How much concrete
do you actually need?
Pick your shape, drop in the measurements, and get the exact volume, the number of bags, and a ready-mix cost estimate — with waste allowance built in, so you never run short mid-pour.
1 · Shape & measurements
Results update as you type. All volumes include your waste allowance where noted.
Rectangular slab
2 · Quantity & waste
Pouring more than one? Set the quantity — footing runs, columns and identical slabs multiply automatically.
Pros order 5–10% extra for slabs on grade, 10–15% for footings in uneven trenches. Running short costs far more than a little extra.
3 · Bag size & prices
Used for the bag count and cost comparison. Edit the prices to match your local store or supplier.
Exact volume before waste: 0.00 yd³ · Slab × 1
Bags by size — same pour
How the maths works
Every result above comes from one idea: volume = the space the pour fills. Measure in feet, multiply, then divide by 27 to get cubic yards — the unit ready-mix trucks sell by.
Length × Width × Thickness
A 10 × 10 ft patio at 4 inches thick is 10 × 10 × 0.333 = 33.3 ft³, which is 1.23 yd³. Thickness is where most people slip up — always convert inches to feet first (inches ÷ 12).
π × Radius² × Depth
For circles, halve the diameter to get the radius, square it, and multiply by π (3.1416) and the depth. A 12 ft diameter pad, 4 in thick: 3.1416 × 6² × 0.333 = 37.7 ft³ ≈ 1.40 yd³.
Each step stacks on the last
Step 1 is one rise tall, step 2 is two rises, and so on. Total volume = width × tread × riser × (1 + 2 + … + n). Four steps add up much faster than most people expect.
Bag yield reference
One bag never covers a full cubic foot. These are the standard yields for premixed concrete bags — the same numbers this calculator uses.
| Bag size | Yield per bag | Bags per ft³ | Bags per yd³ | Bags per m³ |
|---|---|---|---|---|
| 40 lb (18.1 kg) | 0.30 ft³ | 3.33 | 90 | 118 |
| 50 lb (22.7 kg) | 0.375 ft³ | 2.67 | 72 | 94 |
| 60 lb (27.2 kg) | 0.45 ft³ | 2.22 | 60 | 79 |
| 80 lb (36.3 kg) | 0.60 ft³ | 1.67 | 45 | 59 |
Rule of thumb: once a pour passes about 1 yd³ (45 × 80 lb bags), hand-mixing stops being fun and a truck starts making sense.
Bags or ready-mix truck?
The calculator prices both for your exact pour. Which one wins depends on size, access and how much your back is worth.
Premix bags
- No minimum order, no delivery window to hit
- Mix only what you need, when you need it
- Cheapest for small repairs, posts and pads
- Heavy work: an 80 lb bag covers just 0.6 ft³
- Risk of cold joints if you mix too slowly
Ready-mix truck
- One continuous pour — stronger, more consistent
- Far cheaper per yard at volume
- Typical minimum order is 1 yd³, short-load fees below that
- You need the forms, crew and finishing tools ready when it arrives
- Standard truck carries about 8–10 yd³
Before you order
Measure the real trench, not the plan
Footings dug by hand or machine are almost always wider and deeper than drawn. Measure the actual excavation in three places and use the average — that is what the waste slider is compensating for.
Subgrade eats concrete
An uneven base can quietly add half an inch of thickness across a whole slab. Compact and level the gravel base, and check depth with a string line before the truck is booked.
Round up, never down
Suppliers would rather take back a quarter yard than send a second truck for a shortfall. Order to the nearest quarter yard above your with-waste figure, and have a backup spot (a small pad, post holes) ready for any leftover.
Common questions
How many bags of concrete do I need for a 10×10 slab?
At the standard 4-inch thickness, a 10×10 ft slab is 1.23 yd³ (33.3 ft³). That works out to roughly 56 bags of 80 lb, 74 bags of 60 lb, or 111 bags of 40 lb — before waste. Add 10% and most people order a truck instead: it is one phone call versus fifty-plus bags through a mixer.
How thick should my slab be?
4 inches is standard for patios, walkways and shed bases. Go 5–6 inches for driveways and anywhere a car will park, and 6 inches or more with rebar or mesh for heavy vehicles and garage floors. Footings in cold climates must also reach below the frost line — check your local code.
Why does the calculator add waste?
Because real pours are never perfect boxes: trenches over-dig, forms bow, subgrade dips, and some concrete stays in the wheelbarrow and pump. A 10% allowance absorbs all of that. Running short mid-pour leaves a cold joint — a permanent weak seam — so the extra is cheap insurance.
How do I convert cubic feet to cubic yards?
Divide by 27. One cubic yard is 3 ft × 3 ft × 3 ft = 27 ft³. For metres: one cubic yard is 0.765 m³, and one cubic metre is 1.31 yd³. This calculator shows all three at once so you can order in whichever unit your supplier uses.
How much does a yard of concrete weigh?
About 4,050 lb (roughly 2 tons) per cubic yard when cured, or about 150 lb per cubic foot. That matters for trailers and pickup beds: a single yard in bags is around 3,600 lb of dry mix before you add water — well past most half-ton truck payloads in one trip.
What strength (PSI) should I order?
2,500–3,000 PSI covers most residential flatwork (patios, sidewalks). Driveways and garage floors are usually 3,500–4,000 PSI, often with air entrainment in freeze–thaw climates. Footings commonly run 2,500–3,500 PSI. Your supplier can match the mix to the job — tell them what it is for.