On this page (11)
How to use this calculator#
- Convert the depth into the same unit as the plan dimensionsA 4-inch slab is 4 ÷ 12 = 0.3333 ft, and a 100 mm slab is 0.1 m. Multiplying feet by inches is the single most common concrete error and it overstates the volume twelvefold.
- Measure the excavation, not the drawingSub-grade is never flat and trench sides are never plumb. Take the depth in several places and use the deepest reading, because concrete finds every low spot and a shortfall stops the pour dead.
- Split the pour into simple prismsA slab with a thickened edge is a slab plus a perimeter beam. Steps are stacked rectangles. Footings are a trench times its cross section. Work out each piece separately and add them.
- Add a waste allowanceAdd 10% for a slab on prepared gravel and 15% for machine-dug trench footings, where over-dig at the edges is unavoidable. Forms also bow outward under the weight of wet concrete.
- Decide between bags and ready-mixPast about one cubic yard — roughly 45 to 50 eighty-pound bags — mixing by hand takes a full day for two people and risks cold joints. A truck is cheaper per yard and places the whole slab in one continuous pour.
The formula#
Volume of a rectangular pour
V = L × W × D • cubic yards = cubic feet ÷ 27 • bags = (V × (1 + waste)) ÷ yield per bag
- L, W
- Plan length and width of the slab, footing or column base
- D
- Depth or thickness, converted to the same unit as L and W
- 27
- Cubic feet in a cubic yard (3 × 3 × 3)
- yield per bag
- Finished concrete from one bag of pre-mix: 0.60 cu ft for 80 lb, 0.45 for 60 lb, 0.30 for 40 lb
- waste
- Allowance for over-dig, spillage and form movement: 0.10 for a slab, 0.15 for trench footings
Volume factors are length factors cubed: 1 ft = 0.3048 m, so 1 cu ft = 0.3048³ = 0.0283168 m³, and 1 cu yd = 0.764555 m³. The trap that ruins a pour is mixing units — entering a 4-inch depth as the number 4 alongside dimensions in feet gives 400 cu ft for a 10 × 10 slab instead of 33.33, a twelvefold error you will not notice until the truck arrives.
Volume, bags and ready-mix#
Concrete is sold by volume: length times width times depth, with the depth converted to the same unit as everything else. A 10 by 10 ft slab poured 4 inches deep is 10 x 10 x 0.333, or 33.3 cubic feet, which is 1.23 cubic yards. Pre-mix bags yield about 0.60 cu ft for 80 lb, 0.45 cu ft for 60 lb and 0.30 cu ft for 40 lb, so that slab swallows 56 eighty-pound bags.
When to stop bagging and order a truck#
Mixing 56 bags by hand is a full day for two people, and the pour will end up with cold joints where one batch stiffened before the next arrived. Past roughly one cubic yard — about 50 eighty-pound bags — ready-mix delivered by truck is cheaper per yard and lets you place the whole slab in one continuous pour. Most suppliers add a short-load fee under 3 yards, so price both options.
Waste, over-dig and why you add 10%#
Sub-grade is never perfectly level and timber forms bow outward under the weight of wet concrete, so real pours consume 5-10% more than the arithmetic promises. Machine-dug footing trenches over-dig at the edges by even more. Add 10% for a slab on prepared gravel and 15% for trench footings — running out mid-pour leaves a cold joint you can see and a weak plane you cannot.
Worked examples#
A 10 × 10 ft patio slab, 4 inches thick
A garden patio on a compacted gravel base.
- Depth in feet: 4 ÷ 12 = 0.3333 ft
- Volume = 10 × 10 × 0.3333 = 33.33 cubic feet
- Cubic yards = 33.33 ÷ 27 = 1.235 cu yd
- Cubic metres = 33.33 × 0.0283168 = 0.944 m³
- With 10% waste: 1.235 × 1.10 = 1.36 cu yd
- In 80 lb bags at 0.60 cu ft each: 33.33 × 1.10 ÷ 0.60 = 62 bags
1.36 cubic yards with waste. That is 62 eighty-pound bags — well past the point where ready-mix is the better call.
A strip footing for a garden wall
A trench 12 m long, 0.45 m wide and 0.3 m deep.
- Volume = 12 × 0.45 × 0.30 = 1.62 m³
- Trench footings over-dig, so add 15%: 1.62 × 1.15 = 1.86 m³
- In cubic yards: 1.86 ÷ 0.764555 = 2.44 cu yd
- In 25 kg bags at about 0.0117 m³ each: 1.86 ÷ 0.0117 = 160 bags
1.86 m³ (2.44 cu yd) — order ready-mix. At 160 bags, hand mixing would take two people the best part of two days.
