What this calculator does
Find out how much concrete you need, how many bags to buy and roughly what it will cost. Big jobs use ready-mix concrete, delivered by truck. In the US it's sold by the cubic yard, and almost everywhere else by the cubic meter. Small jobs are usually poured from premixed bags. Either way, the first step is measuring the space the concrete will fill.
This calculator handles the four shapes that come up most often. There's a flat slab or footing (the concrete base under a wall). There's a round column cast in a cardboard tube form, and a hollow round tube. And there's a row of post holes with posts set in them.
You get an estimate of the material, with a little extra added for waste. It's shown in cubic yards, cubic meters and the number of 40, 60 and 80 lb bags. You'll also see the total bag weight you'll have to haul. Enter your local prices to compare a ready-mix order with buying 80 lb bags.
This is not an engineering design. Loads, soil and frost depth decide how thick a slab, how big a footing and how deep a post hole should be. So follow your local building code and the instructions printed on the product you buy.
Where it's used
Why this math matters outside the classroom:
- Patio slabs. A 10 × 10 ft patio poured 4 in thick needs 33⅓ ft³, which is about 1.23 cubic yards of concrete.
- Fence posts. A hole 10 in wide and 2 ft deep is a cylinder of about 1.09 ft³ of concrete before the post is subtracted.
- Round columns. A 12 in diameter tube form 8 ft tall holds π × 0.5² × 8, about 6.28 ft³ of concrete.
- Bag counts. DIYers divide the volume by a bag's yield, such as about 0.6 ft³ for an 80 lb premix bag, to count bags for a small pour.
- Ready-mix orders. Contractors divide a footing's cubic feet by 27 because ready-mix plants sell concrete by the cubic yard.
Grade level and standards: Grades 7–8; Common Core State Standards for Mathematics 7.G.B.6, 8.G.C.9. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.
How to use it
- Choose US units (feet and inches) or metric units (meters and centimeters).
- Pick what you're pouring: a slab or footing, a round column, a hollow tube, or post holes.
- Enter the measurements. For post holes, give the number of holes, the hole size and depth, and the post's real size. The calculator takes out the space the post fills.
- Set a waste allowance. If your bags list a different yield (how much concrete one bag makes), change the typical figures.
- Enter the ready-mix price per cubic yard (or meter) and the price of an 80 lb bag, or clear them. Then read the amount to order, the bag counts, both costs and the bag table.
What the inputs mean
- Thickness (depth)
- The finished thickness of the slab, or the depth of a footing. A trench footing is really a long, narrow slab, so enter its length, width and depth here.
- Column diameter
- The inside diameter (the distance straight across) of the tube form. Common cardboard forms run from about 6 to 24 inches.
- Inner diameter (hollow core)
- For a hollow tube or ring, the diameter of the empty core. It must be smaller than the outer diameter.
- Hole diameter
- A common rule is about three times the post width.
- Post width or diameter
- The real width of a square post, or the diameter of a round one. Lumber is smaller than its name: a 4×4 measures about 3.5 in, and a 6×6 about 5.5 in.
- Extra for waste and spillage
- Ground is never perfectly flat, forms bow outward, and some concrete stays in the mixer or wheelbarrow. 5–10% extra is common for ready-mix. Small bagged jobs often use 10%.
- Ready-mix price per cubic yard (optional)
- Ask your local plant; delivery and short-load fees are extra. Clear it to skip the cost.
- Ready-mix price per cubic meter (optional)
- Ask your local plant; delivery and short-load fees are extra. Clear it to skip the cost.
- Price per 80 lb bag (optional)
- The shelf price of an 80 lb (36 kg) bag of premix at your store.
- Yield of a 40 lb bag
- Typical value: about 0.30 ft³. Check the bag.
- Yield of a 60 lb bag
- Typical value: about 0.45 ft³. Check the bag.
- Yield of an 80 lb bag
- How much mixed concrete one bag makes. For ordinary premix, about 0.30, 0.45 and 0.60 ft³ for 40, 60 and 80 lb bags is typical. Blends differ, though, so use the figure on your bag.
- Yield of a 40 lb (18 kg) bag
- Typical value: about 8.5 liters. Check the bag.
- Yield of a 60 lb (27 kg) bag
- Typical value: about 12.7 liters. Check the bag.
- Yield of an 80 lb (36 kg) bag
- Typical value: about 17 liters. Check the bag.
Formula and method
slab or footing: V = length × width × thickness round column: V = π × (diameter ÷ 2)² × height × count hollow tube: V = π ÷ 4 × (outer diameter² − inner diameter²) × height × count post holes: V = holes × (π × (hole diameter ÷ 2)² − post area) × depth square post area = width²; round post area = π × (diameter ÷ 2)² to order = V × (1 + waste % ÷ 100); bags = to order ÷ yield per bag, rounded up 1 yd³ = 27 ft³ = 0.764554857984 m³ ready-mix cost = volume to order × price per yd³ (or m³); bag cost = 80 lb bags × price per bag
In the formulas, V is the volume, the amount of space the concrete fills. π (pi) is about 3.14, the number that links a circle's width to its area. Every shape works the same way: find the area of its face, then multiply by the height or depth.
