Concrete Calculator

What this calculator works out

How much concrete a whole pour needs, even when it mixes shapes: a patio slab with a few post holes, a footing under a wall, or a set of steps. It adds up every part, adds the waste allowance you choose, and gives the total in cubic yards, cubic feet and cubic meters. It then counts the bags for each bag size you enter, using the yield printed on the bag, and rounds a ready-mix order up to the step your supplier sells in. With your prices, it lists the cost of each option side by side.

How to use it

• Units: feet and inches, or meters and centimeters. Switching converts the measures you have typed. The results always show cubic feet, cubic yards and cubic meters.
• Elements: one row for each shape. Choose the shape, then its measures. Lengths, widths and heights are in feet (type 16, 16.5, 16'6" or 16 ft 6 in) or meters. Thicknesses, depths, hole diameters, rises and treads are in inches or centimeters, so a 4 in slab is 4. Quantity repeats a row for identical parts, such as post holes; blank counts as 1.
• Waste allowance: the percentage added to the measured volume for spills, uneven ground and forms that bulge. 0% is allowed.
• Bag sizes: the weight and the yield printed on each bag you might buy, in pounds or kilograms and cubic feet or liters, plus a price per bag if you want the cost. Remove the sizes you won’t buy, or remove them all if you are only ordering ready-mix.
• Ready-mix: the step your supplier rounds orders to, and optionally a price per cubic yard (or meter) and any flat fees, such as delivery.

Results update as you type. The Try buttons load a 10 × 10 ft patio, fence posts set in concrete, porch steps with a landing, a footing under a wall, and a metric slab. To weigh two plans, press Save for comparison, change the inputs and save again.

How much concrete do I need?

Work out the volume of each part in cubic feet, add them up, add the allowance, and divide by 27 for cubic yards. Rectangular parts (slabs, footings, walls) are length × width × thickness. Round parts (columns, post holes, round slabs) are cylinders: π × radius² × depth.

$$\begin{aligned} V_{\text{rectangle}} &= L \times W \times T \\ V_{\text{round}} &= \pi \left(\frac{D}{2}\right)^2 H \\ V_{\text{yd}^3} &= \frac{V_{\text{ft}^3}}{27} \end{aligned}$$

• $L$, $W$ and $H$ are lengths in feet, and $D$ is the diameter in feet.
• $T$ is the thickness in feet: inches ÷ 12, so 4 in is 0.3333 ft.
• A cubic yard is 3 × 3 × 3 = 27 cubic feet. For cubic meters, multiply cubic feet by 0.028316846592.

In metric, measure in meters, divide centimeters by 100, and the result is already in cubic meters.

Worked example: a 16 × 12 ft patio and four pergola footings

A patio slab 16 ft by 12 ft, 4 in thick, plus four round footings for pergola posts, each 10 in across and 3 ft deep, with a 10% allowance:

1. Slab: 16 × 12 × (4 ÷ 12) = 64.000 ft³.
2. One footing: π × (10 ÷ 12 ÷ 2)² × 3 = π × 0.4167² × 3 = 1.636 ft³. Four of them: 6.545 ft³.
3. Measured volume: 64.000 + 6.545 = 70.545 ft³ (2.61 yd³).
4. Allowance: 70.545 × 10% = 7.054 ft³, so buy 77.599 ft³, shown as 77.60 ft³.
5. Cubic yards: 77.60 ÷ 27 = 2.87 yd³. In cubic meters, 2.20 m³.
6. Bags at 0.60 ft³ each: 77.60 ÷ 0.60 = 129.33, rounded up to 130 bags of 80 lb. With 0.45 ft³ bags it takes 173 of 60 lb; with 0.30 ft³ bags, 259 of 40 lb.
7. Ready-mix rounded up to the next 0.25 yd³: 3.00 yd³, 0.13 yd³ more than needed.

Whichever bag size you choose, you would carry more than 10,000 lb: 10,400 lb in 80 lb bags, 10,380 lb in 60 lb bags or 10,360 lb in 40 lb bags.

How many bags of concrete do I need? (40, 60 and 80 lb)

Divide the volume, allowance included, by the yield printed on the bag, and round up to a whole bag. The yield is the volume of mixed concrete one bag makes, and it differs between products, so read it off the bag you will buy. The example uses one manufacturer’s published figures for its concrete mix: 0.30 ft³ for 40 lb, 0.45 ft³ for 60 lb and 0.60 ft³ for 80 lb.

Round up once, for the whole pour. Counting bags for each part and adding them up can buy several bags too many.

How many bags make a cubic yard?

With those yields, a cubic yard (27 ft³) takes:

That is before any allowance. A bag with a different yield gives a different count, and the calculator uses whatever yield you type.

How much extra concrete should I order?

