Concrete Calculator
Work out exactly how much concrete you need for a slab, footing, wall, column, round pad, tube, steps or curb and gutter. Get cubic yards, bag counts, a ready-mix vs bag price comparison and, if you mix on site, the cement, sand and gravel you need. Add several parts to total a whole project.
What are you pouring?
4 in for patios and walks, 5–6 in for driveways and garage floors.
Typical US prices. Replace them with your local quotes for a real comparison.
Concrete needed, including 10% waste
- Cubic feet
- 36.7
- Cubic metres
- 1.04
- Weight
- 5,500 lb
Order 1.5 yd³ of ready-mix
Or in bags: 62 × 80 lb · 82 × 60 lb · 123 × 40 lb
| Option | Estimated cost |
|---|
Mixing it yourself? Cement, sand & gravel
Uses dry volume = wet volume × 1.54. Water is roughly half the cement weight (w/c 0.45–0.55).
Everything is calculated in your browser. Nothing you type is sent or stored.
Concrete volume is length × width × thickness, with every measurement in feet, divided by 27 to give cubic yards. A 10 × 10 ft slab at 4 inches is 10 × 10 × 0.333 = 33.3 cubic feet, or 1.23 cubic yards. Add 10% for waste (1.36 yd³), then order 1.5 yards of ready-mix or 62 bags of 80 lb concrete.
How to use this concrete calculator
- Pick the shape you are pouring. Slabs, patios, driveways and pads are a slab. Strip footings and trenches are a footing. Post holes, piers and Sonotubes are a column. Use round slab for circular pads and hollow tube for pipe-shaped forms.
- Enter the dimensions. Lengths go in feet (or metres) and thickness in inches (or centimetres). The calculator converts everything, so you never divide by 12 yourself.
- Set how many identical pieces you need, such as 12 post holes or 6 pier footings, and a waste allowance. 10% is a safe default.
- Read the order. You get cubic yards, cubic feet, cubic metres, weight, ready-mix rounded to the quarter yard, and bag counts for 40, 60 and 80 lb bags.
- Building more than one shape? Press Add to project after each part. A patio slab plus its footings plus the step at the door add up to one order total.
Concrete formulas for every shape
Every concrete estimate is a volume calculation. Convert all dimensions to feet first: divide inches by 12. Then divide the cubic feet by 27 to get cubic yards.
| Shape | Formula (cubic feet) | Example |
|---|---|---|
| Slab, patio, driveway | Length × Width × Thickness | 12 × 12 ft × 4 in = 48 ft³ (1.78 yd³) |
| Strip footing | Length × Footing width × Depth | 40 ft × 12 in × 8 in = 26.7 ft³ (0.99 yd³) |
| Wall | Length × Height × Thickness | 20 ft × 4 ft × 8 in = 53.3 ft³ (1.98 yd³) |
| Column, post hole, pier | π × (Diameter ÷ 2)² × Depth | 12 in × 3 ft = 2.36 ft³ each |
| Round slab | π × (Diameter ÷ 2)² × Thickness | 10 ft × 4 in = 26.2 ft³ (0.97 yd³) |
| Hollow tube | π × (R² − r²) × Height | 24 in outer, 18 in inner × 4 ft = 5.5 ft³ |
| Steps | Width × Run × Rise × n(n+1) ÷ 2 | 3 steps, 4 ft wide, 7 in rise, 11 in run = 12.8 ft³ |
| Curb & gutter | Length × (Curb width × Curb height + Gutter width × Gutter thickness) | 50 ft, 6 × 18 in curb + 24 × 6 in gutter = 87.5 ft³ (3.24 yd³) |
The one number to remember is 27. A cubic yard is a cube 3 feet on each side, and 3 × 3 × 3 = 27 cubic feet. Dividing by 3 instead of 27 is the classic mistake, and it makes an order nine times too big.
How many bags of concrete do I need?
Bagged concrete is sold by weight but used by volume. The yield printed on the bag is what matters. Standard pre-mixed concrete yields about 0.60 ft³ per 80 lb bag, 0.45 ft³ per 60 lb bag and 0.30 ft³ per 40 lb bag. Divide the cubic feet you need by the yield, then round up to a whole bag.
| Bag size | Yield per bag | Bags per ft³ | Bags per yd³ |
|---|---|---|---|
| 80 lb | 0.60 ft³ | 1.67 | 45 |
| 60 lb | 0.45 ft³ | 2.22 | 60 |
| 50 lb | 0.375 ft³ | 2.67 | 72 |
| 40 lb | 0.30 ft³ | 3.33 | 90 |
Bag counts for common jobs before waste, at 80 lb:
| Project | Volume | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 4 × 4 ft pad, 4 in | 5.3 ft³ | 9 | 12 |
| 8 × 8 ft slab, 4 in | 21.3 ft³ | 36 | 48 |
| 10 × 10 ft slab, 4 in | 33.3 ft³ | 56 | 75 |
| 10 × 12 ft slab, 4 in | 40.0 ft³ | 67 | 89 |
| One 10 in post hole, 2 ft deep | 1.09 ft³ | 2 | 3 |
| One 12 in pier, 4 ft deep | 3.14 ft³ | 6 | 7 |
For post holes, these figures ignore the post itself. A buried 4×4 (3.5 in square) displaces about 0.17 ft³ per 2 feet of depth, so you need slightly less. The fence post concrete calculator subtracts it for you.
