Concrete Calculator
Calculate how much concrete you need for a slab, footing or column — enter your dimensions to get the volume in cubic yards, the number of 40, 60 or 80 lb bags, and an estimated cost.
United States — national average · 2026 prices
Material only unless stated as installed. Excludes tax, permits and local labour. Verify with local suppliers — see how we calculate.
How this calculator works
Every figure above comes from the formula below, applied to the assumptions listed. Nothing is looked up from a hidden table — you can reproduce the result by hand.
Formula
area (sq ft) = length × width · square: side² · round: π × (diameter ÷ 2)²
cubic feet = area × thickness (in) ÷ 12, then × (1 + waste ÷ 100)
cubic yards = cubic feet ÷ 27
bags = cubic feet ÷ bag yield (80 lb = 0.60 · 60 lb = 0.45 · 40 lb = 0.30 cu ft)
Assumptions & limits
- Bag yields are the published QUIKRETE Concrete Mix figures: 0.60 cu ft per 80 lb bag, 0.45 per 60 lb, 0.30 per 40 lb.
- The gravel sub-base is fixed at 4 inches compacted, the usual residential slab detail.
- Welded wire mesh is costed at one 6×6 sheet per 42 sq ft of slab.
- Ready-mix is priced at $140–$165 per cubic yard and forms at $1.40–$2.40 per lineal foot of perimeter.
- Default waste allowance is 5%. The volume shown is concrete only — it does not subtract the volume of a post or pipe cast into it.
- Reinforcement is quantified, not engineered. Bar size, spacing and slab thickness for a load-bearing slab are a design decision, not an estimate.
Worked example
A 20 ft × 16 ft patio slab, 4 inches thick, 5% waste, with mesh and gravel base
- Area = 20 × 16 = 320 sq ft
- Cubic feet = 320 × 4 ÷ 12 = 106.67, plus 5% waste = 112.0 cu ft
- Cubic yards = 112.0 ÷ 27 = 4.15 yd³
- Bags if bagging instead = 112.0 ÷ 0.60 = 187 × 80 lb (or 249 × 60 lb, 374 × 40 lb)
- Gravel base = 320 × 4 ÷ 12 ÷ 27 = 3.95 yd³ · mesh = 320 ÷ 42 = 7.6 sheets
Result:4.15 cubic yards of concrete — about $968 to $1,352 including mesh, gravel base and forms.
Built and maintained by Christoph Ballmer, Honeygoat GmbH
Method and constants last checked against the calculator code on August 17, 2026. Independent trade review is not yet in place for this calculator — see how we handle review.
Read the site-wide calculation methodology · estimate vs. quote and safety limits
How to calculate concrete
Concrete is ordered by volume — cubic yards for ready-mix delivered by truck, or bags for smaller pours. The math is the same for any job: multiply length × width × thickness with every dimension in feet, which gives cubic feet, then divide by 27 to convert to cubic yards (one cubic yard is 27 cubic feet). The calculator above does this for the three shapes that cover almost every residential pour — flat slabs, wall and pad footings, and round columns or post holes — and adds a waste allowance automatically.
cubic feet = length (ft) × width (ft) × thickness (ft) · cubic yards = cubic feet ÷ 27
The one step people get wrong is the thickness. Slab thickness is given in inches, so convert it to feet first by dividing by 12: a 4-inch slab is 4 ÷ 12 = 0.333 ft, a 6-inch slab is 0.5 ft. Skip that and your estimate is off by a factor of twelve.
Worked example: a 20 × 24 ft driveway
Say you are pouring a two-car driveway, 20 ft long by 24 ft wide, at the standard residential thickness of 4 inches. Here is the full calculation, start to finish:
- Area: 20 ft × 24 ft = 480 sq ft
- Thickness in feet: 4 in ÷ 12 = 0.333 ft
- Volume: 480 × 0.333 = 160 cubic feet
- Cubic yards: 160 ÷ 27 = 5.93 yd³
- Add 10% waste: 5.93 × 1.10 ≈ 6.5 yd³ — round up and order 6.5 cubic yards of ready-mix
At a 2026 ready-mix price of roughly $140 per cubic yard, that material runs about 6.5 × $140 ≈ $910, before delivery and any short-load fee. A driveway this size is firmly in ready-mix territory — bagging 6.5 yards would mean mixing close to 290 eighty-pound bags by hand. To price your own pour with delivery and fees, use the concrete cost calculator.
How many bags of concrete in a cubic yard?
