Concrete Calculator
- Updated Jul 2026
This free concrete calculator works out exactly how much concrete your project needs, in cubic yards, cubic feet and pre-mix bags. A cubic yard is 27 cubic feet and takes 45 × 80 lb bags. Enter your dimensions below for slabs, footings, columns, post holes, walls or stairs, plus an estimated cost.
Table of Contents
Concrete Calculator Tool
Enter Your Details
Fill in the fields, then press Calculate.
Fill in your information on the left to see your result.
Results are estimates for planning purposes. Verify quantities for structural or safety-critical work.
How to use this concrete calculator
Concrete is ordered two very different ways depending on job size, and the concrete calculator gives you both figures at once:
- Choose your shape, slab, footing, round column, post hole, wall, stairs or curb.
- Enter dimensions in feet or inches. Thickness is normally in inches; length and width in feet.
- Read both outputs. Cubic yards for ready-mix delivery, bag counts for DIY mixing.
- Add a waste margin and a price to see your total cost.
Running short mid-pour creates a cold joint that permanently weakens the slab, so the concrete calculator adds a safety margin by default rather than returning the bare minimum.
Concrete Calculation formula
Every calculation on this page reduces to volume, then a conversion. The concrete calculator runs these in sequence:
Cubic feet = Length (ft) × Width (ft) × Thickness (ft)Cubic yards = Cubic feet ÷ 27Shortcut for 4″ slabs: Cubic yards = Square feet ÷ 81
That last line is the one contractors use on site. A 4-inch slab is one-third of a foot thick, and 27 × 3 = 81, so dividing square footage by 81 gives cubic yards directly. A 200 sq ft patio at 4 inches needs 200 ÷ 81 = 2.47 yards.
Three numbers worth memorising: 1 cubic yard = 27 cubic feet = 45 × 80 lb bags, and weighs about 4,000 lb.
Concrete Calculators / Concrete Calculation for Every Project Type
Concrete work covers everything from a single fence post to a full foundation, and each job type measures differently. The sections below explain how the calculation changes for each application, so you know which inputs matter before you start measuring.
A concrete slab calculator handles the most common pour of all: a flat rectangular surface with uniform thickness. Multiply length by width by thickness in feet, then divide by 27 for cubic yards. Patios, walkways and shed pads sit at 4 inches, while garage floors move to 5 or 6. If your slab is an odd shape, split it into rectangles and total the sections.
Every concrete estimate starts as a volume calculation, whatever the shape. A concrete volume calculator finds cubic feet first, then converts to the unit your supplier uses. The concrete calculator formula stays constant — length × width × depth — with only the area formula changing between rectangles, circles and triangles. Get the volume right and every other figure follows.
For small pours you’ll buy pre-mix rather than ready-mix, and a concrete bag calculator converts your volume into a bag count. An 80 lb bag yields 0.60 cubic feet, so 45 bags make a cubic yard; 60 lb bags need 60, and 40 lb bags need 90. Knowing how many bags of concrete you need before you reach the store saves a second trip mid-pour.
A Quikrete concrete calculator and a Sakrete concrete calculator work identically, because both brands use the same standard bag sizes and near-identical yields. Quikrete’s 80 lb bag yields 0.60 cubic feet; Sakrete’s matches. Whichever brand your store stocks, the bag count stays the same — just check the printed yield on the bag, since fast-setting and high-strength formulas differ slightly.
Ready-mix suppliers quote in cubic yards, so a concrete yard calculator is what you need before phoning the plant. One cubic yard is 27 cubic feet and weighs around 4,000 lb. Most plants deliver in quarter-yard increments with a minimum of one yard, and charge a short-load fee below three — worth factoring in when your yardage sits close to that line.
Footings carry the load of walls and columns down to stable soil, so a concrete footing calculator works from trench dimensions: width × depth × total run. Depth is usually dictated by your local frost line rather than by choice, and a concrete footer calculator gives the same result under a different name. Always confirm the required depth with your building department before digging.
