Concrete Calculator

A concrete calculator turns the dimensions of a pour into the volume of concrete it needs, then expresses that volume two ways: cubic yards for a ready mix order and bag counts for bagged mix. This tool handles slabs, sidewalks, strip footings, round columns, square columns, stairs and curb and gutter runs. Each result shows the volume before and after waste, the count of 80 lb, 60 lb and 40 lb bags, and a Low, Typical and High cost range for both buying routes. Concrete is sold by the cubic yard, and one yard holds 27 cubic feet. The waste setting covers spillage and uneven subgrade.

What the result numbers mean

The large figure at the top of the result is the order quantity: total volume with the waste allowance added, in cubic yards. Ready mix plants price and batch by this number. The line under it shows the same volume before waste, which is the exact geometric size of the pour.

Bag counts come from the volume with waste divided by the yield of each bag size. An 80 lb bag of standard concrete mix yields 0.60 cubic feet, a 60 lb bag yields 0.45 and a 40 lb bag yields 0.30. Counts always round up, since a partial bag still has to be bought whole.

The cost table has four rows. The ready mix row multiplies the order volume by a national rate of $140 to $190 per cubic yard and adds a flat delivery fee of $60 to $150. The three bag rows multiply each bag count by its shelf price. All four rows cover material only. Forming, placing and finishing labor sit outside this tool, and the Concrete Cost Calculator handles installed pricing. The region setting scales every cost row by ×0.85 for lower cost states, ×1.00 for the national average or ×1.25 for high cost states such as California, New York and Massachusetts. The five steps behind these figures appear on the estimate methodology page.

The formula and the math, step by step

Every shape reduces to cubic feet first. Inches convert to feet by dividing by 12, and the result converts to cubic yards by dividing by 27.

• Slab or sidewalk: length × width × (thickness ÷ 12)
• Strip footing: length × (width ÷ 12) × (depth ÷ 12)
• Round column: π × (diameter ÷ 2)² × height, with the diameter in feet, times the number of columns
• Square column: side × side × height, with the side in feet, times the number of columns
• Stairs: width × tread depth × step rise × n(n + 1) ÷ 2, where n is the number of steps
• Curb and gutter: length × [(gutter width × flag thickness) + (curb width × curb height above the gutter)] ÷ 144, with the cross section measured in inches

The stair formula treats each step as a solid block that reaches down to grade. The bottom step stands one rise tall and the top step stands n rises tall, so the stack holds 1 + 2 + … + n rise heights, and that sum equals n(n + 1) ÷ 2. A landing at the top is a separate slab.

A curb and gutter cross section splits into two rectangles: the flat gutter flag and the curb that rises above it. Their areas in square inches add together, and dividing by 144 turns square inches into square feet. With a 24 inch gutter, a 6 inch flag, a 6 inch curb width and a 6 inch curb height, each linear foot holds 1.25 cubic feet. A 100 foot run along a driveway apron or a street edge therefore needs 5.09 cubic yards once the 10 percent waste allowance is added.

The math for a 10 by 10 foot slab at 4 inches runs like this. Volume equals 10 × 10 × 4 ÷ 12, or 33.33 cubic feet. Dividing by 27 gives 1.23 cubic yards. A 10 percent waste allowance raises the volume to 36.67 cubic feet, or 1.36 cubic yards. Divided by the 80 lb yield of 0.60, that volume equals 61.11 bags, which rounds up to 62. The same volume needs 82 bags of 60 lb mix or 123 bags of 40 lb mix.

How the estimate gets used on a project

1. The process starts with the finished surface. Length and width come from the inside of the forms, since the outside edge of form boards adds nothing to the pour.
2. Thickness gets checked in at least five spots with a tape and a string line pulled tight between form tops. Small errors grow fast at the scale of a cubic yard: a slab planned at 4 inches that averages 5 inches across a dished subgrade needs 25 percent more concrete than the plan shows. The deepest reading matters more than the average on a subgrade with soft spots.
3. The shape setting matches the pour type. Sidewalks use the slab math with a narrow width, post holes poured with tube forms use the round column setting, and a poured curb along a driveway or street uses the curb and gutter setting.
4. The waste setting follows site conditions. Five percent suits a compacted, level base inside straight forms. Ten percent is the default and covers ordinary spillage and minor dips. Fifteen percent fits rough ground, hand mixing in a wheelbarrow or a first pour by an inexperienced crew.
5. The order volume then decides the buying route. At typical national prices, ready mix becomes the cheaper option once the pour passes about 0.69 cubic yards before waste, an order of about 0.76 cubic yards with the 10 percent allowance. Below that point, the flat delivery fee outweighs the savings on material. Bags also suit spots a truck cannot reach. Every 45 bags of 80 lb mix equals a full cubic yard of hand mixing and 3,600 lb of dry material.
6. The supplier call comes last. Many plants batch in quarter or half yard steps, so the order rounds up to the next step above the calculated volume.