Each bag lists a yield in cubic feet, and a cubic yard is 27 cubic feet, so the bag count is just 27 ÷ yield. Use this table to cross-check the calculator or to size a job you are bagging yourself:
| Bag size | Yield | Bags per ¼ yd³ | Bags per ½ yd³ | Bags per cubic yard |
|---|---|---|---|---|
| 80 lb | ≈ 0.60 cu ft | 12 | 23 | 45 |
| 60 lb | ≈ 0.45 cu ft | 15 | 30 | 60 |
| 50 lb | ≈ 0.375 cu ft | 19 | 38 | 72 |
| 40 lb | ≈ 0.30 cu ft | 23 | 45 | 90 |
Yields are typical for standard concrete mix; check the printed yield on your bag, which varies a little by brand.
Concrete for a slab — quick reference
| Slab size | 4 in thick | 6 in thick |
|---|---|---|
| 10 × 10 ft | 1.23 yd³ | 1.85 yd³ |
| 12 × 12 ft | 1.78 yd³ | 2.67 yd³ |
| 20 × 20 ft | 4.94 yd³ | 7.41 yd³ |
Volumes before waste — add 10% for spillage and an uneven subgrade. Enter your exact dimensions in the calculator above for the precise figure.
A handy field shortcut for 4-inch slabs: divide the square footage by 81 to get cubic yards (a yard of concrete covers 81 sq ft at 4 in). For a 6-inch slab the divisor is 54, and for a full 12-inch pour it is 27. So 480 sq ft ÷ 81 ≈ 5.9 yd³ — the same answer as the driveway example above.
What strength (PSI) concrete do you need?
Volume tells you how much; PSI tells you which mix to order. PSI is the cured compressive strength, and the right grade depends on the load and your climate. In freeze-thaw regions the American Concrete Institute's ACI 318 code calls for at least 4,000 PSI air-entrained concrete on anything exposed to deicing salt, such as a driveway. This is the choice that protects a slab long-term, so it is worth getting right before the truck arrives.
| Mix strength | Typical use |
|---|---|
| 2,500–3,000 PSI | Sidewalks, patios, light walkways, fence-post footings |
| 3,000–3,500 PSI | Standard residential slabs, garage floors, foundation footings |
| 4,000 PSI | Driveways, RV pads, workshops, any slab in a freeze-thaw climate (ACI 318 minimum) |
| 4,500–5,000+ PSI | Heavy-traffic and commercial slabs, structural columns |
Cost and weight at a glance
| Item | Typical 2026 range |
|---|---|
| Ready-mix concrete (delivered) | $125–$165 per cubic yard |
| 80 lb bag of concrete mix | $6–$15 per bag |
| 60 lb bag of concrete mix | $4–$14 per bag |
| Short-load fee (under ~1 yd³) | $50–$120 |
| Finished slab, formed and poured | $6–$12 per sq ft |
| Weight of cured concrete | ≈ 150 lb per cu ft (≈ 4,050 lb per yd³) |
Cost figures are estimates and vary by region, mix strength, fuel and material prices, and 2026 market conditions — always confirm with a local supplier.
Weight matters for ordering too: a standard ready-mix truck holds about 8 to 10 cubic yards, and one yard weighs roughly two tons, so plan access for a heavy vehicle on a large pour.
Waste factor: why you order a little extra
Always order 10–15% more than the bare volume. Concrete is unforgiving: running short mid-pour creates a cold joint — a weak seam where fresh concrete meets a batch that has already started to set — and there is no clean way to fix it. The extra covers an uneven or over-excavated subgrade, spillage off the chute, and the bit left in the truck. On a thin slab, every quarter-inch the subgrade dips below grade adds real volume across the whole area, which is why a 10% cushion is the sensible floor. The calculator defaults to a 10% allowance you can adjust.
Common mistakes to avoid
- Forgetting to convert thickness to feet. A 4-inch slab is 0.333 ft, not 4 ft — the single most common ten-fold error.
- Mixing units. Keep every dimension in feet before multiplying; convert inches first.
- Ordering the exact volume. Skip the waste factor and you risk a cold joint. Round up.
- Choosing the wrong PSI. A 3,000 PSI patio mix in a freeze-thaw driveway will spall and crack within a few winters.
- Ignoring the subgrade. Volume math assumes a flat, compacted base; a soft or dished subgrade quietly eats extra yards.
- Bagging a big job. Past about one cubic yard, mixing bags by hand is slow, uneven and rarely cheaper than ready-mix.
