Concrete Calculator
Work out how much concrete a slab, footing, set of post holes, column or flight of stairs needs, in cubic yards for a ready-mix order and in bags of each size, with the post's own volume subtracted from post holes and a waste allowance built in.
What you are pouring
Add a row for each slab, footing, set of post holes, column or flight of stairs.
Prices (optional)
Enter a bag price and a ready-mix quote to compare them.
Concrete estimate
Concrete to buy
2.73 yd³ of concrete
Mix it yourself from 123 bags of 80 lb, or order 2.75 cubic yards of ready-mix.
Includes 10% extra. Ready-mix is ordered in quarter yards.
- Cubic yards
- 2.732.09 m³
- Cubic feet
- 73.767 before the extra
- Ready-mix order
- 2.75 yd³rounded up to ¼ yard
- Weight in place
- 5.16 tons4.68 t, 10,313 lb
| Bag size | Bags |
|---|---|
| 40 lb bag (0.30 cu ft) | 246 |
| 50 lb bag (0.375 cu ft) | 197 |
| 60 lb bag (0.45 cu ft) | 164 |
| 80 lb bag (0.60 cu ft) | 123 |
| 90 lb bag (0.67 cu ft) | 110 |
Your pours, to scale
| Element | Count | Volume |
|---|---|---|
| Patio slabSlab: 12 ft × 10 ft, 4 in thick | 1 | 40 cu ft |
| Deck footingsPost holes: ⌀ 12 in × 36 in deep | 6 | 14.1 cu ft |
| Back stepsStairs: 4 ft wide, 7 in rise, 11 in run | 3 steps | 12.8 cu ft |
| Total | 67 cu ft | |
| With 10% extra | 73.7 cu ft | |
How to use it
- Add each element. A slab (rectangle, circle, triangle or a known area, with its thickness), a footing (length, width, depth and how many), a post hole (diameter, depth, how many, and the post size so its volume is subtracted), a column (diameter and height) or stairs (width, rise and run of each step, number of steps).
- Use real thicknesses. Measure from the compacted base to the top of the forms. A slab on 2 x 4 forms is 3 1/2 in thick unless you set the forms up on stakes. For post holes, measure the hole you will actually dig, not the auger's nominal size.
- Set the waste allowance. Ten percent is standard for slabs and footings: subgrades are never perfectly flat and some concrete stays in the mixer and the wheelbarrow. Use 5 percent for post holes in firm soil and 15 percent for a slab over rough fill.
- Read the result. Cubic yards, cubic metres, the bag count for each bag size, and a ready-mix order rounded up to the next quarter yard, plus a per-element breakdown. Enter bag and ready-mix prices to compare them.
Bag yields
A bag of concrete mix is sold by weight, but what you need to know is how much space the mixed concrete fills. The calculator uses the yields the maker publishes rather than a rule of thumb:
| Bag | Yield (cu ft) | Bags per cu yd | Bags for a 10 x 10 ft slab, 4 in |
|---|---|---|---|
| 40 lb bag (0.30 cu ft) | 0.300 | 90.0 | 112 |
| 50 lb bag (0.375 cu ft) | 0.375 | 72.0 | 89 |
| 60 lb bag (0.45 cu ft) | 0.450 | 60.0 | 75 |
| 80 lb bag (0.60 cu ft) | 0.600 | 45.0 | 56 |
| 90 lb bag (0.67 cu ft) | 0.670 | 40.3 | 50 |
- QUIKRETE Concrete Mix (No. 1101) data sheet (read 2026-10-05)
- Sakrete High-Strength Concrete Mix technical data sheet (read 2026-10-05)
Yields assume the water content on the bag. Different products (fast-setting, high-strength, mortar mix) have their own yields; check the bag you are buying. The slab column has no waste allowance.
Bags or ready-mix: the break-even
Bagged mix costs more per cubic foot than ready-mix and you do all the mixing, but you can buy exactly what you need, mix it at your pace and pour in sections. Ready-mix arrives in one load, already mixed, and is cheaper per yard, but there is a minimum charge and a short-load fee for small orders, and the truck gives you about an hour to place everything it brings. One cubic yard is 45 of the 80 lb bags, which is 3,600 lb lifted off a pallet, into a cart, into the mixer and into the forms.
With the typical prices on file ($6.15 to $6.47 per 80 lb bag as of October 2026, and $119 to $147 per cubic yard of ready-mix as of January 2026, before delivery or short-load fees), a yard in bags costs between $277 and $291. Add your supplier's fees to the ready-mix side and compare.
Slab thickness by use
- Walkways, patios, shed and hot-tub pads: 4 in over a compacted gravel base is the norm. A hot tub full of water is heavy but spreads its load, and 4 in with reinforcement is usual.
- Driveways and garage floors: 4 in as a minimum for cars; 5 to 6 in with reinforcement where trucks, RVs or trailers will park. The extra inch is 25 percent more concrete, so measure the vehicle, not the fear.
- Footings and foundations: sized by code for the load and the frost depth where you live. Enter the dimensions from your plan or the inspector.
- Stairs: each step adds a block the full width of the stair and as deep as the run, stacked on the one below, so the volume grows quickly with the number of steps. The calculator uses the rise, run and width you enter.
