work, and a slab needs to be poured and finished within about 90 minutes of the first batch in warm weather. Two mixers and four people change that arithmetic; one person and a bucket do not.

Waste, subgrade and why 10 percent is the usual allowance

Nobody pours the volume on the drawing. Forms bow, the gravel base is never perfectly flat, a footing trench is dug an inch deeper than planned along half its length, and some concrete stays in the chute and the barrow. The allowance that covers all of this on a well prepared slab is 5 to 10 percent, and 10 percent is the safe default because running out is far more expensive than a part bag left over: a cold joint in the middle of a slab is a crack waiting to happen, and a second truck for half a yard costs the full short load fee.

Use more than 10 percent when the base is rough or unlevelled, when the pour goes straight into a trench cut in soil rather than into forms, and when the ground is soft enough that the first concrete disappears into it. Footings dug by hand or by a mini excavator routinely take 15 to 20 percent more than the drawing says. Use less, 5 percent, on a formed slab over a screeded gravel base where you have measured the actual depth at several points and used the average as the thickness.

Measure thickness, do not assume it. The difference between a nominal 4 inch slab and one that averages 4.5 inches over a slightly low base is 12.5 percent more concrete, more than the whole waste allowance. Push a rule down to the base at a dozen points across the form and put the average in the thickness box.

Mixing on site: cement, sand and gravel

When the job is too small for a truck and you would rather buy cement, sand and gravel than premixed bags, the volumes are set by a nominal mix ratio. The ratios are by volume, cement first: 1:2:4 is the classic general purpose mix, 1:2:3 is a stronger mix that suits driveways, garage floors and anything with vehicles on it, 1:1.5:3 is stronger again for structural work and roughly matches an M20 grade, and 1:3:5 and 1:3:6 are lean mixes for blinding, backfill and mass fill under footings.

The catch is that dry loose materials shrink when they are mixed, because the sand fills the voids in the gravel and the cement paste fills the voids in the sand. The finished volume is about 65 percent of the loose dry volume, so you need 1.54 times the wet volume in dry materials. The calculator applies that factor, splits the dry volume by the ratio, and turns the cement into bags: a 94 lb US bag is one cubic foot, a 50 kg bag is 1.226 cubic feet and a 40 kg bag 0.981.

For the default slab, 36.7 cubic feet with waste, the dry volume is 56.5 cubic feet. At 1:2:4 that is 8.1 cubic feet of cement, nine 94 lb bags, 16.1 cubic feet of sand and 32.3 cubic feet of gravel. Sand and gravel are sold by the cubic yard or the tonne, so the table gives both, using a loose bulk density of 1.6 tonnes per cubic metre, about 100 lb per cubic foot, for both. Order the aggregates with a margin too; a yard of sand delivered loose is rarely a full yard once it has been raked flat.

Water is not in the table because it depends on how wet the sand arrives, but the starting point for a 1:2:4 mix is about half the weight of the cement, 47 lb or 5.6 gallons per 94 lb bag, then less if the sand is wet. More water makes the mix easier to place and weaker in equal measure; a mix that slumps to a puddle has lost a third of its strength.

A worked check you can repeat on paper

Start with the default pour, a 10 by 10 foot slab 4 inches thick. Four inches is 4 ÷ 12 = 0.3333 feet. Volume is 10 × 10 × 0.3333 = 33.33 cubic feet. Divide by 27 and you have 1.235 cubic yards. Add 10 percent for waste: 33.33 × 1.10 = 36.67 cubic feet, 1.358 cubic yards, which rounds to the 1.36 the tool shows. At 150 lb per cubic foot that is 5,500 lb of concrete, 2.75 tons, and at 0.0283 cubic metres per cubic foot it is 1.038 cubic metres.

Bags: 36.67 ÷ 0.60 = 61.1, so 62 bags of 80 lb, always rounding up because a part bag is a whole bag at the till. In 60 lb bags it is 36.67 ÷ 0.45 = 81.5, so 82. Cost at $160 per yard and $7 per bag: 1.358 × 160 = $217 for ready mix before any short load fee, 62 × 7 = $434 in bags. That is the borderline case the tool describes, where the fee decides.

