How to calculate concrete
Concrete is ordered by volume, so every concrete calculation answers one question: how many cubic feet or cubic yards will the forms hold? For any flat pour (a slab, patio, pad, sidewalk or floor) multiply the three dimensions and convert to yards in one step:
Cubic yards = length (ft) × width (ft) × thickness (in) ÷ 324
The 324 is 12 inches per foot × 27 cubic feet per cubic yard. It lets you keep the thickness in inches, which is how slabs are specified, and skip a conversion step where most hand calculations go wrong. If every dimension is already in feet, multiply them and divide by 27.
Four steps cover every job:
- Measure the inside of the forms. Use a tape, not a plan drawing. Forms are often built an inch or two off the drawing.
- Measure the depth in several places. Excavations and gravel bases are rarely flat. Average your readings, or use the deepest one if they differ by more than ½ in.
- Pick the shape in the concrete calculator above and enter the numbers. Each field has its own unit menu (in, ft, yd, cm or m), and it shows cubic yards, cubic feet and cubic meters at once.
- Add a waste allowance and round up. Add 10% for most jobs, then round ready-mix up to the next ¼ yard or bags up to the next whole bag.
Worked example: 12 × 12 ft shed pad, 4 in thick
- 12 × 12 × 4 ÷ 324 = 1.78 yd³ (48 ft³, 1.36 m³)
- With 10% extra: 1.96 yd³, so order 2 yd³ of ready-mix
- Or in bags: 52.8 ft³ ÷ 0.60 ft³ per bag = 88 bags of 80 lb concrete
Worked examples for the most common jobs
Most people arrive with one specific job in mind. These are the calculations behind the most searched projects, worked with the same formulas the calculator uses.
| Job | Dimensions | Concrete | Order or buy |
|---|---|---|---|
| Shed pad | 12 × 12 ft, 4 in thick | 1.78 yd³ | 2 yd³ ready-mix, or 88 × 80 lb bags with 10% |
| Two-car driveway | 40 × 12 ft, 5 in thick | 7.41 yd³ | 8¼ yd³ ready-mix with 10% |
| Fence posts | 10 posts, 10 in hole, 24 in deep, 4×4 post | 9.2 ft³ | 17 × 80 lb or 28 × 50 lb bags with 10% |
| Deck piers | 4 Sonotubes, 12 in wide, 4 ft deep | 12.6 ft³ (0.47 yd³) | 24 × 80 lb bags with 10% |
| Garden steps | 3 steps, 7 in rise, 11 in run, 4 ft wide, no landing | 12.8 ft³ (0.48 yd³) | 24 × 80 lb bags with 10% |
| Strip footing | 40 ft long, 16 in wide, 8 in deep | 1.32 yd³ | 1½ yd³ ready-mix with 10% |
Each job has its own page with size tables and tips: the concrete slab calculator, driveway calculator, fence post concrete calculator, Sonotube calculator, concrete steps calculator and footing calculator.
Concrete volume formulas for every shape
Every shape in the calculator comes down to a simple volume formula. Convert all dimensions to feet first, then divide cubic feet by 27 for cubic yards, or multiply by 0.0283 for cubic meters.
| Shape | Formula (all in feet) | Typical use |
|---|---|---|
| Slab, pad, patio, floor | length × width × thickness | Patios, sheds, garage floors, sidewalks |
| Monolithic slab | slab volume + thickened edge (edge width × extra depth × perimeter, corners counted once) | Shed and garage slabs poured with their footing |
| Round slab | 3.1416 × (diameter ÷ 2)² × thickness | Fire-pit pads, round patios |
| Footing | total length × width × depth | Strip footings under walls |
| Column or Sonotube | 3.1416 × (diameter ÷ 2)² × height × number | Deck piers, porch columns |
| Post holes | (hole volume − post volume) × number | Fence, mailbox and gate posts |
| Steps | each step’s width × run × height above ground, added up, plus the landing | Porch and garden steps |
| Wall | length × height × thickness | Poured walls, retaining walls |
| Curb and gutter | length × (curb width × curb height + gutter width × gutter thickness) | Driveway aprons, street curbs |
For an L-shaped or irregular area, split it into rectangles and add them together with Add to project. For a triangle, calculate the rectangle around it and halve the result.
