Building materials
Concrete Calculator
Work out how many cubic yards of ready-mix or how many bags of concrete to buy for a slab, footing, post holes, Sonotube, steps, or wall.
Maintained by Austin Witherow · Updated
Ready-mix to order
1.30 yd³
- 80 lb bags
- 59 bags
- 60 lb bags
- 78 bags
- 50 lb bags
- 94 bags
- 40 lb bags
- 117 bags
- Concrete volume (no extra)
- 1.23 yd³
- Volume with extra
- 35.00 ft³
- Volume with extra (metric)
- 0.99 m³
- Each extra 1/4 in of depth adds
- +0.08 yd³ (6.3%)
- Truck loads
- 1 load
- Weight of the 80 lb bags
- 4,720 lb
- Water for the 80 lb bags
- 44.3 to 66.4 gal
- Concrete cost
- Add prices
Formula
Slab (cu ft) = Length × Width × Thickness ÷ 12, plus Edge ring area × Extra edge depth ÷ 12Round slab or column (cu ft) = π × (Diameter ÷ 2)² × HeightPost hole (cu ft) = (π × (Hole diameter ÷ 2)² − Post area) × (Hole depth − Gravel − Top gap) ÷ 1,728Steps (cu ft) = Width × (Run × Rise × n(n − 1) ÷ 2 + Landing × n × Rise) ÷ 144, n = risersOrder (cu ft) = Volume × How many × (1 + Extra ÷ 100)Cubic yards = Order ÷ 27Bags = Order ÷ Bag yield, rounded up (80 lb 0.60, 60 lb 0.45, 50 lb 0.375, 40 lb 0.30)
Key caveat
Volumes assume flat, square-edged pours. Sloped ground, rounded corners, and over-dug trenches change the real volume; the extra percentage is there to cover that.
What this calculator is for
Concrete is bought two ways: by the cubic yard from a ready-mix truck, or by the bag. This calculator works out both from the size of what you are pouring, for slabs, footings, post holes, columns, steps, and walls.
Bag counts use the yields the manufacturers print on their data sheets (an 80 lb bag of QUIKRETE or SAKRETE concrete mix makes 0.60 cubic feet), and the extra you add follows the ready-mix industry's own ordering advice. Post holes take the post's volume out of the hole, and the result shows how much one more quarter inch of depth would add, because thin pours run short faster than people expect.
How it works
Each shape's volume is worked out in cubic feet: length × width × thickness for slabs, footings, and walls; π × radius² × height for round slabs and columns; the hole minus the post for post holes; and a stack of steps for stairs.
The volume is multiplied by the number of identical pours, then by 1 + the extra percentage.
Ready-mix to order is that volume divided by 27, because a cubic yard is 27 cubic feet.
Bags are the same volume divided by each bag's yield (0.60, 0.45, 0.375, or 0.30 cubic feet), rounded up to whole bags.
Steps are solid to the ground. With n risers there are n − 1 treads; the last rise steps onto the porch, or onto a new landing if you enter one.
Truck loads are the cubic yards to order divided by the truck size, rounded up.
The quarter-inch figure is the slab or footing area × 1/4 in, with the same extra, and its share of the whole pour, so you can see what a low spot in the ground costs.
What the calculator measures
A slab is counted from the top of its base, so the gravel under it is a separate order; a thickened edge adds a ring as wide as the edge and as deep as its extra depth. In a post hole, the concrete fills the space around the post, above any gravel and below any soil left at the top. Steps are counted by risers: every tread below the top is solid to the ground, and the top is the porch, or a new landing if you enter one.
Quick calculator insights
Why orders run short
NRMCA's list of why ordered concrete does not fill the forms. A note in ASTM C94 says waste, over-excavation, spreading forms, and settlement are not the producer's responsibility.
- Thicker than planned
- 1/8 in = 3%
- On a 4 in slab, an extra 1/8 in of depth is 3% more concrete.
- Forms that bulge
- Brace forms so they do not spread under the weight.
- Uneven or soft ground
- Concrete settles into granular fill and low spots.
- Hardened volume
- About 2% less
- Hardened concrete is about 2% smaller than fresh.
