Skip to main content
Tool Belt Calc

Home improvement

Concrete Calculator

Slab volume in cubic yards, and how many bags that works out to.

Your measurementsEvery assumption editable01

4 in is standard for a patio or shed base, 6 in for a driveway.

An 80 lb bag yields about 0.6 cu ft of mixed concrete.

Covers spillage and an uneven subgrade. 10% is a sensible minimum.

Concrete requiredCalculated live02

Result

1.36cu yd

1.36 cubic yards — or 62 bags

Slab volume10 × 10 × 4 in
33.33 cu ft
With waste allowance+10%
36.67 cu ft
In cubic yardshow ready-mix is ordered
1.36 cu yd
Bags of premixat 0.6 cu ft per bag
62 × 80 lb

Past roughly a cubic yard, ready-mix delivery is usually cheaper and always easier than bags — a cubic yard is around 45 eighty-pound bags, which is well over a ton to carry and mix by hand.

Concrete cannot be poured in stages without leaving a cold joint. Order the whole pour at once, or plan a deliberate control joint where the sections meet.

This figure is the slab only. Footings, thickened edges and a compacted sub-base are separate volumes.

Enter the slab dimensions and thickness and it gives you both figures that matter: cubic yards for ordering ready-mix, and bag count for buying premix. The gap between them decides how you buy — past about a cubic yard, bags stop making sense.

Why use this tool?What it does differently03

Why use this tool?

Yards and bags together

Ready-mix is ordered by the cubic yard, premix is bought by the bag. Both are shown so you can compare before deciding.

All three bag sizes

40, 60 and 80 pound bags have different yields. Pick the one you can actually carry and the count updates.

Waste included

Subgrades are never perfectly flat and some concrete is always lost. The allowance is editable but starts at a sensible 10%.

Thickness in inches

Enter 4 for a patio or shed base, 6 for a driveway. The conversion to feet happens internally so you enter the number you actually know.

How this worksThe method04

How this concrete calculator works

Volume is length times width times thickness, with the thickness converted from inches to feet. That gives cubic feet, which is divided by 27 to reach cubic yards — the unit every ready-mix supplier quotes in.

Bag counts come from the yield of a mixed bag rather than its dry weight. An 80 pound bag of premix yields about 0.6 cubic feet of placed concrete, a 60 pound bag about 0.45, and a 40 pound bag about 0.3. Those yields are why the bag route gets heavy so quickly: a single cubic yard is roughly 45 eighty-pound bags.

The waste allowance covers an uneven subgrade, spillage and the concrete left in the mixer. Ten per cent is a floor rather than a ceiling on a hand-dug excavation, where the base is rarely as level as intended and the low spots absorb material invisibly.

How to use itStep by step05

How to use it

  1. Step 1: Measure the slab

    Length and width of the area to be poured, at the finished dimensions rather than the excavation.

  2. Step 2: Set the thickness

    Four inches is standard for a patio, walkway or shed base. Six inches for anything a vehicle will drive on.

  3. Step 3: Pick a bag size

    If the bag count looks manageable, choose the size you can lift repeatedly. If it does not, the cubic yard figure is what you give the ready-mix supplier.

  4. Step 4: Check the crossover

    Compare the two figures. Around a cubic yard, delivery usually costs less than the bags and saves hours of mixing.

Example usageWorked figures06

Example usage

A 10 by 10 patio at 4 inches
33.3 cubic feet plus 10% is 36.7, which is 1.36 cubic yards or 62 eighty-pound bags. That is well over two tons of material to move by hand — firmly in ready-mix territory.
A small shed base
A 6 by 8 foot base at 4 inches is 16 cubic feet plus waste, about 30 eighty-pound bags. Manageable with a mixer and a helper, and below most suppliers' minimum delivery.
A driveway section
20 by 10 feet at 6 inches is 100 cubic feet, or 4.07 cubic yards with waste. Order ready-mix, and plan the pour in one go — a driveway poured in two sessions has a cold joint through the middle.
Frequently asked questionsCommon questions07

Frequently asked questions

How many bags of concrete are in a cubic yard?

About 45 eighty-pound bags, 60 sixty-pound bags, or 90 forty-pound bags. Those yields are roughly 0.6, 0.45 and 0.3 cubic feet of mixed concrete respectively.

When should I order ready-mix instead of bags?

Around a cubic yard is the usual crossover. Below that, bags avoid a delivery charge and a minimum order. Above it, bags mean moving over two tons of material and mixing continuously for hours, and ready-mix is normally cheaper too.

How thick should a concrete slab be?

Four inches suits patios, walkways and shed bases. Six inches is the minimum where vehicles will park or drive. Anything structural, or any slab on poor or made-up ground, needs proper design rather than a rule of thumb.

Does this include footings?

No. It calculates the slab volume only. Footings, thickened edges and a compacted sub-base are separate volumes — calculate each as its own rectangle and add them.

Can I pour a slab in sections?

Not without consequence. Concrete placed after the previous batch has begun to set forms a cold joint, which is a plane of weakness. Either pour the whole thing in one session or design a deliberate control joint at the boundary.

Related toolsElsewhere on the site08

Browse every tool in Home Improvement Calculators.