Order too little concrete and you get a cold joint halfway across your slab. Order too much and you pay for a truck load that ends up in a pile. The maths is not hard, but a few details decide whether your estimate lands close or misses by a wheelbarrow or ten.
The one formula behind every estimate
Every concrete estimate, from a garden path to a warehouse floor, is a volume calculation: length × width × thickness. The only thing that changes is the shape. A slab is a flat box. A footing is a long narrow box. A column is a cylinder. Once you have the volume, you convert it into whatever unit your supplier sells in.
In the United States, ready-mix concrete is sold by the cubic yard, which is 27 cubic feet. In most other countries it is sold by the cubic metre, and one cubic metre is about 1.308 cubic yards. Bagged mix is sold by weight, and each bag has a stated yield in cubic feet or litres.
The single most common error is mixing units. If your length and width are in feet but your thickness is in inches, you must convert the inches to feet by dividing by 12 before multiplying. A 4-inch slab is 0.333 feet thick. Forget that and your answer is off by a factor of twelve, which is why quick estimates sometimes look absurd.
Let the calculator handle the unitsEnter length, width and thickness in feet, inches or metres and get cubic yards, cubic metres and a bag count.
Open the Concrete Slab CalculatorWorked example: a patio slab
Say you want a patio that is 20 feet long, 10 feet wide and 4 inches thick.
- Convert thickness: 4 in ÷ 12 = 0.3333 ft.
- Volume: 20 × 10 × 0.3333 = 66.67 cubic feet.
- Convert to cubic yards: 66.67 ÷ 27 = 2.47 cubic yards.
- Add a 10% allowance: 2.47 × 1.10 = 2.72 cubic yards.
So you would order about 2.75 to 3 cubic yards from a ready-mix plant. The calculation is the same in metric: a 6 m × 3 m slab at 100 mm thick is 6 × 3 × 0.1 = 1.8 cubic metres, and with 10% extra you would order about 2 cubic metres.
You can use the same approach for any rectangular pour. The Concrete Volume Calculator handles several shapes in one place, and the Cubic Yard Calculator is handy when you only need to convert a volume you already know.
Why thickness matters so much
Thickness is the dimension people guess at, and it has the biggest effect on cost because it multiplies the entire area. Going from 4 to 6 inches does not add 50% more work to the formwork, but it adds exactly 50% more concrete.
Typical thicknesses depend on the load. These are common starting points, and your local building code or engineer always has the final word:
| Use | Typical thickness | Notes |
|---|---|---|
| Garden path, walkway | 3.5 to 4 in (90 to 100 mm) | Foot traffic only |
| Patio, shed base | 4 in (100 mm) | Light loads, add a gravel base |
| Garage floor, driveway | 5 to 6 in (125 to 150 mm) | Vehicle loads often call for reinforcement |
| Heavy-duty or commercial floors | 6 in and up | Designed by an engineer |
Footings and strip foundations
A footing is a wider strip of concrete that spreads the load of a wall or column onto the soil. Its volume uses the same box formula, but with the cross-section (width × depth) multiplied by the total run length.
Example: a footing run of 40 feet in total, 24 inches wide and 12 inches deep.
- Cross-section: 2 ft × 1 ft = 2 square feet.
- Volume: 2 × 40 = 80 cubic feet.
- Cubic yards: 80 ÷ 27 = 2.96 cubic yards.
- With 10% extra: about 3.26 cubic yards.
Measure the centre line of the trench for the run length, and remember to add corners and any pads under columns separately. For a full foundation with several parts, break it into simple pieces, calculate each, and add them together. The Concrete Footing Calculator and the Foundation Calculator are built for exactly that.
Columns and round piers
A round column is a cylinder, so use the circle area formula: π × radius² × height. Take a column that is 12 inches in diameter (radius 0.5 ft) and 8 feet tall.
- Volume: 3.1416 × 0.5 × 0.5 × 8 = 6.28 cubic feet.
- In cubic yards: 6.28 ÷ 27 = 0.233 cubic yards.
Four of those columns need 0.93 cubic yards, plus waste. Piers drilled into the ground are often wider than their cardboard form, so measure the real hole. The Concrete Column Calculator works with round and rectangular shapes. Square columns are simply width × depth × height.
Bags, mix ratios or a truck?
How you buy concrete depends mostly on volume. Here is a practical way to choose.
- Under about half a cubic yard: bagged mix is fine for post holes, small pads and repairs.
- Roughly half to two cubic yards: mixing on site is possible but exhausting. Many people hire a mixer or order a small ready-mix load.
- Above about two cubic yards: order ready-mix. The consistency is better, you can finish in one continuous pour and the labour is far less.
How many bags do you need?
An 80-pound bag usually yields about 0.6 cubic feet, a 60-pound bag about 0.45 cubic feet and a 50-pound bag about 0.375 cubic feet. Always check the label, because yields vary by product. For the 20 × 10 × 4 inch patio:
- 66.67 cubic feet ÷ 0.6 = 111 bags of 80 lb.
- With 10% extra: about 122 to 123 bags.
