Concrete Calculator

The most common concrete question - "how much do I need?" - comes down to one piece of arithmetic and one habit. The arithmetic is volume; the habit is ordering a little extra. A standard 10 ft x 10 ft slab poured 4 inches thick works out to about 33.33 cubic feet, which is roughly 1.23 cubic yards. Add a 10% waste allowance and you would order about 1.36 cubic yards, or hand-mix it with around 82 bags of 60 lb concrete (about 62 of the 80 lb bags). This concrete calculator does that math for slabs, footings and round columns, then tells you the cubic yards, cubic feet and bag count in one step.

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 boundary where ready-mix delivery starts to beat bags on both cost and effort - 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 typical for patios and walkways; 5-6 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.
6. Choose a waste factor: 10% is a sensible default; bump it to 15% for rough subgrade or hard-to-place pours.

Press Calculate concrete and 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 - typically 5-10%.
• Ready-mix: wet concrete delivered by truck, ordered by the cubic yard. The efficient 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 ~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 - order ready-mix (~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.

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 - 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.
• Cross the one-yard line with ready-mix. Past roughly 60 bags, a truck is cheaper, faster and more consistent than hand-mixing.
• 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 waits for no one - it begins setting within an hour, 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 - lean on the waste factor.
• Bag yields of 0.45 and 0.60 cu ft are standard manufacturer figures; check your specific product, as yields vary slightly by brand and mix.
• The weight estimate uses about 4,050 lb per cubic yard 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 - a slab on top of pier footings, for example - calculate each part separately and add the cubic yards together before you apply a single waste factor to the total.

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, which is well over 1.5 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: once you cross about a yard, the per-yard price of delivery typically undercuts 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.

Sources & references

The yields, weights and conversions used here follow standard, publicly documented figures rather than any single brand:

• Unit conversion: 1 cubic yard = 27 cubic feet, and thickness in inches ÷ 12 = feet. These are exact definitional conversions.
• Bag yields: 0.45 cu ft for a 60 lb bag and 0.60 cu ft for an 80 lb bag are the yields published by major pre-mix manufacturers; always confirm the figure printed on your specific product.
• Concrete weight: normal-weight concrete is taken at about 150 lb per cubic foot (roughly 4,050 lb per cubic yard), the standard density for ordinary structural mixes.
• Geometry: rectangular volume = length × width × height; cylinder volume = π × radius² × height.
• 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.