Reference tables#
| Bag size | Yield (cu ft) | Yield (litres) | Bags per cubic yard | Bags per cubic metre |
|---|---|---|---|---|
| 40 lb (18 kg) | 0.30 | 8.5 | 90 | 118 |
| 50 lb (23 kg) | 0.375 | 10.6 | 72 | 95 |
| 60 lb (27 kg) | 0.45 | 12.7 | 60 | 79 |
| 80 lb (36 kg) | 0.60 | 17.0 | 45 | 59 |
| 20 kg | 0.33 | 9.4 | 82 | 107 |
| 25 kg | 0.41 | 11.7 | 66 | 86 |
| 40 kg | 0.66 | 18.7 | 41 | 54 |
Bag counts are rounded up, because you cannot buy part of a bag. An 80 lb bag needs roughly 3 quarts (2.8 litres) of water; adding more than the label states to make it easier to work is the fastest way to weaken the finished concrete.
| Slab size | Cubic feet | Cubic yards | Cubic metres | 80 lb bags | Cu yd with 10% waste |
|---|---|---|---|---|---|
| 4 × 4 ft | 5.33 | 0.198 | 0.151 | 9 | 0.22 |
| 8 × 8 ft | 21.33 | 0.790 | 0.604 | 36 | 0.87 |
| 10 × 10 ft | 33.33 | 1.235 | 0.944 | 56 | 1.36 |
| 12 × 12 ft | 48.00 | 1.778 | 1.359 | 80 | 1.96 |
| 10 × 20 ft | 66.67 | 2.469 | 1.888 | 112 | 2.72 |
| 20 × 20 ft | 133.33 | 4.938 | 3.776 | 223 | 5.43 |
| 20 × 30 ft | 200.00 | 7.407 | 5.663 | 334 | 8.15 |
Bag counts here exclude waste. Anything past about 50 bags is a ready-mix job — most suppliers charge a short-load fee under 3 cubic yards, so price both options.
| Thickness | Cu ft per sq ft | Sq ft per cubic yard | m² per cubic metre | Typical use |
|---|---|---|---|---|
| 2 in (50 mm) | 0.167 | 162 | 19.7 | Non-structural topping or overlay |
| 3 in (75 mm) | 0.250 | 108 | 13.1 | Footpaths, light shed bases on good sub-base |
| 4 in (100 mm) | 0.333 | 81 | 9.8 | Patios, walkways, standard shed and garage floors |
| 5 in (125 mm) | 0.417 | 65 | 7.9 | Light-vehicle driveways |
| 6 in (150 mm) | 0.500 | 54 | 6.6 | Driveways, RV pads, light truck loading |
| 8 in (200 mm) | 0.667 | 41 | 4.9 | Heavy vehicle and commercial slabs |
Thickness is only half the story — anything carrying vehicles also needs reinforcement, a compacted sub-base and correctly placed control joints. Check local code and a structural specification for load-bearing work.
Common mistakes#
- Leaving the depth in inchesA 10 × 10 ft slab at '4' deep computes as 400 cubic feet if the 4 is taken as feet, against a true 33.33 cubic feet. That is 14.8 cubic yards instead of 1.24 — an order twelve times too big, and a delivery you cannot send back.
- Ordering the exact calculated volumeSub-grade dips, forms bow outward under the weight of wet concrete, and trenches always over-dig at the edges. Real pours consume 5–10% more than the arithmetic promises. Running short mid-pour leaves a cold joint — a visible line and a permanently weak plane through the slab.
- Bagging a pour that should have been ready-mix56 bags is a full day for two people, and each batch stiffens while the next is being mixed. The result is a slab full of cold joints that cracks along them. Past roughly one cubic yard, a truck is both cheaper per yard and structurally better.
- Adding extra water to make the mix easier to placeWater beyond the label figure raises the water-to-cement ratio, and strength falls sharply with it — a mix noticeably wetter than specified can lose a quarter or more of its design strength, along with gaining shrinkage cracks and a dusty surface. Use a plasticiser if the mix is too stiff, never a hose.
Frequently asked questions#
How many 80 lb bags of concrete are in a cubic yard?
About 45. One 80 lb bag yields roughly 0.60 cubic feet and a cubic yard is 27 cubic feet, so 27 divided by 0.6 gives 45 bags.
How thick should a concrete slab be?
Four inches, or 100 mm, suits patios, walkways and shed bases. Go to 5-6 inches with reinforcement for driveways or anything that carries vehicles.
How much water does a bag of concrete need?
Roughly 3 quarts, about 2.8 litres, per 80 lb bag. Adding extra water to make the mix easier to work is the fastest way to weaken the finished concrete.
Does this include rebar or the gravel base?
No — the calculator gives concrete volume only. Reinforcing mesh or rebar, form lumber and the compacted sub-base are all priced separately.
Key terms#
- Cubic yard
- 27 cubic feet, or 0.764555 m³. The standard unit for ordering ready-mix concrete in the US. A typical mixer truck carries 8 to 10 cubic yards.
- Yield
- The finished volume of concrete one bag of pre-mix produces once water is added — about 0.60 cu ft for an 80 lb bag. It is less than the dry bag volume because the powder packs into the aggregate voids.
- Cold joint
- The plane where fresh concrete is placed against a batch that has already begun to stiffen. The two do not knit, leaving a visible line and a weak plane. It is the direct consequence of running out mid-pour.
- Sub-base
- The compacted layer of crushed stone or gravel beneath a slab. It spreads load, provides drainage and gives a flat surface to pour against — a slab poured straight onto soft ground will crack regardless of its thickness.
- Water-to-cement ratio
- The weight of mixing water divided by the weight of cement. It is the single strongest control on concrete strength: the lower the ratio, the stronger and more durable the result, and every extra litre of water reduces it.
- Short-load fee
- A surcharge ready-mix suppliers add for deliveries below a minimum, commonly around 3 cubic yards, because a part-full truck costs nearly as much to run as a full one.
Sources#
- Portland Cement Association — concrete technology and design resources — Portland Cement Association
- American Concrete Institute — codes and technical resources — American Concrete Institute
- Special Publication 811: Guide for the Use of the International System of Units (SI) — NIST
Figures last checked .
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