To work out how much concrete you need, multiply length × width × depth in feet to get cubic feet, divide by 27 to get cubic yards, then add about 10% for waste. A 10 × 10 ft slab 4 inches thick is 33.3 ft³, or 1.23 yd³ (0.94 m³), so you would order roughly 1.35 yd³.
That one line covers most driveways and patios. The rest of this guide shows how the same idea works for footings, fence post holes and poured steps, how many premix bags that volume turns into, and when it makes more sense to call a ready-mix truck. If you would rather skip the arithmetic, the concrete calculator handles slabs, footings, columns and holes in any unit.
The basic formula: volume in cubic yards
Concrete is sold by volume. In the US that means cubic yards (often just called “yards”); almost everywhere else it means cubic metres. The math is the same for both:
- Volume = length × width × thickness, with every measurement in the same unit.
- Cubic feet ÷ 27 = cubic yards, because a cube 3 ft on each side holds 3 × 3 × 3 = 27 ft³.
- Cubic yards × 0.7646 = cubic metres, and cubic metres × 1.308 = cubic yards.
The trap is the thickness. Slabs are specified in inches but the length and width are in feet, so convert first: 4 in = 0.333 ft, 5 in = 0.417 ft, 6 in = 0.5 ft. Multiplying 10 × 10 × 4 and dividing by 27 gives 14.8 “yards”, a figure that would have you ordering a full truck for a small patio. If you already know the area of an odd-shaped pour, the square feet to cubic yards calculator converts area and depth straight to yards. For help measuring that area, see our guide on how to calculate square footage.
How much concrete do I need for a slab?
Slabs are the simplest pour: a box of uniform thickness. Patios and walkways are usually 4 in thick, driveways and garage floors 5 to 6 in, and anything that will carry trucks or heavy equipment thicker still. The table shows volume per 100 ft² (a 10 × 10 ft area), so you can scale it to your own job.
| Thickness | Cubic feet per 100 ft² | Cubic yards | Cubic metres | 80 lb bags (no waste) |
|---|---|---|---|---|
| 3.5 in (89 mm) | 29.2 | 1.08 | 0.83 | 49 |
| 4 in (102 mm) | 33.3 | 1.23 | 0.94 | 56 |
| 5 in (127 mm) | 41.7 | 1.54 | 1.18 | 70 |
| 6 in (152 mm) | 50.0 | 1.85 | 1.42 | 84 |
| 8 in (203 mm) | 66.7 | 2.47 | 1.89 | 112 |
A handy rule falls out of the 4 in row: one cubic yard covers about 81 ft² at 4 inches. At 6 in it covers 54 ft².
Worked example: a 12 × 16 ft patio
You want a 12 × 16 ft patio poured 4 in thick.
- Convert the thickness: 4 ÷ 12 = 0.333 ft.
- Volume: 12 × 16 × 0.333 = 64 ft³.
- Cubic yards: 64 ÷ 27 = 2.37 yd³ (1.81 m³).
- Add 10% waste: 2.37 × 1.10 = 2.61 yd³ (70.4 ft³, or 1.99 m³).
- Round up to the supplier’s increment: order 2.75 yd³.
In bags, 70.4 ft³ ÷ 0.6 ft³ per 80 lb bag = 117.3, so 118 bags. That is more than four tons of material to lift, mix and place by hand, which is why a pour this size almost always goes to ready-mix. The finished slab weighs roughly 9,600 lb (64 ft³ at about 150 lb/ft³); the concrete weight calculator gives the exact figure for other mixes.
The same slab in metric
A 4 m × 5 m patio at 100 mm thick is 4 × 5 × 0.1 = 2.0 m³. With 10% waste that becomes 2.2 m³. A 25 kg bag of premix yields about 0.0117 m³, so you would need 188 bags. Order 2.25 or 2.5 m³ of ready-mix instead.
Footings and foundation walls
A continuous strip footing is just a long, narrow slab, so the formula does not change. A footing 40 ft long, 16 in wide and 8 in deep is:
- 40 × (16 ÷ 12) × (8 ÷ 12) = 40 × 1.333 × 0.667 = 35.6 ft³
- 35.6 ÷ 27 = 1.32 yd³ (1.01 m³)
- Plus 10%: 1.45 yd³, so order 1.5 yd³
Footings are dug into soil rather than formed with boards, and trench walls are never as neat as the plan. Measure the trench after it is dug, not the drawing, and lean toward 10 to 15% extra. Footing size and depth are set by your local building code (frost depth matters a lot in cold climates), so check with your building department before you dig. If the footing carries a block wall, the concrete block calculator will count the blocks for the wall on top.
Fence post holes and round columns
Round holes use the cylinder formula: volume = π ÷ 4 × diameter² × depth. For a fence post you then subtract the volume taken up by the post itself and any gravel at the bottom of the hole.
Worked example: ten 4 × 4 fence posts
- Holes: 10 in diameter, 30 in deep, with 6 in of gravel at the bottom, leaving 24 in of concrete.
- Hole area: π ÷ 4 × 10² = 78.54 in². A 4 × 4 post actually measures 3.5 × 3.5 in, which takes up 12.25 in², leaving 66.29 in².
- Concrete per post: 66.29 × 24 = 1,591 in³ = 0.92 ft³ (1,728 in³ per ft³).
- Ten posts: 9.2 ft³; with 10% waste, 10.1 ft³.
- Bags: 10.1 ÷ 0.375 = 27 bags of 50 lb, or 10.1 ÷ 0.6 = 17 bags of 80 lb.