How much area does a yard of concrete cover?
One cubic yard spread at a given thickness covers 324 ÷ thickness (in inches) square feet. This is the fastest way to sanity-check a quote or a calculator result.
| Thickness | Coverage per yd³ | Typical use |
|---|---|---|
| 3 in | 108 sq ft | Overlays, shed floors on good base |
| 4 in | 81 sq ft | Patios, sidewalks, shed and AC pads |
| 5 in | 65 sq ft | Driveways (cars), garage floors |
| 6 in | 54 sq ft | Heavy driveways, RV pads, aprons |
| 8 in | 40.5 sq ft | Equipment pads, commercial floors |
Square feet to cubic yards of concrete
Contractors quote flatwork in square feet, while suppliers sell by the cubic yard. To convert, multiply the area in square feet by the thickness in inches, then divide by 324. For example, a 600 sq ft driveway at 5 inches needs 600 × 5 ÷ 324 = 9.26 yd³ (about 10.2 yd³ with waste, so roughly one full truck).
| Area | 4 in thick | 5 in thick | 6 in thick |
|---|---|---|---|
| 100 sq ft | 1.23 yd³ | 1.54 yd³ | 1.85 yd³ |
| 200 sq ft | 2.47 yd³ | 3.09 yd³ | 3.70 yd³ |
| 400 sq ft | 4.94 yd³ | 6.17 yd³ | 7.41 yd³ |
| 600 sq ft | 7.41 yd³ | 9.26 yd³ | 11.11 yd³ |
| 1,000 sq ft | 12.35 yd³ | 15.43 yd³ | 18.52 yd³ |
How to calculate 1 m³ of concrete (cement, sand and aggregate)
When you mix concrete on site from cement, sand and gravel (common for RCC work in South Asia, the UK and anywhere ready-mix is not practical), the ingredients have to fill more than the finished volume. Dry materials contain air gaps that disappear once water is added. The standard allowance is to multiply the wet volume by 1.54 to get the dry volume, then split it by the mix ratio.
- Wet volume
- 1 m³
- Dry volume (× 1.54)
- 1.54 m³
- Mix 1 : 2 : 4 (7 parts)
- 1.54 ÷ 7 = 0.22 m³ per part
- Cement: 0.22 m³ × 1,440 kg/m³
- 317 kg = 6.3 × 50 kg bags
- Sand: 2 parts
- 0.44 m³ (15.5 ft³)
- Aggregate: 4 parts
- 0.88 m³ (31.1 ft³)
For a stronger 1 : 1.5 : 3 (M20) mix, the same 1 m³ needs about 8 bags of cement, 0.42 m³ of sand and 0.84 m³ of aggregate. The calculator’s Mixing it yourself panel does this for any shape and ratio. For a deeper explanation, see why dry volume uses the 1.54 factor and the concrete mix ratio calculator.
Bags or ready-mix: which is cheaper?
At around $6.50 for an 80 lb bag, one cubic yard of bagged concrete (45 bags) costs about $290 in material alone, before you rent a mixer or lift 3,600 lb by hand. Ready-mix commonly runs $150–$200 per yard in the US, but small loads often carry a short-load or delivery fee. Use the cost comparison in the calculator with your own local quotes.
- Under about 0.5 yd³ (up to roughly 20 bags)Bags win: post holes, small pads, steps and repairs.
- About 0.5–1 yd³Close call. Bags are cheaper on paper, but a truck saves a full day of mixing and gives a more consistent pour.
- Over about 1 yd³Ready-mix is almost always cheaper per yard and far less work. For anything over 2 yd³, order a truck.
A standard mixer truck carries roughly 8–10 yd³. Suppliers usually sell in quarter- or half-yard increments, which is why the calculator rounds your order up.
How thick should concrete be (and what strength)?
Thickness is set by load, not by area. Strength is measured in PSI after 28 days of curing.
| Project | Thickness | Typical strength |
|---|---|---|
| Sidewalks, patios, paths | 4 in | 2,500–3,000 PSI |
| Shed base, AC and hot tub pads | 4 in (6 in for hot tubs) | 3,000 PSI |
| Residential driveway | 4–5 in (cars), 6 in (trucks) | 3,500–4,000 PSI |
| Garage floor | 4–6 in | 3,500–4,000 PSI |
| RV pad, heavy equipment | 6–8 in | 4,000+ PSI |
| Footings | 8–12 in deep, twice the wall width | 2,500–3,000 PSI |
In freeze–thaw climates, use air-entrained concrete of at least 4,000 PSI for any exterior slab. Always check local code for footing depth. It is set by your frost line.