Truck or bags
- 1 cubic yard
- 45 bags of 80 lb
- 3,600 lb to lift and mix.
- Truck mixer
- 8 to 12 yd³
- A loaded truck is about 80,000 lb and may not cross a residential curb (NRMCA).
- Last load
- Re-measure before ordering the last load so it is not a short one (NRMCA).
Bags in one cubic yard
27 cubic feet divided by each bag's published yield, before any extra.
| Bag | Yield per bag | Bags per cubic yard | Source |
|---|---|---|---|
| 40 lb | 0.30 ft³ | 90 | QUIKRETE and SAKRETE |
| 50 lb | 0.375 ft³ | 72 | QUIKRETE |
| 60 lb | 0.45 ft³ | 60 | QUIKRETE and SAKRETE |
| 80 lb | 0.60 ft³ | 45 | QUIKRETE and SAKRETE |
| 90 lb (QUIKRETE) | 0.675 ft³ | 40 | QUIKRETE |
| 90 lb (SAKRETE) | 0.66 ft³ | 41 (40.9) | SAKRETE |
Area one cubic yard covers
27 cubic feet spread at each thickness, before any extra.
| Thickness | Area per cubic yard | 80 lb bags per 10 sq ft |
|---|---|---|
| 3-1/2 in | 92.6 sq ft | 4.9 |
| 4 in | 81 sq ft | 5.6 |
| 5 in | 64.8 sq ft | 6.9 |
| 6 in | 54 sq ft | 8.3 |
| 8 in | 40.5 sq ft | 11.1 |
Published thickness minimums
Minimums from each source. Soil, loads, and frost can call for more; your local code and plans decide.
| Source | Floor slab, walk, or patio | Driveway |
|---|---|---|
| IRC R506.1 (building code) | 3-1/2 in for floor slabs in a house; walks and patios not covered | Not covered |
| Michigan Concrete Association | Not covered | 4 in; 5 in with delivery trucks |
| SAKRETE High-Strength data sheet | 4 in for flatwork | Not covered |
| QUIKRETE Concrete Mix | 2 in minimum pour | Not covered |
How to use this calculator
- 01Pick what you are pouring.
- 02Measure it: length and width in feet, thickness and depth in inches, as each field says.
- 03Enter how many identical pours there are, such as the number of post holes.
- 04Set the extra: NRMCA's range is 4 to 10%.
- 05Order the cubic yards from a ready-mix supplier, or buy the bag count for the bag size you will use.
Inputs explained
- What are you pouring?
- Chooses the shape and shows its measurements.
- Slab length, width, and thickness
- The slab's outline in feet and its thickness in inches.
- Thickened edge
- A strip around the outside of the slab that goes deeper, entered as its width and how far it goes below the slab.
- Diameter and round thickness
- Across a round slab in feet with its thickness in inches, or inside a column form in inches.
- Footing length, width, and depth
- The total run in feet and the concrete's cross-section in inches.
- Column height
- The height of concrete in the form.
- Post hole sizes
- Hole diameter and depth, the post's shape and size, and any gravel at the bottom or gap at the top.
- Steps
- Width, the number of risers (every step up, including the last one), each step's rise and run, and an optional new landing at the top.
- Wall
- Length and height in feet and thickness in inches.
- How many
- Identical pours of the same size.
- Extra to order
- A percentage added to the volume before yards and bags are worked out.
- Truck size
- Cubic yards per truck, for the number of loads.
- Prices
- Optional. Ready-mix per cubic yard and per 80 lb bag, to compare the two.
Outputs explained
- Ready-mix to order
- Cubic yards including the extra, the figure to give a supplier.
- Bags
- Whole bags of each size for the same volume, from the manufacturers' yields.
- Concrete volume (no extra)
- The measured volume, before the extra.
- Volume with extra
- The order in cubic feet.
- Volume with extra (metric)
- The same order in cubic meters.
- Each extra 1/4 in of depth adds
- For slabs and footings: the cubic yards one more quarter inch of depth would need, and that as a share of the whole pour.