Post holes
The common rules are a hole about three times the post's width and a depth of a third of the post's height above ground, and always below the frost line where the ground freezes. For a 4 x 4 post (3 1/2 in actual) that is a 10 to 12 in hole; for a 6 ft fence, 2 to 2 1/2 ft deep. Many people put 4 to 6 in of gravel in the bottom for drainage and set the post on that, which shortens the concrete column; enter the concrete depth, not the hole depth. The calculator subtracts the post's volume, which matters: in a 12 in hole 30 in deep a 3 1/2 in post is about a tenth of the volume.
Worked example: a patio slab and six pergola posts
A 12 by 10 ft patio slab 4 in thick, plus six post holes 12 in across and 30 in deep for 4 x 4 pergola posts, with 10 percent extra:
Result
| Element | Count | Cubic feet |
|---|---|---|
| Patio slab, 12 x 10 ft, 4 in | 1 | 40.00 |
| Pergola post holes, 12 in x 30 in, 4 x 4 posts | 6 | 10.50 |
- Volume with 10% extra55.6 cu ft
- Cubic yards2.06
- Ready-mix order2.25 cu yd
- 40 lb bags186
- 50 lb bags149
- 60 lb bags124
- 80 lb bags93
- 90 lb bags83
The slab is 40.0 cubic feet and the six holes add 10.5 after the posts are subtracted. With the allowance that is 55.6 cubic feet or 2.06 cubic yards, which is 93 of the 80 lb bags or 124 of the 60 lb bags. A ready-mix supplier would bill 2.25 yards. This is the awkward size: too many bags to enjoy mixing, a small load for a truck. Many people would set the posts from bags one weekend and order ready-mix for the slab the next.
Mistakes people make
- Calling a 2 x 4 form a 4 in slab. It is 3 1/2 in. Set forms on stakes or enter 3.5.
- Skipping the waste allowance. Running out mid-pour means a cold joint or a frantic trip for bags. Ten percent.
- Rounding ready-mix down. Suppliers bill by the quarter yard and will not come back with a quarter yard for free. The calculator rounds up.
- Too much water. A sloppy mix pours easily and cracks early. Follow the water on the bag; the yields assume it.
- Forgetting the post displaces concrete. Count the hole minus the post, which the calculator does: for a 4 x 4 post that is 2 of the 80 lb bags in a 10 in hole 24 in deep and 3 in a 12 in hole 30 in deep.
- Pouring on loose soil. Compact the base, or the slab settles and the waste allowance turns into a short pour.
Frequently asked questions
How many 80 lb bags of concrete make a cubic yard?
An 80 lb bag yields 0.60 cubic feet, so 45 bags make a cubic yard (27 cubic feet). 90 of the 40 lb bags, 72 of the 50 lb bags, 60 of the 60 lb bags, 41 of the 90 lb bags do the same. That is 3,600 lb of bags to carry and mix for one yard, which is the real argument for ready-mix on larger pours.
How many bags of concrete for a 10 x 10 slab?
At 4 in thick a 10 by 10 ft slab is 33.3 cubic feet, 1.23 cubic yards. Without any waste allowance that is 75 of the 60 lb bags or 56 of the 80 lb bags; add 10 percent for an uneven subgrade and you are at 82 of the 60 lb bags or 62 of the 80 lb bags. The table above has the other bag sizes. At that size most people order ready-mix.
How thick should a patio or shed slab be?
Four inches is the standard for patios, walkways, sidewalks and shed floors, over a compacted gravel base. A slab that will carry cars is normally 4 in as a minimum and 5 to 6 in where trucks, RVs or heavy equipment will sit, with reinforcement. Remember that a 2 x 4 form board is 3 1/2 in tall, so a slab screeded to the top of 2 x 4 forms is 3 1/2 in thick, not 4; enter the real thickness. Footings and anything structural are sized by your local code.
How much concrete does a fence post need?
It depends on the hole, not the post. A 12 in diameter hole 30 in deep around a 4 x 4 post (3 1/2 in actual) holds 1.75 cubic feet once the post's own volume is subtracted, which is 4 of the 60 lb bags or 3 of the 80 lb bags per hole. Smaller holes, or holes with 6 in of gravel in the bottom, take less. The fence calculator counts the holes and the bags for the whole fence.
When is ready-mix cheaper than bags?
Compare the bag count times the bag price with the ready-mix price per yard plus any short-load fee, and then weigh the labour. Bags are cheap per bag but you mix every one; ready-mix has a minimum charge that makes small loads expensive but arrives mixed and ready to pour. Most people find the crossover somewhere around a cubic yard: below it, bags; above it, ready-mix or a mixing trailer from the supply yard. Enter your local prices and the calculator shows both totals.
How much does one bag of concrete yield?
From the maker's data: the 40 lb bag makes 0.30 cubic feet, the 50 lb bag makes 0.375 cubic feet, the 60 lb bag makes 0.45 cubic feet, the 80 lb bag makes 0.60 cubic feet, the 90 lb bag makes 0.67 cubic feet. Yields are for the mix at the water content on the bag; more water makes slightly more volume and much weaker concrete.
Can I pour a slab in sections with bags?
Yes, and for a one-person job it is sensible: form the slab, mix and pour one section at a time to a control joint, and finish each section before the next. Each section should be poured in one go, so divide the slab into sections you can mix and place within about an hour. Enter each section as its own element to see the bags per section, or the whole slab for the total.