Fence and deck posts need a post hole concrete calculator that subtracts the post itself from the hole volume, since the timber displaces concrete. A 4×4 post in a 10-inch hole 30 inches deep needs roughly 1.15 cubic feet — about two 80 lb bags. Over-ordering here is harmless, so round up rather than risk a partly filled hole.
Round piers poured into cardboard forms use a sonotube concrete calculator, which is the same maths as any concrete cylinder calculator: π × radius² × height. Sonotubes come in standard diameters from 6 to 24 inches, and because the form is round, volume climbs fast with diameter — a 12-inch tube holds four times the concrete of a 6-inch one at the same depth.
Poured walls use a concrete wall calculator working from length × height × thickness. Foundation walls typically run 8 to 10 inches thick, retaining walls thicker still depending on the load behind them. Remember to deduct window and door openings from the total, and calculate the footing beneath the wall as a separate pour.
Block walls need two numbers, and a concrete block calculator gives both: the block count and the concrete needed to fill the cores. A standard 8×8×16 block covers 0.89 square feet with its mortar joint, so a 100 square foot wall takes 113 blocks. Filling those cores adds roughly one cubic yard of concrete per 100 blocks.
Once you know your yardage, a concrete cost calculator turns it into a budget. Ready-mix runs roughly $125 to $180 per cubic yard for material alone, while a finished slab installed by a contractor costs $6.50 to $10.50 per square foot. A concrete price calculator is only as good as the local rate you feed it, so get a quote from your nearest plant first.
A concrete slab cost calculator combines volume with a per-yard or per-square-foot rate. For DIY pours, cost is simply yardage × price per yard plus any short-load fee. For contracted work, expect the installed rate to include base preparation, forms, reinforcement and finishing — which is why a concrete pad cost calculator returns a much higher figure than material alone.
Driveways are the largest residential pour most people ever order, so a concrete driveway calculator matters. A single-car drive at 10 × 20 feet and 4 inches thick needs about 2.5 cubic yards. Tapered drives should be measured at the average width. A concrete driveway cost calculator adds reinforcement and a thicker slab, pushing installed rates to $8–$15 per square foot.
Foundations combine footings and walls, so a concrete foundation calculator totals both pours separately before adding them. Measure the footing trench first, then the wall above it. Because foundations are structural and usually engineer-specified, a concrete foundation cost calculator should be treated as a planning estimate only — final quantities come from the approved drawings.
Batching from raw materials rather than bags calls for a concrete mix calculator. The standard general-purpose ratio is 1:2:4 — one part cement, two parts sand, four parts aggregate — yielding around 3,000 PSI. A 1:2:4 concrete mix calculator scales those proportions to your total volume: one cubic yard needs roughly six 94 lb bags of cement, 0.45 yd³ of sand and 0.9 yd³ of aggregate.
For anything above a cubic yard, a ready mix concrete calculator is the right tool. Plants batch to your specified strength and deliver by truck, typically carrying up to 10 cubic yards. Ordering ready-mix avoids the cold joints that form when hand-mixed batches set at different times, which is the main reason it beats bags on quality as well as cost.
Weight matters for site access and structural loading, and a concrete weight calculator converts volume into pounds or tons. Standard concrete weighs about 150 lb per cubic foot, which works out to roughly 4,000 lb — two tons — per cubic yard. A fully loaded 10-yard mixer truck therefore carries around 40,000 lb, enough to crack an existing driveway on the way in.
Interior floor levelling uses a thin pourable compound rather than structural concrete, so a self leveling concrete calculator works in much smaller depths — typically ⅛ to 1 inch. Coverage is quoted per bag at a given thickness, usually around 25 square feet per 50 lb bag at ¼ inch. Measure the deepest low spot, not the average, or you’ll come up short.
Recycled concrete broken down into aggregate makes a cheap, well-compacting base material, and a crushed concrete calculator estimates it much like gravel. Density runs around 2,400 lb per cubic yard, slightly lighter than virgin crushed stone. It compacts tightly thanks to its fines, making it a good sub-base under driveways and pads where appearance doesn’t matter.