- Truck loads
- Loads at the truck size you entered. Under 1 cubic yard it says so instead, since ready-mix suppliers can have a minimum order.
- Weight of the 80 lb bags
- What you would lift and haul if you buy 80 lb bags.
- Water for the 80 lb bags
- From QUIKRETE's 6 to 9 pints per 80 lb bag.
- Concrete cost
- Ready-mix and bag costs at the prices you enter.
Worked examples
10 × 10 ft slab at 4 in
- Volume = 10 × 10 × 4 ÷ 12 = 33.33 cubic feet, or 1.23 cubic yards.
- With 5% extra: 35 cubic feet, or 1.30 cubic yards of ready-mix.
- Bags: 35 ÷ 0.60 = 59 of 80 lb, 78 of 60 lb, 94 of 50 lb, or 117 of 40 lb.
- The 59 bags weigh 4,720 lb. One more 1/4 in of depth over the slab adds 0.08 cubic yards, 6.3%.
12 × 40 ft driveway at 5 in
- Volume = 12 × 40 × 5 ÷ 12 = 200 cubic feet, or 7.41 cubic yards.
- With 5% extra: 7.78 cubic yards, one truck load.
- In 80 lb bags it would be 350 bags, 28,000 lb.
- Each extra 1/4 in of depth adds 0.39 cubic yards (5%).
10 fence posts, 3-1/2 in square, in 10 in holes 32 in deep
- One hole holds π × 5² × 32 = 2,513 cubic inches; the post takes 3.5 × 3.5 × 32 = 392, leaving 2,121, or 1.23 cubic feet.
- Ten holes with 5% extra: 12.89 cubic feet.
- That is 22 bags of 80 lb, or 35 of 50 lb. Bags are counted for the whole job, so one opened bag can finish the next hole.
12 in Sonotube, 4 ft tall
- Volume = π × 0.5² × 4 = 3.14 cubic feet.
- With 5% extra: 3.30 cubic feet, or 6 bags of 80 lb.
Three risers up to a porch, 4 ft wide, 7 in rise and 11 in run
- Three rises reach a 21 in porch, so there are two treads, 7 and 14 in tall, each 11 in deep: 11 × 7 × (1 + 2) = 231 square inches of side profile.
- 231 ÷ 144 × 4 ft = 6.42 cubic feet; with 5% extra, 6.74 cubic feet, or 12 bags of 80 lb.
- Pour a new 36 in landing at the top instead of stepping onto a porch, and it adds 36 × 21 = 756 square inches: 27.42 cubic feet, or 48 bags of 80 lb with 5% extra.
When to use it
- Ordering ready-mix for a patio, garage slab, or driveway.
- Buying bags for fence posts, deck piers, or a small pad.
- Checking a contractor's yardage.
- Deciding between a truck and bags for a mid-sized job.
Tips
- Measure the forms after they are set, not the plan: NRMCA lists miscalculated form volume and slab thickness first among the reasons concrete runs short.
- Run a slab and its footings separately and add them.
- Use the yield printed on your bag if it differs from the figures here; a 90 lb bag yields 0.675 cubic feet for QUIKRETE but 0.66 for SAKRETE.
- For ready-mix, ask the supplier about their truck size, small-load minimum, and how they want the last load called in.
Limitations
- Volumes assume flat, square-edged pours. Sloped ground, rounded corners, and over-dug trenches change the real volume; the extra percentage is there to cover that.
- Steps are treated as solid concrete down to the ground, with the last rise onto the porch or landing. Steps poured over fill or a slab need less.
- Bag counts use the published yields of QUIKRETE Concrete Mix and SAKRETE High-Strength Concrete Mix. Fiber-reinforced, crack-resistant, and other specialty mixes can yield differently; check the bag.
- For post holes the calculator uses the bag's printed yield. QUIKRETE's own Fast-Setting example sets a 3 in post in a 9 in hole 30 in deep with two 50 lb bags; by the printed yield that hole needs more than two (see the FAQ).
- This is a material estimate. Thickness, reinforcement, footing size, and frost depth come from your plans and local code.
Methodology & assumptions
- Volumes are standard geometry in cubic feet; cubic yards are cubic feet ÷ 27.