Site mix at 1:2:4: 36.67 × 1.54 = 56.47 cubic feet of dry material. One seventh is cement, 8.07 cubic feet, nine 94 lb bags; two sevenths is sand, 16.13 cubic feet, 0.60 cubic yards; four sevenths is gravel, 32.27 cubic feet, 1.20 cubic yards. Change the thickness to 6 inches and every figure grows by half: 2.04 cubic yards and 92 bags, which is the quickest way to see why thickness deserves a tape measure rather than a guess.

Mistakes that catch people out

Multiplying feet by inches. A 10 by 10 slab at 4 inches is not 400 of anything. Convert the thickness to feet first, or use the tool, which does it for you and shows the conversion in the how it was worked out line.

Forgetting to divide by 27. A cubic yard is 3 feet on each side, 27 cubic feet, not 9. Ordering 33 yards for a 33 cubic foot slab has happened.

Ordering the drawing volume with no waste. Running out in the middle of a slab leaves a cold joint that will crack, and a second delivery of half a yard costs the full short load fee. Add the allowance, then round up to the plant's quarter yard increment.

Using the outside diameter of a sonotube. Tubes are sold by inside diameter, and that is the concrete size. Using the outside adds 5 to 10 percent you will not pour.

Counting bags by weight instead of yield. An 80 lb bag makes 0.60 cubic feet, not 80 of anything, and a 60 lb bag makes 0.45, so 45 of the big bags or 60 of the small ones make a yard. Bags of mortar, sand mix and fast setting mix have different yields again; check the bag.

Treating steps as a wedge. Formed steps are solid blocks, not a ramp, and the top step is a platform. The block model here gives the formed volume; if you plan to fill the core with rubble, subtract the fill rather than starting from a triangle.

Guessing the thickness. A slab that averages half an inch thicker than nominal takes 12 percent more concrete than the calculation, which is more than the waste allowance. Measure down to the base at several points and use the average.

Popular concrete calculations

The pours people look up most. Each opens the calculator on that shape and size with its own address, so you can bookmark or share the exact answer.

Slabs at 4 inches thick

Patios, walkways, shed and hot tub bases. A 4 inch slab takes 1.23 cubic yards per 100 square feet before waste.

Slabs at 5 and 6 inches thick

Driveways, garage floors and pads that will carry vehicles. Six inches is half again as much concrete as four.

Round pads

Sonotubes and round piers

Deck footings, fence and pergola posts, pier foundations. Sizes are the inside diameter of the tube; set the count to the number of holes.

Strip footings

Width and depth in inches, length in feet. Footings dug in soil usually need 15 to 20 percent waste rather than 10.

Square columns and piers

Concrete steps

Solid formed steps at a 7 inch rise and 11 inch run. Open one and change the rise, run and platform to match your drawing.

How many bags of concrete in a cubic yard

Bag counts at all four sizes for a given number of yards, with the weight to carry.

How many bags of concrete for a number of cubic feet

Metric slabs in metres and centimetres

Cubic metres and 25 kg bags. Change the bag size on the tool for 20, 30 or 40 kg bags.

How this was checked

The volume engine was tested against hand calculations for every shape. The step volume is computed two ways, once by adding the blocks one at a time and once with the closed formula width × rise × (run × n(n − 1) ÷ 2 + n × platform), and the two agree to the last digit across every combination of 1 to 12 steps, rises from 5 to 8 inches, runs from 9 to 14 inches and widths from 2 to 12 feet. Bag counts always round up, with a small tolerance so that exactly 27 cubic feet gives exactly 45 bags of 80 lb rather than 46.

The bag yields are the manufacturer figures printed on standard premixed concrete, 0.30, 0.375, 0.45 and 0.60 cubic feet for 40, 50, 60 and 80 lb bags, and 0.0094, 0.0117, 0.0141 and 0.0187 cubic metres for 20, 25, 30 and 40 kg bags. The site mix figures were compared with the standard quantity tables for nominal mixes: for one cubic metre at 1:2:4 the engine gives 317 kg of cement, 0.44 m³ of sand and 0.88 m³ of gravel, which matches the published 6.3 bags of 50 kg for that mix. The conversion between cubic yards and cubic metres uses the exact foot, 0.3048 m, so one cubic yard is 0.764555 cubic metres.

