Concrete Calculator

Calculate the cubic yards and bags a slab needs, plus the point where ready-mix gets cheaper.

Your measurements

The area you are filling

Measure the finished area you want covered, not the excavation. If the shape is irregular, split it into rectangles and add the results together.
Both measurements are in feet. For anything measured in inches, divide by twelve first. A 30 inch path is 2.5 feet wide.
In inches, because that is how depth is specified on site. What counts as usual depends on the material: an inch of bedding sand under pavers, 2 to 3 inches of gravel or mulch over a prepared bed, 4 inches for a concrete patio slab, 5 or 6 for a driveway carrying vehicles, and 6 to 12 inches of topsoil depending on what has to grow in it.
Use this when the same measurement repeats, such as four identical pads or three beds of the same size. Leave it at one for a single area.
Ten percent is the usual allowance, and what running short actually costs is rarely the material. On a bulk delivery it is a second haulage charge for a part load. On a concrete pour it is worse, because a slab finished in two goes has a cold joint through it. 10.00%
Material and price
Bags are sold by weight but used by volume, and the two do not track each other. An 80 lb bag of concrete mix yields about 0.6 cubic feet; mulch and soil are labeled in cubic feet directly.
Enter the price and then say what it is per, below. Delivery and any minimum load charge are not included here.
Bagged material is priced per bag, delivered aggregate per ton, and ready-mix per cubic yard. Pick whichever matches the quote you are working from.
Material to order
1.63 cu yd
9%
is spare
The job itself 1.48 cu yd
Waste allowance 0.15 cu yd
Order this much 1.63 cu yd
Area covered
120 sq ft
Same in cubic feet
44 cu ft

What to buy

Bags to buy 74 bags
Material cost $261
Works out per cubic yard $160.00

Every figure here is a planning estimate, not a professional takeoff. Yields, waste factors and code requirements vary by product, supplier and jurisdiction, so confirm quantities against the manufacturer’s data and your local building department before you order.

Generated by Mathterial

Concrete is ordered in cubic yards and measured in feet and inches, and that conversion is where most orders go wrong. This concrete calculator gives you the volume, the bag count and the cost, so the bags-or-ready-mix decision is made on numbers rather than on instinct.

The bag arithmetic is worse than it looks

An 80 pound bag of concrete mix yields about 0.6 cubic feet, the figure Quikrete prints on its concrete mix data sheet, alongside 0.45 for the 60 pound bag and 0.30 for the 40. A cubic yard is 27 cubic feet. So one cubic yard is 45 bags, and at $6.50 a bag that is $292.50 of material against roughly $160 for the same yard delivered ready-mixed.

You pay nearly twice as much for the privilege of mixing it yourself, and 45 bags is one and a half tons to move, open and mix inside the working time of the first batch.

The practical line sits around half a cubic yard. Below that, bags win: no delivery fee, no short-load charge, and you can work at your own pace. Above it, ready-mix wins on cost, on consistency and on the fact that a slab poured in one go has no cold joints through it. A 12 by 10 patio at 4 inches is 1.63 cubic yards, or 74 bags. That is a ready-mix job.

Depth is the number to get right first

Area is fixed by the site, so depth is the lever, and on concrete it is not one to trim. That same 12 by 10 slab:

  • 4 inches is 1.63 cubic yards, about $261. Standard for a patio, walkway or shed base.
  • 5 inches is 2.04 cubic yards, about $326.
  • 6 inches is 2.44 cubic yards, about $391. What a driveway carrying vehicles wants.

Saving an inch on a slab that will carry a car is a cracking problem you cannot repair, only replace. Spend it here rather than in the finish.

Ordering ready-mix without a mistake

Short-load fees apply below a full truck, typically under about 3 cubic yards, and they can add substantially to a small pour. Ask for the figure when you quote, because it may move the bags-or-mix line for your job.

Order slightly over rather than exactly. The truck will not wait while you find more, a second delivery for a quarter yard costs more than the concrete, and a pour that stops halfway leaves a cold joint that is a permanent weakness. The 10% allowance above covers an uneven subgrade, which is where concrete quietly disappears.

What sits under it

A slab needs a compacted aggregate base beneath it, typically 4 inches, and the gravel calculator works out that layer. For setting posts rather than pouring flat, the post hole concrete calculator handles cylinders and subtracts the post, which this page does not.

Concrete calculator FAQ

How many bags of concrete are in a cubic yard?

About 45 bags at 80 pounds each, since a bag yields roughly 0.6 cubic feet and a cubic yard is 27. At $6.50 a bag that is $292.50, against about $160 for the same yard delivered ready-mixed.

How much concrete do I need for a 12 by 10 slab?

About 1.63 cubic yards at 4 inches deep, including a 10% allowance, which is roughly 74 bags. At that size ready-mix is both cheaper and better, since the whole slab can be poured without a cold joint.

Should I use bags or ready-mix concrete?

Bags below about half a cubic yard, ready-mix above it. Bags avoid delivery and short-load fees and let you work at your own pace. Ready-mix is cheaper per yard, more consistent, and lets a slab be poured in one continuous go.

How thick should a concrete slab be?

Four inches for a patio, walkway or shed base. Five to six for a driveway carrying vehicles. On a 12 by 10 slab the difference between 4 and 6 inches is about 0.8 cubic yards, or roughly $130 of material.