Concrete shapes and formulas
Not every pour is a rectangle. The concrete calculator covers eight shapes, and the formulas below let you verify any result by hand.
| Shape | Volume formula (cubic feet) | Typical use |
|---|---|---|
| Slab / pad | Length × Width × Thickness | Patios, driveways, garage floors |
| Footing | Width × Depth × Length | Foundations, wall bases |
| Round column | π × (Diameter ÷ 2)² × Height | Sonotube piers, posts |
| Post hole | Hole volume − Post volume | Fence and deck posts |
| Wall | Length × Height × Thickness | Retaining and foundation walls |
| Stairs | Σ (Run × Rise × Width) per step | Porch and entry steps |
| Curb & gutter | Profile area × Length | Driveway edges, kerbing |
| Tube / ring | π × (R outer² − R inner²) × Height | Circular slabs, ring beams |
For irregular pours, split the area into rectangles and add the volumes, our square footage calculator handles the area step, and the concrete calculator turns each section into yards.
Worked examples
How much concrete for a 10×10 slab?
| Thickness | Cubic yards | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 4 inches | 1.23 yd³ | 56 bags | 75 bags |
| 5 inches | 1.54 yd³ | 70 bags | 93 bags |
| 6 inches | 1.85 yd³ | 84 bags | 112 bags |
At 4 inches: 10 × 10 × 0.333 = 33.3 cubic feet, ÷ 27 = 1.23 cubic yards. Or use the shortcut — 100 sq ft ÷ 81 = 1.23. The concrete calculator applies whichever route matches your inputs.
Post hole example
Post holes are the one shape most tools get wrong, because the post itself displaces concrete. For a 4×4 post in a 10-inch hole, 30 inches deep:
- Hole volume: π × (5 ÷ 12)² × 2.5 = 1.36 cu ft
- Post volume: 0.29 × 0.29 × 2.5 = 0.21 cu ft
- Concrete needed: 1.36 − 0.21 = 1.15 cu ft ≈ 2 × 80 lb bags
Check your local code before pouring footings or post holes. Minimum depth is usually set by the local frost line, and requirements vary considerably by region. The concrete calculator gives you quantities, not code compliance.
How many bags of concrete do I need?
Pre-mix bags are sold by weight, but concrete is calculated by volume, so the first thing the concrete calculator does is convert your yardage into bag counts at whatever size you’re buying.
| Bag size | Yield per bag | Bags per cubic yard | Covers at 4″ | Common brands |
|---|---|---|---|---|
| 40 lb | 0.30 cu ft | 90 bags | ~0.9 sq ft | Quikrete, Sakrete |
| 50 lb | 0.375 cu ft | 72 bags | ~1.1 sq ft | Fast-setting mixes |
| 60 lb | 0.45 cu ft | 60 bags | ~1.35 sq ft | Quikrete, Sakrete |
| 80 lb | 0.60 cu ft | 45 bags | ~1.8 sq ft | Quikrete, Sakrete |
Yields are approximate and vary slightly by brand and mix type — Quikrete and Sakrete both print the exact figure on the bag. Enter your bag size in the concrete calculator and it returns the count for your specific pour.
Bags or ready-mix: which should you order?
| Pre-mix bags | Ready-mix delivery | |
|---|---|---|
| Best for | Under ~1 cubic yard | Over ~1 cubic yard |
| Typical jobs | Post holes, small pads, repairs | Driveways, patios, foundations |
| Cost per yard | $140–$220 in bags | $125–$180 delivered |
| Labour | Mixing 45+ bags by hand | Poured straight from the chute |
| Watch for | Cold joints between batches | Short-load fees under 3 yards |
The break-even sits around one cubic yard. Below that, bags win on convenience; above it, ready-mix wins on both cost and pour quality. Note that most suppliers charge a short-load fee under three yards, which the concrete calculator’s cost field lets you factor in.