That is nearly five tonnes of dry material to move. At that point a truck usually wins on both cost and sanity. If you do mix your own, the Cement Bag Calculator converts a volume into bags, and the Sand Calculator and Gravel Calculator cover the other ingredients.
Mixing from scratch
A common nominal mix by volume is 1 part cement, 2 parts sand and 4 parts coarse aggregate. Dry materials occupy more space than the finished concrete, so you multiply the wet volume by about 1.54 to find the dry volume needed. For one cubic metre of concrete in a 1:2:4 mix, that works out to roughly 0.22 cubic metres of cement (about 6 bags of 50 kg), 0.44 of sand and 0.88 of aggregate. Mix designs vary with strength requirements, so treat this as a rough guide and follow the specification for structural work.
Waste, slope and uneven ground
An estimate of exactly 2.47 cubic yards is almost never enough. Real pours lose concrete in many small ways:
- Subgrade that is not perfectly level uses more concrete in low spots.
- Formwork can bow slightly outward under the weight of the wet mix.
- Some concrete stays behind in wheelbarrows, chutes and pump lines.
- Spillage and over-excavation both add up.
Most builders add 5 to 10% for waste, and 10% is a sensible default for DIY. For rough ground, trenches dug by hand or pours with complex shapes, 10 to 15% is safer. Having a little left over is much better than running short mid-pour, because a stop in the middle can leave a cold joint, which is a weak line where fresh concrete meets concrete that has begun to set.
If the surface slopes for drainage, calculate the average thickness. A slab that is 4 inches at one edge and 5 inches at the other averages 4.5 inches.
Gravel base, excavation and rebar
Concrete rarely goes straight onto bare soil. A gravel base gives drainage, a uniform support and some protection against frost heave. It also affects your dig, your material orders and your total budget.
Excavation
If the slab is 4 inches thick and the gravel base is another 4 inches, you must dig out about 8 inches. For the 20 × 10 ft area, that is 20 × 10 × (8 ÷ 12) = 133.3 cubic feet, or 4.94 cubic yards of soil to remove. Loose soil takes up more room than undisturbed soil, so the pile will look larger than the hole. The Excavation Calculator works this out and helps you plan skips or trailer trips.
Gravel
A 4-inch gravel base over the same area is 66.67 cubic feet or 2.47 cubic yards after compaction. Because compaction reduces volume, order about 10 to 15% more than the finished depth requires.
Reinforcement
Steel does not make concrete stronger in compression. Its job is to hold cracks tight and carry tension. For a residential slab, designers often specify welded wire mesh or small-diameter bars in a grid, commonly with spacing between 16 and 24 inches. As an example, a 20 × 10 ft slab with bars on an 18-inch grid needs about 8 bars running the 20-foot direction and 15 bars running the 10-foot direction. That is 160 + 150 = 310 linear feet of bar, before overlaps and waste. Use the Reinforcement Calculator for the layout and the Rebar Weight Calculator when you need the tonnage for a quote.
Whatever you choose, keep the steel in the middle or upper third of the slab, held up on chairs or spacers. Rebar lying on the dirt does almost nothing.
Mistakes that cost money
- Measuring the form, not the hole. If you dig deeper than planned, the extra thickness is filled with expensive concrete instead of cheap gravel.
- Forgetting the units. Inches and feet, millimetres and metres. Convert first, then multiply.
- No allowance for waste. Plan for the extra and cancel the surplus if you do not need it.
- Ordering at the wrong time. Hot, windy or freezing conditions affect finishing and curing. Check the forecast and arrange for help.
- Skipping the base. A good base is cheap compared with replacing a cracked slab.
- Ignoring delivery rules. Plants have minimum loads, fixed time slots on site and charges for waiting time. Ask before you book.
Concrete projects tend to expand once you start pricing materials, tools and labour. Put the whole job together with the Construction Material Estimator and the Construction Cost Estimator, and if you are financing the work, read our guide to how loan EMIs are calculated before you commit.
Frequently asked questions
How much concrete is in a cubic yard?
A cubic yard is 27 cubic feet. At a 4-inch thickness, one cubic yard covers about 81 square feet.
How long should concrete cure?
Concrete gains strength over weeks. Many sources suggest keeping it damp and protected for about a week and avoiding heavy loads for at least 28 days, though the details depend on the mix and the weather.
Do I need rebar in a small patio?
Not always, but reinforcement helps control cracking, particularly on weak soil or where vehicles will drive. Follow your local code and the advice of a professional for anything structural.
Should I order more than I calculated?
Yes. Add roughly 5 to 10% for waste. If the ground is uneven or the job is complicated, add more.
Related guides and tools
References
- Portland Cement Association, resources on concrete, cement and construction practice. cement.org
- American Concrete Institute, standards and guidance for concrete construction. concrete.org
- Wikipedia, “Concrete slab”: types, thickness and construction methods. en.wikipedia.org/wiki/Concrete_slab
- Wikipedia, “Rebar”: how reinforcing bar works in concrete. en.wikipedia.org/wiki/Rebar
The figures here are general estimates for planning and budgeting. Bag yields, mix designs, thicknesses and footing sizes depend on your product, soil and local building code. For structural work, consult an engineer or your local building authority.