- Bag yields are from the QUIKRETE Concrete Mix, QUIKRETE Fast-Setting, and SAKRETE High-Strength data sheets. The 40, 60, and 80 lb yields are the same on both makers' sheets.
- The default 5% extra sits inside NRMCA's 4 to 10% range. The quarter-inch figure follows NRMCA's point that a small thickness error is a proportional volume error.
- Truck sizes are NRMCA's 8 to 12 cubic yards. Mixing water is QUIKRETE's 6 to 9 pints per 80 lb bag.
Sources
Last reviewed 2026-10-06
- QUIKRETE Concrete Mix (No. 1101) product data sheet, revised October 2022
Yields: 40 lb 0.30 ft³, 50 lb 0.375, 60 lb 0.45, 80 lb 0.60, 90 lb 0.675. About 6 pt of water per 80 lb bag to start, about 9 pt at most. For pours 2 in thick or more. Post holes about 3 times the post diameter and 1/3 of the post height deep.
- QUIKRETE Fast-Setting Concrete Mix (No. 1004) product data sheet, revised November 2025
Yields: 50 lb 0.375 ft³, 60 lb 0.45. States that two 50 lb bags set a 3 in square post in a 9 in diameter hole 30 in deep; fill the dry mix to 3 to 4 in below the top of the hole.
- SAKRETE High-Strength Concrete Mix technical data sheet (2023)
Yields: 40 lb 0.30 ft³, 60 lb 0.45, 80 lb 0.6, 90 lb 0.66. Flatwork: a minimum of 4 in of concrete over several inches of gravel. Posts: 3 to 4 in of gravel at the base of the hole.
- SAKRETE: Estimating bagged concrete jobs
It takes 45 bags of 80 lb mix to make a cubic yard; one cubic yard at 4 in deep covers 81 sq ft; add 10% to every estimate.
- NRMCA Concrete in Practice CIP 8: Discrepancies in Yield
Order 4 to 10% more than the plan volume. A 1/8 in difference in a 4 in slab is a 3% shortage. Hardened concrete is about 2% smaller than fresh. Re-measure near the end of the pour and avoid short loads.
- NRMCA Concrete in Practice CIP 31: Ordering Ready Mixed Concrete
Ready-mix is sold by the cubic yard of fresh concrete. Truck mixers carry 8 to 12 cubic yards. Order 4 to 10% over the plan estimate. A loaded truck weighs about 80,000 lb and may not cross residential curbs. 3,500 to 5,000 psi is typical.
- Michigan Concrete Association: Driveway & Sidewalk Design and Placement Guidelines
Driveways at least 4 in thick, 5 in where delivery vehicles use them, on at least 4 in of compacted sub-base. Control joints no farther apart than 2 times the slab thickness in feet (8 ft for 4 in) and 1/4 of the thickness deep.
- 2015 International Residential Code, Section R506 (as published by the City of Richmond, KY, Building Inspection)
R506.1: concrete slab-on-ground floors at least 3-1/2 in thick. R506.2.2: a 4 in base course of sand, gravel, or crushed stone where the slab is below grade. R506.2.3: no vapor retarder needed under driveways, walks, and patios.
FAQs
How do I calculate how much concrete I need?+
Multiply length × width × thickness, with the thickness in feet (inches ÷ 12), to get cubic feet. Divide by 27 for cubic yards, and add 4 to 10% extra as NRMCA advises. A 10 × 10 ft slab at 4 in is 33.33 cubic feet, 1.23 cubic yards, or 1.30 with 5% extra.
How many bags of concrete do I need for a 10 × 10 slab?+
At 4 in thick, 56 bags of 80 lb or 75 of 60 lb with no extra, and 59 or 78 with 5% extra. At 6 in thick it is 1.85 cubic yards: 84 bags of 80 lb before any extra.
How many 60 lb bags of concrete for a 10 × 10 slab?+
75 bags at 4 in thick with no extra (33.33 cubic feet ÷ 0.45), or 78 with 5% extra.