Never order the exact amount. A pour that runs short creates a cold joint — a permanent weak seam. Contractors add 5–10%; on a 20-yard order, finishing with half a yard spare is a good order.
How thick should a concrete slab be?
| Application | Thickness | Base | Reinforcement |
|---|---|---|---|
| Walkway, patio | 4″ | 4″ gravel | Optional wire mesh |
| Standard driveway | 4″ | 4–6″ gravel | Wire mesh or rebar |
| Garage floor | 5″ | 4–6″ gravel | Rebar grid |
| Heavy driveway, RV pad | 6″ | 6″ gravel | #4 rebar at 18″ |
| Shed or workshop pad | 4–6″ | 4″ gravel | Wire mesh |
| Commercial / equipment | 8″+ | 6–8″ gravel | Engineered rebar |
Every slab needs a compacted base beneath it, our gravel calculator works out the tonnage for that layer once you know the depth.
Concrete strength: which PSI do you need?
| Strength | Use | Notes |
|---|---|---|
| 2,500 PSI | Non-structural fill, footings in mild climates | Minimum for most residential work |
| 3,000 PSI | Patios, walkways, footings, foundation walls | Standard residential mix |
| 3,500–4,000 PSI | Driveways, garage floors, exterior slabs | Better freeze-thaw resistance |
| 4,500+ PSI | Heavy traffic, commercial, structural | Usually engineer-specified |
PSI doesn’t change the volume the concrete calculator returns, only the price per yard and the mix design. In freeze-thaw regions, ask for air-entrained concrete regardless of strength grade.
Concrete mix ratios
If you’re batching from raw cement, sand and aggregate rather than pre-mix bags, these are the standard proportions by volume:
| Ratio | Cement : Sand : Aggregate | Approx. strength | Use |
|---|---|---|---|
| 1:2:4 | 1 part : 2 parts : 4 parts | ~3,000 PSI | General purpose — slabs, footings, paths |
| 1:1.5:3 | 1 part : 1.5 parts : 3 parts | ~3,500–4,000 PSI | Driveways, structural beams, columns |
| 1:3:6 | 1 part : 3 parts : 6 parts | ~2,000 PSI | Mass fill, non-structural work |
| 1:1:2 | 1 part : 1 part : 2 parts | ~4,500+ PSI | High-strength structural elements |
For a 1:2:4 mix, one cubic yard needs roughly 6 bags of cement (94 lb each), 0.45 yd³ of sand and 0.9 yd³ of aggregate. Water sits around 0.5 by weight of cement, more water means easier placement but weaker concrete. Once you know your total yardage from the concrete calculator, multiply each component proportionally.
How much does concrete cost?
Ready-mix is quoted per cubic yard; finished work is quoted per square foot. The concrete calculator handles both once you enter a rate.
| Item | Typical cost | Notes |
|---|---|---|
| Ready-mix concrete | $125–$180 per yd³ | Material only, delivered |
| Short-load fee | $50–$150 | Applies under ~3 yards |
| 80 lb pre-mix bag | $5–$8 each | ≈ $225–$360 per yard equivalent |
| Plain slab, installed | $6.50–$10.50 per sq ft | Includes labour, base, finishing |
| Stamped / decorative | $12–$25 per sq ft | Color, pattern, sealing |
| Driveway, installed | $8–$15 per sq ft | Thicker slab plus reinforcement |
Prices vary sharply by region and season. Enter your local quote into the concrete calculator for an accurate figure, and if a supplier offers a volume discount our discount calculator shows what it saves across the full order.
How much does concrete weigh?
| Quantity | Weight | Equivalent |
|---|---|---|
| 1 cubic foot | ~150 lb | Standard mix, cured |
| 1 cubic yard | ~4,000 lb | 2 tons |
| 4″ slab, 100 sq ft | ~5,000 lb | 1.23 yd³ |
| Full mixer truck (10 yd³) | ~40,000 lb | 20 tons |
Weight matters for access: a loaded mixer truck can crack an existing driveway or sink into soft ground, so confirm the approach before delivery day.