The ready-mixed concrete industry association suggests ordering at least 4 to 10% more than the volume calculated from the forms. Most of the reasons are about the size of the pour, not spills: forms that bow under the weight, uneven or settling ground, and slabs thicker than drawn. Its example: 1/8 in extra on a 4 in slab is a 3% shortfall. Hardened concrete can also take about 2% less room than the fresh concrete delivered.

The If the slab ends up thicker or thinner table shows what that means for your project. In the worked example, a slab 1/8 in thicker than planned needs 2.96 yd³ and 133 bags of 80 lb, and one 1/2 in thicker needs 3.20 yd³, 144 bags and a 3.25 yd³ order. So measure the depth in several places and use the average; the allowance is for spills and surprises, not for a wrong thickness.

Concrete for a slab, footing, wall, post hole or steps

• Slab or patio: length × width × thickness. For an L-shaped slab, add two rectangles as two elements.
• Footing or curb: length in feet, width and depth in inches. A continuous footing under a wall is one long footing.
• Wall: length and height in feet, thickness in inches.
• Column or hole (a post hole or a tube form): the diameter in inches and the depth in feet. For a post set in concrete, enter its width under Post width: the post’s volume is taken out, since it fills part of the hole. In the fence preset, a 3.5 in square post takes 0.213 ft³ of each 1.364 ft³ hole. The calculator refuses a post too wide for its hole.
• Round slab: the diameter in feet and the thickness in inches.
• Steps: the width, the number of steps (count the risers), the rise and the tread depth. Steps are treated as solid concrete on the ground: the first step is one rise tall, the second two rises, and so on. A top landing replaces the top tread with a deeper platform. Adding up those blocks gives width × rise × (tread × n(n − 1) ÷ 2 + top depth × n) for n risers, where the top depth is the landing (or one more tread without a landing). The porch preset (3 risers of 7 in, 11 in treads, 4 ft wide, with a 3 ft landing) comes to 27.417 ft³ before the allowance.

Bags or ready-mix: comparing the cost with your prices

Enter a price per bag, a ready-mix price per cubic yard and any flat fees, and the Cost with your prices table shows each option with its arithmetic. The calculator doesn’t choose for you; besides the price, weigh the time to mix bag after bag against booking a delivery, and the weight to haul (the bags table shows it).

Ready-mix is sold by the volume of fresh concrete, in cubic yards or cubic meters. Ask your supplier for the step they sell in, their minimum load and any fees, and enter them. The industry association advises re-checking the volume near the end of a pour and telling the supplier in good time, rather than waiting for a small extra load.

Reading the result

• Concrete to buy is the total with the allowance, in cubic yards (or cubic meters in metric), with the other units and the first bag size under it.
• Measured volume, Waste allowance and Ready-mix order split that total. The order is rounded up to your step, and the extra it adds is shown.
• Bags by size gives, for each bag, how many to buy, the concrete they make, how much is left over and the total weight to carry, and it downloads as a CSV file.
• Cost with your prices appears when you enter a price.
• Concrete by element lists the volume of each row, the allowance and the total, and Where the concrete goes shows each part’s share.
• If the slab ends up thicker or thinner reruns the calculation with every slab a little thicker or thinner, keeping everything else as entered.
• How this was calculated shows every step with your numbers.

Assumptions and limitations

• Each element is a simple shape: straight-sided cylinders, flat slabs of even thickness, and solid steps on the ground. Thickened slab edges, sloped ground or tapered holes need to be split into shapes or averaged.
• A post runs the full depth of its hole. Gravel at the bottom of a hole, reinforcing bar and mesh aren’t subtracted, and a gravel base under a slab isn’t counted.
• Bag yields are the ones you enter. The example’s yields come from one manufacturer’s data sheet; other mixes differ.
• Nothing here decides how thick a slab, footing or wall must be, or whether it needs reinforcement. Those depend on the load, the soil and local building codes, and are for the people designing and inspecting the work.
• Prices are only the ones you type. Volumes are rounded for display; the calculation keeps full precision.

Common mistakes

• Typing the thickness in feet. The thickness box takes inches, so 0.33 means a third of an inch. The calculator points this out when a thickness is under an inch.
• Dividing by 9 instead of 27. Nine converts square feet to square yards; cubic feet to cubic yards is 27.
• Using the planned thickness when the ground was dug deeper. Measure the finished depth in several places. A 4 in slab poured 1/2 in thicker everywhere needs 12.5% more concrete, more than a 10% allowance.
• Rounding up for each part. Round bags once, for the total. Rounding each post hole to whole bags buys several extra.
• Reusing another product’s yield. The yield belongs to the mix in the bag, so read it off the bag you buy instead of reusing a figure from another product or another page.
• Leaving out small parts. Post holes, a thickened edge or a step add up, and running short in the middle of a pour is the costly way to find out.