How many bags of concrete do I need for a 4 × 8 slab?+
At 4 in thick, 10.67 cubic feet: 18 bags of 80 lb or 24 of 60 lb with no extra, or 19 and 25 with 5% extra.
How many 80 lb bags of concrete make a yard?+
45. An 80 lb bag yields 0.60 cubic feet on both the QUIKRETE and SAKRETE data sheets, and 27 ÷ 0.60 = 45. It takes 60 bags of 60 lb, 72 of 50 lb, or 90 of 40 lb.
How much does a 50 lb bag of concrete cover?+
A 50 lb bag yields 0.375 cubic feet, so it covers about 1.1 sq ft at 4 in thick or 2.25 sq ft at 2 in. An 80 lb bag covers about 1.8 sq ft at 4 in.
How much will 2 yards of concrete cover at 4 inches thick?+
162 sq ft. One cubic yard covers 81 sq ft at 4 in, because 27 cubic feet ÷ (4 ÷ 12) ft = 81.
How many yards of concrete for a 20 × 20 slab?+
4.94 cubic yards at 4 in thick, or 5.19 with 5% extra. At 6 in it is 7.41 cubic yards before extra.
How many bags of concrete do I need per fence post?+
It depends on the hole. A 3-1/2 in square post in a 10 in hole 32 in deep leaves 1.23 cubic feet of concrete: 3 bags of 80 lb or 4 of 50 lb. QUIKRETE's Fast-Setting sheet says two 50 lb bags set a 3 in post in a 9 in hole 30 in deep. By the bag's printed yield (0.375 cubic feet), that hole, filled to 3-1/2 in below the top as QUIKRETE describes, holds 0.84 cubic feet, which is 2.2 bags. Buy the extra bag rather than run short mid-pour. QUIKRETE's Concrete Mix method also puts 6 in of mix in the bottom of the hole before the post goes in. To count that, enter 6 in as the gravel in the bottom, so the run around the post covers only the rest of the hole, then run the holes again with post size 0 and a 6 in hole depth, and add the two: 1.27 cubic feet per 10 in hole 32 in deep with a 3-1/2 in post.
How do I calculate concrete for a Sonotube or round pier?+
π × radius² × height, with the radius in feet. A 12 in tube 4 ft tall is 3.14 × 0.5² × 4 = 3.14 cubic feet, or 6 bags of 80 lb with 5% extra. An 8 in tube 4 ft tall is 1.40 cubic feet, or 3 bags.
How do I calculate concrete for steps?+
Count the risers, including the last step up onto the porch or landing. Each tread below that is a solid block down to the ground, so the side profile is run × rise × n(n − 1) ÷ 2 for n risers; add the landing's depth × its height if you are pouring one, then multiply by the width. Three 7 in risers with 11 in treads, 4 ft wide, up to a porch are 6.42 cubic feet; with a new 36 in landing, 27.42 cubic feet.
How thick should a concrete slab be?+
For a concrete floor slab in a house, the building code's minimum is 3-1/2 in (IRC R506.1); the code does not set one for patios or walks. The Michigan Concrete Association's minimum for driveways is 4 in, or 5 in where delivery vehicles will drive on it, and SAKRETE's data sheet calls for at least 4 in of concrete for flatwork. Follow your local code and plans.
How much extra concrete should I order?+
NRMCA, the ready-mix industry association, says to order at least 4 to 10% more than the volume from the plan dimensions, and more when a job alternates between slabs, footings, walls, and small fills. SAKRETE adds 10% to bagged estimates.
How much concrete is in a truck?+
Truck mixers carry 8 to 12 cubic yards, according to NRMCA. Ask your supplier for their truck size and any minimum or small-load charge.
Is it cheaper to buy ready-mix or mix bags yourself?+
It depends on local prices, so enter both in the calculator. One cubic yard is 45 bags of 80 lb, which is 3,600 lb to lift and mix by hand; a truck brings the same yard already mixed but may have a minimum order.
How much water do I need for an 80 lb bag of concrete?+
QUIKRETE's data sheet starts at about 6 pints (0.75 gallons) per 80 lb bag and expects no more than about 9 pints. SAKRETE lists 3.5 quarts (7 pints) for its 80 lb bag.
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