Concrete Calculator

How much concrete do I need? Multiply length × width × thickness in feet, then divide by 27 for cubic yards. A 10 ft × 10 ft slab at 4 inches is 33.33 cubic feet, or about 1.23 cubic yards (order about 1.36 yd with 10% waste). This concrete calculator does that math for slabs, footings and round columns, showing cubic yards, cubic feet and bag count.

Building the whole structure, not just the pour? The Roofing and Roof Pitch calculators size shingles and slope up top, the Paint Calculator handles wall coverage, and the Mulch Calculator finishes the beds around a cured slab. Use this concrete tool only for the poured volume itself.

The formula behind the result

Every concrete estimate is just volume. The shape decides which formula applies, but they all end the same way: convert to cubic feet, then divide by 27 to reach cubic yards (since a cubic yard is 3 ft x 3 ft x 3 ft = 27 cubic feet):

The only trap is units. Thickness and depth are almost always given in inches, so you must divide by 12 to get feet before multiplying. A 4-inch slab uses 4÷12 = 0.333 ft, not 4. The calculator handles this conversion for you, so you can enter length and width in feet and thickness in inches the way you would actually measure them.

A worked example, step by step

Say you are pouring a 12 ft x 8 ft patio at 4 inches thick. Here is the full calculation:

1. Convert thickness: 4 in ÷ 12 = 0.333 ft.
2. Volume: 12 × 8 × 0.333 = 32 cubic feet.
3. To cubic yards: 32 ÷ 27 = 1.19 cubic yards.
4. Add 10% waste: 1.19 × 1.10 = 1.30 cubic yards to order.
5. Or in bags: 35.2 cu ft (with waste) ÷ 0.45 = 79 bags of 60 lb, or ÷ 0.60 = 59 bags of 80 lb.

At roughly 1.3 cubic yards, this patio sits right at the point where ready-mix delivery is worth pricing against bags, which is exactly the kind of decision the bag count is meant to inform.

How to measure and use the calculator

You only need a tape measure and the right shape mode. Work through it in order:

1. Pick the shape: Slab/Wall for flat pours, Footing for trenches and pads, or Round Column for tube footings and posts.
2. Measure in feet: enter length and width (or column height) in feet. For odd shapes, break the area into rectangles and add them up, or pour them as separate quantities.
3. Enter thickness in inches: 4 inches is common for patios and walkways, 5 to 6 inches for driveways. Footings use depth instead of thickness.
4. Round columns: enter the tube's diameter in inches and its height in feet.
5. Set quantity: if you are pouring several identical footings or piers, enter how many (a whole number).
6. Choose a waste factor: 10% is a sensible default; bump it to 15% for rough subgrade or hard-to-place pours.

The results update as you type: read the headline cubic-yards figure, then the bag counts and the weight estimate below it.

Who this calculator is for

The volume math is the same for a homeowner and a pro; the calculator just removes the unit-conversion mistakes. It fits:

• DIY homeowners pouring a patio, shed pad, walkway or set of fence-post footings.
• Landscapers sizing steps, mowing strips, edging and small retaining elements.
• Contractors and handymen double-checking a takeoff before phoning in a ready-mix order.
• Anyone pricing a job who needs a defensible material quantity to plug into a supplier quote.

Key terms explained

• Cubic yard: the unit ready-mix concrete is sold in. One cubic yard = 27 cubic feet and weighs about two tons.
• Cubic foot: the unit used to rate bag yield. A 60 lb bag makes about 0.45 cu ft; an 80 lb bag about 0.60 cu ft.
• Waste factor: the percentage you add on top of the exact volume to cover spillage, over-excavation, form bulge and uneven ground; 5 to 10% is a common allowance.
• Ready-mix: wet concrete delivered by truck, ordered by the cubic yard. Usually the practical choice past about one yard.
• Footing: the wider, deeper concrete base that spreads a structure's load into the soil; sized by depth rather than slab thickness.
• Yield: how much finished concrete one bag produces once mixed with water, expressed in cubic feet.

Three common scenarios

Here is how the numbers shake out for projects people pour most often (exact volumes shown before waste; bag counts shown with the standard 10% added):

• Patio, 10 ft x 12 ft at 4 in: 40 cu ft = about 1.48 cubic yards. Add 10% and order about 1.63 yd, or 98 bags of 60 lb. (Lay the footprint out first with the Square Footage Calculator.)
• Driveway section, 20 ft x 10 ft at 6 in: 100 cu ft = about 3.70 cubic yards. Far past the bag threshold, so order ready-mix (about 4.1 yd with waste).
• Four post footings, 12 in diameter, 4 ft deep each: each tube holds about 3.14 cu ft, so four total about 12.6 cu ft = 0.47 cubic yards. Small enough for 24 bags of 80 lb with the 10% waste allowance.

How much concrete for a 10x10, 12x12 or 20x20 slab?

This is the reference for the most-searched slab sizes, all at the standard 4-inch thickness. "Order (+10%)" is what to actually buy after a 10% waste allowance; bag counts already include that waste and are rounded up to whole bags.

Pouring thicker than 4 inches? Scale any row directly: a 5-inch slab is the 4-inch figure × 1.25, and a 6-inch slab is × 1.5. So a 10 × 10 ft slab at 6 inches is 1.23 × 1.5 = about 1.85 cubic yards before waste.

What changes the result the most

If you watch the headline number move as you adjust inputs, a few factors dominate:

• Thickness: the most overlooked lever. Going from 4 to 6 inches adds 50% to the volume on the same footprint.
• Footprint: length and width scale the volume directly, so doubling either doubles the concrete.
• Quantity: identical footings or piers add up fast; six small pads can rival a slab.
• Waste factor: a few percent rarely changes whether you order a truck, but it can change the bag count by several bags.
• Over-excavation: a trench dug deeper or wider than planned silently increases the real volume beyond your measured numbers.