When to hire a pro
The calculator sizes the order; pouring it well is a separate skill. For a small pad, a set of fence-post footings, or a repair under about half a yard, a confident DIYer with bagged mix is fine. Bring in a concrete contractor when the job is structural (a foundation or load-bearing footing), when it is large enough to need ready-mix and a finishing crew working against the clock, when local code requires rebar, a vapor barrier or an inspection, or when the finish matters — a driveway, garage floor or stamped patio shows every trowel mistake. A pro also handles formwork, subgrade compaction and curing, the steps that decide whether a slab lasts decades or cracks in a year.
Sizing the cost or the bag count instead? Use the concrete cost calculator to price a pour by the yard, or the concrete bags calculator if you are bagging it yourself. Comparing driveway surfaces before you commit? See the asphalt calculator.
These figures are estimates only and do not replace project-specific advice — verify quantities, mix strength and cost with a local concrete supplier or licensed contractor before you order.
Frequently asked questions
What PSI of concrete do I need for a driveway?
For a residential driveway, order at least 4,000 PSI concrete. In any region with freeze-thaw winters, the ACI 318 building code treats 4,000 PSI air-entrained concrete as the minimum for surfaces exposed to deicing salt. Sidewalks and patios can use 3,000 PSI, and standard slabs or footings 3,000–3,500 PSI, but a driveway carries vehicle loads and weather, so step up to 4,000 PSI.
How much does a cubic yard of concrete weigh?
Cured concrete weighs about 150 pounds per cubic foot, so one cubic yard (27 cubic feet) comes to roughly 4,050 pounds — just over two tons. A standard ready-mix truck holds about 8 to 10 cubic yards, which means a full load can weigh 30,000–40,000 pounds. Plan driveway and yard access for a heavy vehicle on any large pour.
What is the 81 rule for concrete?
It is a field shortcut for 4-inch slabs: one cubic yard of concrete covers 81 square feet at 4 inches thick, so dividing your total square footage by 81 gives the cubic yards needed. A 480 sq ft slab ÷ 81 ≈ 5.9 yd³. For a 6-inch slab divide by 54 instead, and for a 12-inch pour divide by 27. Add a 10% waste allowance to whatever the shortcut gives.
How long does concrete take to cure?
Concrete is usually firm enough to walk on after 24–48 hours, but it keeps gaining strength for weeks. It reaches roughly 70% of its design strength in about 7 days and over 90% by 28 days, which is why engineers quote PSI at 28 days. Keep new concrete damp during the first week, and wait the full curing period before driving vehicles onto a slab or driveway.
How deep should a concrete footing or fence post be?
For footings, depth is set by your local frost line — the footing base must sit below it, often 12–48 inches depending on climate, and codes also specify a minimum width. For a typical fence post, dig a hole about three times the post width and one-third to one-half the post height deep; a 6-foot-tall post commonly sits in a hole 2–3 feet deep. Enter the hole as a round column in the calculator above to size the concrete, and subtract the post itself for a closer figure.
How much concrete do I need?
Multiply length × width × thickness (all in feet) to get cubic feet, then divide by 27 for cubic yards. For a slab, convert the thickness from inches to feet first (4 inches = 0.333 ft). The calculator above does this for slabs, footings and round columns, then converts the volume into bags and ready-mix yards with a waste allowance.
How many bags of concrete are in a cubic yard?
A cubic yard is 27 cubic feet. Since an 80 lb bag yields about 0.60 cu ft, a 60 lb bag about 0.45 cu ft, and a 40 lb bag about 0.30 cu ft, one cubic yard takes roughly 45 × 80 lb bags, 60 × 60 lb bags, or 90 × 40 lb bags. The calculator shows the bag count for each size.
How much concrete do I need for a 10×10 slab?
A 10 ft × 10 ft slab is 100 sq ft. At 4 inches thick that is 100 × 0.333 = 33.3 cubic feet, or about 1.23 cubic yards — roughly 56 × 80 lb bags. At 6 inches it is about 1.85 cubic yards. Enter your exact thickness above for the precise figure.
Should I use bags or order ready-mix?
Bagged concrete is usually the cheaper, simpler choice for small jobs up to about half a cubic yard — fence posts, small pads, repairs. For anything from roughly one cubic yard up, ready-mix delivered by truck is normally cheaper per yard and far less work than mixing dozens of bags. The calculator shows both so you can compare.
How much extra concrete should I add for waste?
Add about 5–10% extra to cover spillage, uneven subgrade and over-excavation. The calculator defaults to a 10% allowance, which you can adjust. Running a little short mid-pour means a cold joint, so ordering slightly over is the safer choice.