The fence post concrete calculator does this subtraction for any post size, and for deck piers poured in cardboard tubes the tube estimator is quicker. A common rule of thumb for fence posts is a hole three times the post width and about one third of the post length deep, but frost line and soil type can push that deeper.
Concrete steps
Poured steps look complicated but they are just stacked boxes. The bottom step is as deep as the whole staircase, the next one is one tread shorter, and so on. For n steps with riser height R, tread depth T, width W and a top landing of depth P:
Volume = W × [T × R × n(n + 1) ÷ 2 + P × R × n]
Three steps with 7 in risers, 11 in treads, a 48 in width and a 36 in landing:
- Steps: 11 × 7 × (3 × 4 ÷ 2) = 462 in²
- Landing: 36 × 7 × 3 = 756 in²
- Volume: 48 × (462 + 756) = 58,464 in³ = 33.8 ft³ = 1.25 yd³ (0.96 m³)
- With 10% waste: 37.2 ft³, or 63 bags of 80 lb
The landing is the surprise: it holds more concrete than the steps. Our concrete stairs calculator runs this formula for you, and the stair calculator helps you pick riser and tread sizes that meet code before you build forms.
Bags or ready-mix?
Once you know the volume, the next question is how to buy it. Premix bags are sold by weight, but what you care about is the volume each bag yields once mixed:
| Bag size | Yield per bag | Bags per cubic yard | Bags per cubic metre |
|---|---|---|---|
| 40 lb | 0.30 ft³ | 90 | about 118 |
| 50 lb | 0.375 ft³ | 72 | about 94 |
| 60 lb | 0.45 ft³ | 60 | about 79 |
| 80 lb | 0.60 ft³ | 45 | about 59 |
| 25 kg | 0.0117 m³ | about 65 | about 85 |
As a rough guide, bags make sense below about 1 yd³ (0.75 m³): post holes, small pads, a single step, repairs. You can mix a few bags at a time in a wheelbarrow and take breaks. Above that, ready-mix is usually cheaper per yard and far less work. Many suppliers charge a short-load fee for orders under 3 to 4 yd³, so call around; for a 1.5 yd³ pour the bag route and the truck route can come out close, and the truck wins on your back. Ready-mix is also delivered at a consistent strength, which matters for structural work. The concrete driveway cost calculator can help you compare the full job, including labor.
If you are mixing your own from cement, sand and gravel, the cement calculator splits the volume into each ingredient. The Portland Cement Association publishes background on mix proportions, curing and slab thickness if you want to read further.
How much extra concrete to order
Every estimate needs a waste allowance. Concrete gets lost to uneven subgrade, forms that bow under the weight, spillage from the chute, and material left in the drum. Running short is far worse than having a little left over: if the truck leaves before the forms are full, the second load meets concrete that has already started to set and you end up with a weak “cold joint”.
- 5% for formed slabs on a carefully compacted, level base.
- 10% for typical DIY slabs, steps and post holes.
- 10–15% for footings dug into soil, where trench width varies.
Adding a percentage is just multiplication: 2.37 yd³ × 1.10 = 2.61 yd³. Our guide on how to calculate percentage covers that step if it is not second nature. Ready-mix plants usually sell in quarter-yard increments, so always round up after adding waste.
Common mistakes
- Leaving thickness in inches. Always convert to feet (or metres) before multiplying.
- Measuring the plan, not the hole. Dig and form first, then measure what you actually built.
- Forgetting the base. A slab usually sits on 4 in of compacted gravel; that is a separate order, and the gravel calculator works it out.
- Ignoring a sloping site. If the depth varies from 4 to 6 in, use the average (5 in), not the minimum.
- Skipping rebar or mesh in the plan. It does not change the volume much, but you need it on site before the truck arrives. The rebar calculator lays out the grid.
- Mixing up dry and wet volume. An 80 lb bag does not make 80 lb worth of “space”; use the yield figure printed on the bag.
For structural pours such as foundations and retaining walls, follow the drawings and your local code, which in most of the US is based on standards from the American Concrete Institute. Unit conversions in this guide use the exact definitions published by NIST.
Frequently asked questions
How many bags of concrete are in a yard?
45 bags of 80 lb, 60 bags of 60 lb, 72 bags of 50 lb or 90 bags of 40 lb make one cubic yard. In metric, about 85 bags of 25 kg make one cubic metre.
How much concrete do I need for a 10 × 10 slab?
At 4 in thick, a 10 × 10 slab is 33.3 ft³, or 1.23 yd³. With 10% waste, order about 1.35 yd³, which rounds to 1.5 yd³ at most plants.
How many square feet does a yard of concrete cover?
About 81 ft² at 4 in thick, 65 ft² at 5 in and 54 ft² at 6 in. Divide 324 by the thickness in inches to get the coverage for any depth.
How much does a yard of concrete weigh?
Normal-weight concrete weighs roughly 4,000 lb per cubic yard (about 2,400 kg per cubic metre), which is why even small slabs are heavy work by hand.
Is it cheaper to mix my own concrete?
For under about a cubic yard, usually yes, especially once short-load fees are counted. Above that, ready-mix normally wins on cost and almost always on labor and consistency.
How thick should a concrete driveway be?
Most residential driveways are 4 to 5 in thick for cars, and 6 in where heavier vehicles such as RVs or delivery trucks will park.
Measure the real dimensions, convert everything to feet or metres, multiply, divide by 27 for yards, add 10% and round up. Do that and you will order the right amount the first time, with just enough left over to fill a stepping-stone mold instead of a dumpster.