Tips for ordering and pouring

• Always order about 10% extra. Coming up short can ruin a pour with a cold joint; surplus is cheap by comparison.
• Price ready-mix once you cross one yard. Past roughly 60 bags of 60 lb mix, a truck is faster and more consistent than hand-mixing; compare its delivered price with the bags.
• Check your subgrade depth before you order. A trench that is an inch deeper than planned across a big slab is a surprising amount of extra concrete.
• Have help and tools ready. Concrete will not wait: it starts to stiffen soon after mixing, faster in hot weather, so screed boards, floats and labor must be on site before the truck arrives.
• Round bag counts up, never down. The calculator already rounds up; buy whole bags and keep a spare.

Limitations and assumptions

This is a planning estimate for concrete volume, not an engineering spec or a price. Keep these in mind:

• It assumes clean, regular geometry. Sloped grades, thickened edges and irregular outlines hold more concrete than the simple formula shows, so lean on the waste factor.
• Bag yields of 0.45 and 0.60 cu ft come from the QUIKRETE Concrete Mix data sheet; check your specific product, as yields vary slightly by brand and mix.
• The weight estimate uses 150 lb per cubic foot (4,050 lb per cubic yard), the upper end of the 140 to 155 lb range for normal-weight concrete; lightweight or high-density mixes differ.
• It does not size structural elements. Rebar, mesh, slab thickness for loads and footing dimensions should follow local code or an engineer.
• It does not include the gravel base, vapor barrier, forms or finishing. Estimate the compacted sub-base with the Gravel Calculator and price the rest separately.

Related materials and tools

A concrete pour rarely stands alone. Once you have the concrete volume, plan the surrounding materials with sister calculators:

• Use the Square Footage Calculator to lay out and total the area before you assign a thickness.
• Use the Gravel Calculator to size the compacted sub-base under the slab.
• Use the Cubic Yard Calculator for any other material sold by the yard, from sand to fill.
• Use the Mulch Calculator to finish the surrounding beds after the concrete cures.
• Use the Tile and Paint calculators for the finishing layers on or around the new surface.

Slab, footing or column: which mode fits your job?

The three shape modes are not interchangeable, and picking the wrong one is the most common reason an estimate comes out wildly off. Here is how to choose:

• Slab / Wall handles any flat, rectangular pour of roughly even thickness: patios, shed pads, walkways, garage floors and basement walls. You enter length, width and a thickness in inches. This is the right mode for the great majority of homeowner pours.
• Footing is for the deeper, load-bearing concrete that sits under a wall, deck post or column and spreads weight into the soil. Footings are sized by depth rather than slab thickness, and they are usually far thicker than a slab, so do not reuse a 4-inch figure here.
• Round Column covers cylindrical pours: cardboard form tubes for deck and fence posts, pier footings and lally columns. It uses the diameter of the tube, not its radius, and is the only mode that relies on π.

When a project mixes shapes, such as a slab on top of pier footings, calculate each part separately and add the cubic yards together before you apply a single waste factor to the total.

How much concrete per round tube footing?

For deck posts, fence posts and pier footings poured in cardboard form tubes, this table gives the concrete per tube by diameter and depth (using π × radius² × height). Multiply by the number of identical tubes, then add your waste factor.

Bag counts here use the 80 lb bag (0.60 cu ft yield), rounded up per tube, with no waste added. Because rounding happens on each tube, four 12-inch tubes at 4 ft deep need 24 bags (4 × 6), not the 21 you would get from the pooled volume, so buying by the tube always errs safely high.

Bags vs. ready-mix: the real break-even

The bag count is not just trivia: it is the number that tells you which way to buy. Bagged concrete that you mix on site makes sense for genuinely small work: a handful of post footings, a small repair, a step or a single pad under a unit. The moment a job creeps toward a full cubic yard, three things turn against you. First, the labor: a single cubic yard is roughly 60 bags of 60 lb mix, about 1.8 tons of material to haul, open and mix by hand. Second, consistency: every batch you mix by hand can vary in water content and strength, while a ready-mix truck delivers one uniform mix. Third, cost: compare the delivered price per yard, including any short-load or delivery fee, with the price of the equivalent stack of bags. A practical rule is to bag anything under about half a yard, plan ready-mix for anything over a yard, and judge the middle by how much help and time you have. The Cubic Yard Calculator is handy here for converting a supplier's per-yard quote into the same units the bag count uses.

This quick reference converts any cubic-yard amount into cubic feet, bag counts and delivery weight, so you can see the bag-vs-truck break-even at a glance (weight uses 4,050 lb per cubic yard for normal-weight concrete):

Sources & references

• QUIKRETE: Concrete Mix No. 1101 product data sheet (PDF) (yields of about 0.30 cu ft per 40 lb bag, 0.375 per 50 lb, 0.45 per 60 lb and 0.60 per 80 lb). Always confirm the figure printed on your specific product.
• Federal Highway Administration: FHWA-HRT-05-057, Appendix D (unit weights of normal-weight concrete of about 140 to 155 lb per cubic foot).
• National Institute of Standards and Technology: NIST Handbook 44 (2026), Appendix C, General Tables of Units of Measurement (PDF) (27 cubic feet = 1 cubic yard, 2,000 pounds = 1 ton).
• International Code Council: International Residential Code, Chapter 5, Section R506 (concrete floors on ground: 3.5 inch minimum thickness, 4 inch base course).
• Structural sizing (slab thickness for loads, footing depth, reinforcement) should follow your local building code or a licensed engineer; this tool estimates quantity only, not structural adequacy.