Concrete calculator in square feet
If you know the area in square feet, you only need the thickness. Multiply square feet by thickness in inches and divide by 324. A 500 ft² patio at 4 in is 500 × 4 ÷ 324 = 6.17 yd³.
The reverse is just as useful when planning: one cubic yard covers 324 ÷ thickness square feet.
| Thickness | Square feet per cubic yard | Cubic yards per 100 ft² | 80 lb bags per 100 ft² |
|---|---|---|---|
| 3 in | 108 | 0.93 | 42 |
| 4 in | 81 | 1.23 | 56 |
| 5 in | 64.8 | 1.54 | 70 |
| 6 in | 54 | 1.85 | 84 |
| 8 in | 40.5 | 2.47 | 112 |
Bag counts are rounded up to whole bags and do not include a waste allowance. The square feet to cubic yards calculator does this conversion for any material.
How many bags of concrete do you need?
Premixed concrete is sold by weight, but what matters is the volume a bag makes once mixed. QUIKRETE publishes these yields for its standard Concrete Mix. Other brands can differ slightly: for example, Sakrete lists 0.66 ft³ for its 90 lb bag, which is 41 bags per cubic yard (54 per cubic meter) once rounded up. Check the yield printed on your bag:
| Bag size | Yield per bag | Bags per cubic yard | Bags per cubic meter |
|---|---|---|---|
| 40 lb (18.1 kg) | 0.30 ft³ | 90 | 118 |
| 50 lb (22.7 kg) | 0.375 ft³ | 72 | 95 |
| 60 lb (27.2 kg) | 0.45 ft³ | 60 | 79 |
| 80 lb (36.3 kg) | 0.60 ft³ | 45 | 59 |
| 90 lb (40.8 kg) | 0.675 ft³ (regional) | 40 | 53 |
Divide your volume in cubic feet by the yield and always round up. The calculator shows the 40, 50, 60, 80 and 90 lb counts side by side, so you can compare what your store stocks. Brand pages cover the Quikrete calculator and Sakrete calculator, and the bags per yard chart covers common volumes.
How thick should concrete be?
Thickness drives volume and cost more than anything else. Going from 4 in to 6 in uses 50% more concrete for the same area.
| Project | Common thickness | Notes |
|---|---|---|
| Sidewalk, walkway, patio | 4 in | On a compacted gravel base (depth per your plans or local code) |
| Shed pad | 4 in | Thickened edge for heavier sheds |
| Driveway for cars | 4–5 in | 5 in is a common choice for homes |
| Driveway for trucks or RVs | 6 in | Often with rebar or mesh |
| Garage floor | 4–6 in | Follow your local building code |
| Hot tub or equipment pad | 6 in | Check the maker’s load figure |
These are common practice, not code requirements. Garages, house slabs, footings and retaining walls must follow your plans and your local building department. Bagged mixes such as QUIKRETE Concrete Mix are made for pours 2 in thick or more.
Ready-mix or bags: which is cheaper?
Ready-mix plants deliver concrete already mixed, priced per cubic yard plus fees. Bags are mixed on site with water, by hand or in a mixer. The volume usually decides it:
- Under about ½ yd³ (around 23 × 80 lb bags): bags are practical. There is no delivery fee, no short-load charge and no truck to schedule.
- About ½ to 1 yd³: compare a short-load delivery with buying 23–45 bags and renting a mixer. Prices vary by region, so get both quotes.
- Over 1 yd³: ready-mix is usually cheaper per yard and much less work. One yard is 45 bags, about 3,600 lb of dry mix to lift, and large pours mixed by hand risk cold joints where one batch starts setting before the next arrives.
The calculator shows this advice once your pour passes one cubic yard, and gives the ready-mix order rounded up to the ¼ yard most plants sell in.
Which concrete mix and strength to buy
Concrete strength is measured in psi (pounds per square inch) after 28 days of curing. Standard bagged concrete is a general-purpose mix: QUIKRETE’s data sheet lists 2,500 psi at 7 days and 4,000 psi at 28 days for its Concrete Mix. Ready-mix plants will quote whatever strength you ask for, and structural work should follow the strength on your plans.
- General slabs, patios, steps and footings: standard concrete mix or a standard ready-mix.
- Fence and mailbox posts: a fast-setting mix poured dry into the hole and then watered is quicker.
