Ordering concrete is one of those jobs where a small measuring mistake turns into a real problem. Order too little and the pour stops halfway, leaving a cold joint in the middle of your slab. Order too much and you have paid for a pile of concrete you cannot use. The good news is that the maths behind it is simple: measure, multiply, convert, and then add a little extra for waste. This guide walks through each step with a worked example you can follow for your own project.
Step 1: Measure the job
For a rectangular slab or footing you need three measurements: length, width and thickness. For a round column you need the diameter and the height. Measure in feet where you can, because concrete is sold by the cubic foot and cubic yard, and working in feet keeps the numbers clean.
Thickness is the measurement people most often get wrong, because it is usually given in inches. A 4-inch slab is not 4 feet thick — it is 4 divided by 12, or 0.33 feet. A 6-inch slab is 0.5 feet. Always convert inches to feet before you multiply, or your volume will be twelve times too big.
Measure the formwork, not the drawing. Timber forms bow, ground settles and hand-dug footings are rarely perfectly straight, so the hole you actually pour into is usually a little bigger than the plan says. Measuring the real opening is the first step to an accurate order.
Step 2: Work out the volume in cubic feet
For a slab or footing, multiply length by width by thickness, all in feet. For a round column, multiply pi (3.14) by the radius squared, then by the height. The radius is half the diameter, so a column 1 foot across has a radius of 0.5 feet.
Take our worked example: a patio slab 10 feet long, 10 feet wide and 4 inches thick. Convert the thickness first — 4 inches is 0.33 feet. Then multiply: 10 x 10 x 0.33 = 33.33 cubic feet. That is the volume of concrete the slab itself needs, before any allowance for waste.
Step 3: Convert cubic feet to cubic yards
Ready-mix concrete is sold by the cubic yard, and there are 27 cubic feet in one cubic yard. Divide your cubic feet by 27. For our slab, 33.33 divided by 27 is 1.23 cubic yards. In metric terms that is 0.94 cubic metres.
Suppliers usually sell in fractions of a yard, and many have a minimum order or a short-load fee for small amounts. Knowing your figure in cubic yards lets you compare quotes properly, because that is the unit the price is quoted in.
Step 4: Work out how many bags you need
If the job is small, bags of ready-mix concrete are often more practical than a truck. The number of bags depends on the size of the bag, because each size makes a different amount of concrete. The yields printed on most US bags are:
- A 40 lb bag makes 0.30 cubic feet
- A 60 lb bag makes 0.45 cubic feet
- An 80 lb bag makes 0.60 cubic feet
Divide your volume by the yield per bag. Our slab needs 33.33 cubic feet, so that is 33.33 divided by 0.45, which is 74.07 — so 75 bags of 60 lb mix. The same slab would need 56 bags of 80 lb mix, or 112 bags of 40 lb mix. As a handy rule, one cubic yard needs 90 bags of 40 lb, 60 bags of 60 lb, or 45 bags of 80 lb.
As a general guide, if the job needs more than about one cubic yard, ready-mix delivery is usually cheaper than bags and far less work. Under that, bags are the practical choice for posts, small footings and repairs.
Step 5: Add a waste allowance
Never order the exact volume you calculated. Some concrete spills, some stays in the mixer and the wheelbarrow, the ground is never perfectly level, and a hand-dug footing is always a little wider than planned. A 10% waste allowance is a sensible default for most jobs.
For our slab, adding 10% takes the volume from 1.23 to 1.36 cubic yards. That small extra is cheap insurance against running short in the middle of a pour, which is the one outcome that is hard to fix afterwards.
Step 6: Work out the cost
The cost has three parts: the concrete itself, delivery, and labour. The concrete cost is your volume with waste multiplied by the price per cubic yard. At $150 per yard, our slab’s 1.36 cubic yards cost $203.70. Add a delivery fee if your supplier charges one — say $75 — and labour for the pour and finish, which might be $6 per square foot, or $600 for a 100 square foot slab. The installed total is $878.70.
Always ask for an itemised quote. Delivery fees, minimum-load charges, reinforcement, formwork and finishing are often priced separately, and the headline price per yard is only part of the job.
Worked example summary
| Measurement | Value |
|---|---|
| Slab size | 10 ft x 10 ft x 4 in |
| Volume | 33.33 cu ft |
| Volume in cubic yards | 1.23 cu yd (0.94 m³) |
| Bags needed (60 lb) | 75 bags |
| Volume with 10% waste | 1.36 cu yd |
| Installed cost example | $878.70 |
How to use the calculator
Open the concrete calculator, choose Slab, Footing or Column, and enter your measurements. Set your waste allowance — 10% is the default — and the calculator shows the volume in cubic feet, cubic yards and cubic metres, plus the bag counts for all three bag sizes. Enter your price per cubic yard, delivery fee and labour rate to see the installed cost. If you already know the slab size and only want the price, the concrete slab cost calculator does the same job in fewer steps, and the concrete bag calculator is there when you are buying bags instead of ready-mix. For reinforcement, the rebar calculator works out the steel your slab needs.
Common mistakes to avoid
The most common mistake is forgetting to convert inches to feet, which makes the volume twelve times too large. The second is ordering the exact calculated volume with no waste allowance — a short pour in the middle of a slab is the most expensive mistake on this list. The third is measuring from the drawing instead of the formwork. And the fourth is forgetting the extras: delivery fees, reinforcement and finishing labour are real costs, and a quote that leaves them out is not the full price of the job.
Measure twice, add your waste, and check the quote line by line. Concrete is forgiving about many things, but it is not forgiving about running out halfway through.