Worked example: a 12 by 16 foot patio

A backyard patio measures 12 by 16 feet and is formed for a 4 inch thickness on compacted gravel. Waste stays at the 10 percent default and the region stays at the national average.

Output	Result
Volume before waste	64.00 cu ft (2.37 cu yd)
Volume with 10% waste	70.40 cu ft (2.61 cu yd)
80 lb bags	118
60 lb bags	157
40 lb bags	235
Ready mix with delivery	$425 to $645 (typical $530)
80 lb bagged mix	$590 to $944 (typical $767)

The patio covers 192 square feet. At 4 inches thick, that area holds 64.00 cubic feet, and the waste allowance lifts the order to 2.61 cubic yards. Ready mix lands at a typical $530 including delivery, while 118 bags of 80 lb mix run a typical $767. The gap of $237 favors the truck, and the labor gap is larger still: those bags weigh 9,440 lb in total, and each one has to be lifted, opened, mixed and placed before the first batch starts to set. A plant that batches in half yard steps fills this order as 3.0 cubic yards.

Cost drivers and conditions that move the price

Delivery is the first swing factor on small pours. The flat fee of $60 to $150 stays the same whether the truck carries one yard or six, so it adds about $100 per yard to a 1 yard order and only $20 per yard to a 5 yard order at the typical rate. Plants that charge a separate short load fee push the small order gap wider.

Bag size changes the price per cubic foot. At typical shelf prices, 80 lb bags cost $10.83 per cubic foot of concrete, 60 lb bags cost $12.22 and 40 lb bags cost $15.00. The smaller bags are easier to carry but cost 38 percent more per cubic foot than the 80 lb size.

Region moves every line. A high cost state adds 25 percent to the national figure, and a low cost state takes 15 percent off. Fuel surcharges, plant distance and local demand during spring and summer building seasons push quotes toward the High column.

Weather adds cost without adding volume. Concrete placed in air below 40°F needs blankets, heated water or an accelerating admixture, and summer heat may call for a retarder or an early morning pour. Each additive raises the per yard price on the ticket. Access matters too. A pump truck becomes necessary when the mixer cannot back within reach of its chute, and pump time bills separately from the concrete.

Planning scenarios: Basic, Standard and Premium

Scenario	Pour	Order volume	80 lb bags	Ready mix, typical	80 lb bags, typical
Basic	4 × 4 ft pad, 4 in thick	0.22 cu yd	10	$136	$65
Standard	10 × 20 ft patio, 4 in thick	2.72 cu yd	123	$548	$800
Premium	20 × 20 ft garage floor, 6 in thick	8.15 cu yd	367	$1,440	$2,390

The Basic pad suits an air conditioner base or a trash can pad. At 10 bags, bagged mix costs $65 against $136 for a truck, so bags win by a wide margin. The Standard patio flips that result: ready mix saves $252 at the typical rate and spares the crew 123 bags of hand mixing. The Premium garage floor needs 8.15 cubic yards. That volume fits in one full mixer load, and bagged mix stops being a realistic option at 367 bags and 29,360 lb of dry material.

Buying notes for bags and ready mix

• One cubic yard equals 45 bags of 80 lb mix, 60 bags of 60 lb mix or 90 bags of 40 lb mix before waste.
• A yard of 80 lb bags weighs 3,600 lb dry. That load exceeds the rated payload of most half ton pickups, so large bag orders usually need store delivery or several trips.
• Bags stay dry and off the ground on a pallet or plywood. Mix that has absorbed moisture forms hard lumps and loses strength.
• A ready mix quote should state the strength in psi, the slump, any fiber or admixtures, the short load policy and the allowed unloading time before overtime charges start.
• Truck access needs a clear path. A loaded mixer is heavy enough to crack an existing driveway or sidewalk, and plants often ask the buyer to accept that risk in writing.
• A washout spot for the chute and tools keeps leftover slurry out of storm drains, which many cities regulate.