- Thin toppings under 2 in: use a topping or resurfacing product, not standard concrete.
Water is the most common way to weaken good concrete. QUIKRETE says to start with about 6 pints (2.8 L) of water per 80 lb bag and add more only as needed for a workable mix, up to a maximum of about 9 pints (4.3 L). Adding more makes the mix easier to place but lowers its strength and increases shrinkage cracking.
Pre-pour checklist
Before the truck arrives or the first bag is opened:
- Forms are staked, level and braced so they will not bulge under the weight.
- The base is compacted, and the depth has been checked in several places.
- Reinforcement is on chairs, not lying on the ground.
- You have enough help, tools (wheelbarrows, screed board, floats, edger) and a clear path for the truck or barrows.
- The weather allows it: avoid pouring onto frozen ground, and plan for hot days, when concrete sets faster.
- Your order is your calculated volume plus 10%, rounded up to the supplier’s increment.
What concrete costs: using the price estimate
Concrete prices vary widely by region, season, mix strength and order size, so the calculator does not guess a national average. Enter the price per cubic yard your local plant quotes and it multiplies by your rounded order.
When you call for prices, ask about:
- Price per yard for the strength you need (for example 3,000 or 4,000 psi).
- Short-load fees for orders below the plant’s minimum.
- Delivery and waiting time. Trucks include a set unloading time and bill extra minutes.
- Extras such as fiber, color, accelerators in cold weather or retarders in heat.
For a whole project, including forms, base, reinforcement and finishing labor, use the concrete slab cost calculator or driveway cost calculator.
How much extra concrete should you order?
This calculator adds 10% by default, a common rule of thumb rather than a published standard. Allow more on rough or sloping ground, and ask your supplier what they recommend. Concrete is lost to spillage, form bulges, low spots in the base and what stays in the chute and wheelbarrow. Running short is far worse than a little left over: if the second delivery or the last bags arrive after the first concrete has started to set, you get a cold joint, a weak seam that tends to crack.
Common ordering mistakes the calculator helps you avoid:
- Using plan dimensions instead of measuring the forms.
- Forgetting thickened edges and footings under slabs. Add them as separate items with Add to project.
- Mixing up inches and feet in the thickness. A “4” entered as feet instead of inches orders twelve times too much.
- Not rounding to the supplier’s increment. Ready-mix is sold in ¼ or ½ yard steps.
Metric concrete calculation
Switch the calculator to Metric to set every field to meters and centimeters, or pick cm or m on any single field. In metric the formula is simply length (m) × width (m) × thickness (m) = cubic meters.
A 3.6 × 3.6 m slab 10 cm thick is 3.6 × 3.6 × 0.10 = 1.30 m³, or 1.43 m³ with 10% extra. That is about 77 bags of 80 lb (36 kg) premix before the extra. One cubic meter is 35.3 ft³, or 1.31 cubic yards.
How much does concrete weigh?
Normal-weight concrete weighs about 140–155 lb per cubic foot (FHWA); WSDOT uses 150 lb/ft³ for plain concrete in its bridge design loads. The calculator uses 150 lb/ft³ by default (you can switch to 145 or 140), so one cubic yard is about 4,050 lb (about 2,400 kg per cubic meter). QUIKRETE lists about 140 lb/ft³ for its bagged Concrete Mix. Weight matters when you plan trailer loads, hand-carry bags or check what a deck or floor can support.
Using this instead of a concrete calculator app
This concrete calculator runs in any phone or desktop browser with nothing to install. Add it to your home screen from the browser’s share or menu button and it opens like an app. Your inputs never leave your device, the Copy link to this result button saves the exact numbers to send to a supplier or crew, and Add to project totals several pours, such as a slab plus its footing, in one place.
Why you can trust these numbers
Every formula on this site is tested in code against hand-worked examples like the ones above. Bag yields come from manufacturer data sheets, and the concrete density from state and federal highway agency figures (WSDOT, FHWA). Loose-material densities are typical planning values (sand matches a QUIKRETE data sheet), so use your supplier’s figure; the methodology page explains each one. The calculators are researched and written by Izhar Ahmad Danish and reviewed by M Bilal. Results are estimates: confirm sizes on site, follow your local code for structural work, and get supplier quotes before you order.