The page was then opened at every linked address in the hubs above and each one was checked for a distinct title, a correct headline and clean CSV, image and print output containing only that pour. The same checks were run on a phone width screen.

Common questions

How many bags of concrete do I need for a 10x10 slab?

A 10 by 10 foot slab poured 4 inches thick is 33.3 cubic feet, or 1.23 cubic yards. Without any allowance that is 56 bags of 80 lb or 75 bags of 60 lb. With the usual 10 percent added for waste and an uneven base it becomes 36.7 cubic feet, 1.36 cubic yards, which is 62 bags of 80 lb or 82 bags of 60 lb. At 6 inches thick the same slab needs 2.04 cubic yards, or 92 bags of 80 lb with waste.

How many 80 lb bags of concrete make a cubic yard?

45. An 80 lb bag yields 0.60 cubic feet and a cubic yard is 27 cubic feet, so 27 ÷ 0.60 = 45 exactly. A 60 lb bag yields 0.45 cubic feet, so 60 bags make a yard; a 50 lb bag yields 0.375, so 72 bags; a 40 lb bag yields 0.30, so 90 bags. Whichever size you pick, a yard in bags weighs about 3,600 lb before water and around 4,050 lb once it has cured.

Is it cheaper to buy bags or order a ready mix truck?

Bags cost roughly two to three times as much per yard as ready mix, but a truck comes with a minimum charge and a short load fee for small orders, so below about half a yard bags always win and above two yards the truck nearly always wins. In between it depends on the fee, on whether you have a mixer and helpers, and on whether the pour must be finished in one go. The calculator shows both costs for every pour and says which side of the line it falls on.

How thick should a concrete slab be?

Four inches is the standard for patios, walkways, shed bases and most residential floors. Driveways that carry cars are usually poured 4 to 5 inches; anything that will take trucks, an RV or a lift is better at 6 inches. Footings and structural slabs follow the drawings and the local code rather than a rule of thumb, and thickness matters more than most people expect: going from 4 to 6 inches adds half again as much concrete.

How much concrete does a sonotube hold?

Per foot of height, an 8 inch tube holds 0.35 cubic feet, a 10 inch tube 0.55, a 12 inch tube 0.79, a 16 inch tube 1.40 and a 24 inch tube 3.14. So a 12 inch sonotube 4 feet tall takes 3.14 cubic feet, which is six 80 lb bags with a little left over, and ten of them for a deck is 1.16 cubic yards before waste. Sonotubes are sized by inside diameter, so use the size printed on the tube.

What is a 1:2:4 mix and how much cement does a cubic yard take?

One part cement to two parts sand to four parts gravel by volume, the classic general purpose mix that lands around 2,200 to 3,000 psi depending on the water. Because the loose dry materials shrink when they are mixed, you need about 1.54 times the finished volume in dry ingredients. For one cubic yard that is 41.6 cubic feet of dry material: 5.9 cubic feet of cement, six 94 lb bags, plus 0.44 cubic yards of sand and 0.88 cubic yards of gravel. Per cubic metre it works out at about 317 kg of cement, 6.3 bags of 50 kg, with 0.44 m³ of sand and 0.88 m³ of gravel.

Related Calculators and Reference

• Slab on Grade Takeoff Calculator: the full slab package, concrete plus gravel base, vapour barrier, mesh or rebar and formwork, for a quote rather than a single pour.
• Rebar Weight and Spacing Calculator: the steel that goes in before the concrete, by bar size and spacing, with weight for the order.
• Isolated Footing Sizing Calculator: work out how big the footing has to be for the load and the soil before you work out its concrete.
• Cubic Yard Calculator: the same volume arithmetic for any material, gravel, soil, mulch or sand.
• Gravel Calculator: the compacted base that goes under the slab, by depth and area.
• Concrete Pour Record Generator: log the delivery tickets, slump and cylinders once the truck arrives.
• Brick Calculator: bricks and mortar for a wall, with openings taken off.
• National Ready Mixed Concrete Association: industry guidance on ordering, placing and curing ready mixed concrete.