Concrete block walls
Block walls need two separate figures, the blocks themselves, and the concrete or mortar filling them.
| Block size | Blocks per 100 sq ft of wall | Concrete to fill cores |
|---|---|---|
| 8″ × 8″ × 16″ | 113 blocks | ~1.0 yd³ per 100 blocks |
| 6″ × 8″ × 16″ | 113 blocks | ~0.75 yd³ per 100 blocks |
| 12″ × 8″ × 16″ | 113 blocks | ~1.5 yd³ per 100 blocks |
Standard blocks cover about 0.89 sq ft each including the mortar joint, so 100 ÷ 0.89 = 113 blocks per 100 square feet. Use the wall shape in the concrete calculator for the footing beneath, then add core fill separately.
Concrete block walls
Block walls need two separate figures, the blocks themselves, and the concrete or mortar filling them.
| Block size | Blocks per 100 sq ft of wall | Concrete to fill cores |
|---|---|---|
| 8″ × 8″ × 16″ | 113 blocks | ~1.0 yd³ per 100 blocks |
| 6″ × 8″ × 16″ | 113 blocks | ~0.75 yd³ per 100 blocks |
| 12″ × 8″ × 16″ | 113 blocks | ~1.5 yd³ per 100 blocks |
Standard blocks cover about 0.89 sq ft each including the mortar joint, so 100 ÷ 0.89 = 113 blocks per 100 square feet. Use the wall shape in the concrete calculator for the footing beneath, then add core fill separately.
Common concrete estimating mistakes
- Ordering the exact calculated amount. Running short mid-pour creates a permanent cold joint.
- Forgetting the post displaces concrete in post holes — though over-ordering here is harmless.
- Mixing feet and inches. Thickness in inches must be divided by 12 before multiplying.
- Ignoring uneven subgrade. Add ¼ inch to slab thickness in your budget to cover dips.
- Skipping the gravel base, which leads to settling and cracking.
- Not planning for short-load fees on orders under three yards.
Adding a waste margin in the concrete calculator covers most of these before the truck arrives.
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Frequently asked questions
How do I calculate how much concrete I need?
Multiply length × width × thickness in feet, then divide by 27 for cubic yards. For a 4-inch slab, divide the square footage by 81 instead, it gives the same answer in one step.
How many 80 lb bags of concrete are in a cubic yard?
45 bags. Each 80 lb bag yields about 0.60 cubic feet, and a cubic yard is 27 cubic feet.
How much concrete do I need for a 10x10 slab?
1.23 cubic yards at 4 inches thick, which is 56 × 80 lb bags. At 6 inches it rises to 1.85 cubic yards or 84 bags.
How thick should a concrete slab be?
Both are abbreviations for board feet. BDFT and BF are shorthand; FBM stands for “foot, board measure.” A thousand board feet is written MBF, MFBM, or MBFT.
How much does a cubic yard of concrete weigh?
Four inches for patios, walkways and standard driveways; five inches for garage floors; six inches or more for heavy vehicles and RV pads.
Should I use bags or order ready-mix?
Bags make sense under about one cubic yard. Above that, ready-mix is cheaper, faster and avoids cold joints between hand-mixed batches.
What is a 1:2:4 concrete mix?
One part cement to two parts sand to four parts aggregate by volume, yielding roughly 3,000 PSI. It’s the standard general-purpose ratio for slabs, footings and paths.
How much concrete do I need for a post hole?
Calculate the hole volume, then subtract the post. A 4×4 post in a 10-inch hole 30 inches deep needs about 1.15 cubic feet — roughly two 80 lb bags.
Sources and further reading: Strength grades and structural requirements on this page follow ACI 318 building code requirements, and ordering guidance aligns with the National Ready Mixed Concrete Association. Always confirm footing depth and reinforcement with your local building department before pouring.