Concrete pour quote checklist

• Total cubic yards ordered, the overage the contractor adds and who pays for a short load or a second truck
• Mix strength in psi, slump and any air entrainment for freeze and thaw exposure
• Pump truck, buggy or wheelbarrow placement when the truck cannot reach the forms
• Form setting, stripping and disposal of form lumber
• Reinforcement, chairs and any dowels into existing concrete
• Finish type for each element: broom slab, smooth steps, exposed aggregate or stamped
• Washout location for the truck chute and cleanup of overspray and splatter
• Payment schedule, rain delay terms and a written workmanship warranty

What this estimate does not include

• Labor to excavate, form, place, finish and cure the concrete
• Gravel base under the slab and the compaction work it needs
• Rebar, wire mesh and chairs (the Concrete Slab Calculator adds a rebar count and cost)
• Form lumber, stakes, expansion joint strips and curing compound
• Pump truck time, short load fees beyond the flat delivery fee, and overtime unloading charges
• Permits, inspections and haul away of old concrete or excavated soil
• Tool and mixer rental for bagged pours

When a licensed contractor becomes necessary

Footings that carry a structure, such as an addition, a garage or a deck ledger, fall under building codes in most jurisdictions. Depth below the local frost line, rebar placement and an inspection before the pour are standard requirements, and an inspector can order a failed footing removed. Retaining walls taller than local limits and any slab under a living space usually need stamped drawings as well.

Pours larger than a few cubic yards move fast. A truck delivers concrete faster than two people can spread and level it, and a slab that starts to set before finishing ends with a rough surface or cold joints. Crews with screeds, bull floats and power trowels handle these pours in one pass.

Safety facts apply to every pour. Wet concrete is caustic and causes chemical burns through clothing and boots, so waterproof gloves, long sleeves and eye protection are standard. Cutting or grinding cured concrete releases crystalline silica dust, which calls for water suppression and a rated respirator.

Cost data note

Prices in this tool are United States national planning ranges, updated October 2026. Ready mix runs $140 to $190 per cubic yard before delivery. Bagged concrete runs $5.00 to $8.00 per 80 lb bag, $4.50 to $7.00 per 60 lb bag and $4.00 to $6.00 per 40 lb bag. The rates are reviewed every three months, and the date above changes with each review. Local supplier quotes take priority over any figure here.

Frequently asked questions

How many bags of concrete does a 10 by 10 slab need?

62 bags of 80 lb mix for a 4 inch thick slab, or 82 bags of 60 lb mix. The slab holds 33.33 cubic feet, and the 10 percent waste allowance raises that to 36.67 cubic feet before the division by bag yield.

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

45 bags. A cubic yard holds 27 cubic feet and each 80 lb bag yields 0.60 cubic feet. With a 10 percent waste allowance the count rises to 50 bags.

How much concrete does a 20 by 20 slab need?

5.43 cubic yards at 4 inches thick with 10 percent waste. The geometric volume is 4.94 cubic yards. At 5 inches thick the order grows to 6.79 cubic yards.

Is ready mix cheaper than bagged concrete?

Ready mix is cheaper once a pour passes about 0.69 cubic yards before waste at typical national prices, with the 10 percent waste allowance on both routes. A 2.61 cubic yard patio costs a typical $530 by truck against $767 in 80 lb bags. For a small pad the flat delivery fee reverses the result, and a 0.22 cubic yard pad costs $65 in bags against $136 by truck.

How thick should a concrete patio or sidewalk be?

Four inches is the standard thickness for patios and walkways that carry foot traffic. Driveways and garage floors that carry vehicles commonly run 5 to 6 inches, and each added inch raises the volume of a 4 inch slab by 25 percent.

Related calculators

• Concrete slab calculator with rebar and installed cost for slabs that need a reinforcement count and a labor estimate
• Bag count calculator for premixed concrete for orders entered as a known volume
• Footing and pier volume calculator for strip footings and round piers
• Installed concrete cost estimator for material, labor and total pricing on slabs, patios, driveways and footings
• Concrete block and mortar counter for block walls built on a poured footing

The full set of concrete calculators sits on one hub page. Sidewalk projects priced as a hired job are covered in the sidewalk replacement cost guide, which pairs with the sidewalk setting of this tool.