Thicknesses and diameters are entered in inches or centimeters. The calculator divides them by 12 or 100 first, so every length is in feet or meters before multiplying.
In a post hole, concrete fills the ring between the hole wall and the post. So the post's area is taken away before multiplying by the depth. For bags, the calculator divides the volume, with waste, by how much one bag makes. Then it rounds up, since you can't buy part of a bag.
Worked example
A 10 ft × 10 ft patio slab, 4 inches thick, with a 10% waste allowance and the typical bag yields.
- Volume: 10 × 10 × (4 ÷ 12) = 33.33 ft³.
- Cubic yards: 33.33 ÷ 27 = 1.2346 yd³.
- With 10% waste: 1.2346 × 1.1 = 1.358 yd³, which is 36.67 ft³.
- 80 lb bags: 36.67 ÷ 0.60 = 61.1, rounded up to 62 bags.
- 60 lb bags: 36.67 ÷ 0.45 = 81.5, so 82 bags; 40 lb bags: 36.67 ÷ 0.30 = 122.2, so 123 bags.
- Ready-mix at $165 per yd³: 1.358 × $165 = $224.07 before delivery fees.
- Bags at $6.50 each: 62 × $6.50 = $403.00, and 62 × 80 lb = 4,960 lb to lift and mix.
Result: Order about 1.36 cubic yards (1.04 m³) of ready-mix for roughly $224 plus delivery, or buy 62 bags of 80 lb premix for about $403.
Understanding the result
As a rough guide, jobs over about one cubic yard are usually cheaper and easier with a ready-mix delivery. That's because one cubic yard is about 45 bags of 80 lb premix. Suppliers often sell in quarter or half yard steps. Some charge a short-load fee for small orders, so round up to what they offer.
The two cost lines compare the material alone, side by side. Ready-mix usually wins on price per yard. But small orders often carry delivery and short-load charges that this calculator doesn't know about. So ask the plant for an all-in quote before you decide.
If you're close to a whole number of bags, buy one or two more. It's cheap insurance. Running short mid-pour leaves a weak joint where fresh concrete meets concrete that has started to set.
Assumptions and limitations
- Slabs and footings are rectangles with the same thickness all over.
- Columns and tubes are straight cylinders, the same width from top to bottom.
- Post holes have straight sides and are filled with concrete to the depth you enter. Each post runs the full depth of the concrete.
- The waste allowance is one percentage applied to the whole volume.
- This is a material estimate only. It doesn't check strength, steel reinforcement, thickness, footing size or frost depth. Those come from local codes or an engineer.
- Bag yields vary by product. The defaults are typical figures, so replace them with the yield printed on your bag.
- Belled footings (wider at the bottom), thickened slab edges, steps and odd shapes aren't covered. Split them into simple pieces and add the volumes together.
- Gravel placed in the bottom of a post hole for drainage means you need less concrete. Subtract the gravel's depth from the depth you enter.
- Prices are whatever you enter. The defaults are placeholders, not live market prices. Delivery, pump hire, short-load fees, rebar, forms and tax aren't included.
Frequently asked questions
How many bags of concrete are in a cubic yard?
A cubic yard is 27 ft³. At the typical yields used here, that's 45 bags of 80 lb premix, 60 bags of 60 lb, or 90 bags of 40 lb. That's before any waste allowance.
Is it cheaper to buy bags or order ready-mix?
Compare the two cost lines using your local prices. For the default 10 × 10 ft patio, bags come to about $403, against about $224 of ready-mix. But the truck's delivery or short-load fee can close that gap on a small order. Bags also mean mixing and lifting: 62 bags of 80 lb is almost 5,000 lb.
How many 80 lb bags of concrete do I need for a 10 × 10 slab?
At 4 inches thick, it's 33.3 ft³, or 36.7 ft³ with 10% waste. At 0.60 ft³ per bag that's 61.1, so buy 62 bags. At 6 inches thick, it's 55 ft³ with waste, or 92 bags.
How thick should a concrete slab be?
Walkways and patios are often 4 inches (about 10 cm) thick. Driveways and slabs that carry vehicles are often thicker. The right figure depends on the weight it carries, the ground beneath and your local code. So check those rather than relying on a rule of thumb.
How much concrete does a fence post hole need?
Choose post holes, then enter the hole diameter, the depth of concrete and the post size. For example, a 10 inch hole 36 inches deep around a 4×4 post takes about 1.38 ft³ of concrete. That's a little over two 80 lb bags at 0.60 ft³ each, before waste.
What is the difference between the column and hollow tube options?
A column is solid, like a pier cast in a cardboard tube form. A hollow tube has an empty core, like a round drain pipe (culvert) or a ring wall. So the core's area is taken away from the outer circle.
Why does the calculator round bags up?
Bags are sold whole, and running short partway through a pour is worse than having part of a bag left over. The cubic yard and cubic meter figures aren't rounded, so you can round them to your supplier's ordering step.
How this calculator is checked
Method: Prism and cylinder volume formulas (cross-section × height); annulus area for hollow tubes and post holes; exact US–metric factors (1 yd³ = 27 ft³ = 0.764554857984 m³). The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.
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