Worked examples
- Patio: 12 × 16 ft at 4 in
- 12 × 16 × 0.333 = 64 ft³
- Convert to yards
- 64 ÷ 27 = 2.37 yd³
- Add 10% waste
- 2.61 yd³
- Order
- 2.75 yd³ ready-mix
- Deck footings: 8 piers, 12 in × 4 ft
- π × 0.5² × 4 = 3.14 ft³ each
- All 8 piers
- 25.1 ft³ (0.93 yd³)
- Add 10% waste
- 27.6 ft³
- Order
- 47 × 80 lb bags, or 1 yd³ ready-mix
- Front steps: 3 steps, 4 ft wide, 7 in rise, 11 in run
- 4 × 0.917 × 0.583 × 6 = 12.8 ft³
- Add 10% waste
- 14.1 ft³ (0.52 yd³)
- Order
- 24 × 80 lb bags
How much extra concrete should you order?
The geometry assumes a perfectly flat, perfectly square excavation, and no site is. Running short mid-pour leaves a cold joint, which is a weak line where fresh concrete meets concrete that has started to set. It costs far more to fix than a few extra bags.
- Formed slabs on compacted base: 5–8%Forms are straight and the subgrade is level, so little is lost.
- Excavated ground, footings, trenches: 10–15%Dug sides are never square. Over-digging adds volume you cannot see.
- Post holes and piers: 10–20%Augered holes bell out at the bottom. Measure a few holes before you order.
- Pump or long wheelbarrow runs: add 0.25–0.5 yd³Pump lines and chutes hold concrete that never reaches the form.
How much does concrete weigh?
Normal-weight concrete weighs about 150 lb per cubic foot, so a cubic yard is roughly 4,050 lb (2,400 kg per m³). That matters for hauling. A half-ton pickup should not carry even one yard of wet concrete, and 45 bags of 80 lb is 3,600 lb of dry mix. Plan two or three trips, or have the store deliver.
All 67 concrete calculators
Need something more specific? Every concrete calculator on the site, grouped by project. Each one has defaults and checks built for that exact job.
Slabs, patios & driveways
Footings, piers & posts
Steps, walls & structures
Bags, volume & mix
Cost estimators
Reinforcement & joints
Mortar, grout & repair
Asphalt & paving
More calculators
Frequently asked questions
How do I calculate how much concrete I need?
Multiply length × width × thickness, with every measurement in feet, to get cubic feet. Divide by 27 for cubic yards. Convert thickness from inches by dividing by 12 first. Add 5–10% for waste and round up to the next quarter yard or whole bag.
How many 80 lb bags of concrete make a yard?
About 45. An 80 lb bag yields roughly 0.60 ft³, and a yard is 27 ft³. A yard also takes about 60 bags of 60 lb or 90 bags of 40 lb.
How many bags of concrete do I need for a 10×10 slab?
At 4 inches thick, a 10 × 10 slab is 33.3 ft³ (1.23 yd³). That is about 56 bags of 80 lb concrete before waste, or 62 with a 10% allowance. At that size, ready-mix is usually the better buy.
How many square feet does a yard of concrete cover?
81 sq ft at 4 inches, 65 sq ft at 5 inches and 54 sq ft at 6 inches. Divide 324 by the thickness in inches.
How much extra concrete should I order?
Add 5–10% for formed slabs and 10–20% for excavated footings and post holes, then round up to the increment your supplier sells.
How much does a yard of concrete weigh?
About 4,050 lb (roughly 2 tons), based on 150 lb per cubic foot. A cubic metre weighs about 2,400 kg.
Is it cheaper to use bags or order ready-mix?
Bags are cheaper and simpler below about half a yard. Above about one yard, ready-mix is normally cheaper per yard and far less labor, even after delivery and short-load fees. Enter your local prices in the calculator to compare.
How much cement, sand and aggregate are in 1 m³ of concrete?
For a 1 : 2 : 4 mix, about 6.3 bags (50 kg) of cement, 0.44 m³ of sand and 0.88 m³ of aggregate. This comes from dry volume = 1.54 × wet volume, split into 7 parts.
How do I convert square feet to cubic yards of concrete?
Multiply the square feet by the thickness in inches and divide by 324. For example, 400 sq ft at 4 inches is 400 × 4 ÷ 324 = 4.94 cubic yards.
How thick should a concrete slab be?
4 inches for patios, walkways and shed bases. 5–6 inches for driveways and garage floors. 6 inches or more for RVs, trucks and equipment.
Concrete guides
What this calculator assumes
- Bag yields: 80 lb = 0.60 ft³, 60 lb = 0.45 ft³, 40 lb = 0.30 ft³. These are typical for standard pre-mixed concrete. Check your bag label, as high-strength and fast-setting mixes yield slightly less.
- Density 150 lb/ft³ (2,400 kg/m³) for normal-weight concrete.
- Ready-mix orders are rounded up to the next 0.25 yd³ (0.25 m³), and one truck carries up to about 10 yd³.
- Waste is applied to the total volume. Results are estimates for planning and ordering, not a structural design.