How to Estimate Concrete for a Slab

Concrete is the least forgiving material you can order for a home project. Mulch can be topped up and gravel can be raked thinner, but once a pour starts, it has to finish. Run short and you are left waiting for a second truck while the first load stiffens, and the join between the two batches can show for the life of the slab. A careful estimate prevents all of that. This guide walks through one real world slab from tape measure to final order, covering the details simple calculators leave out: measuring inside the forms, uneven ground, thickened edges and the bag versus truck decision.

What You Need Before You Start

• The slab outline, ideally with the forms already set, so you measure what will actually be filled.
• A planned thickness, based on what the slab will carry.
• A base plan. Most slabs sit on a compacted layer of gravel, which is ordered separately.
• Local requirements. Driveways, garage floors and anything attached to a building may fall under building codes or permits, which can set the thickness and reinforcement for you.

Step 1: Measure the Slab Footprint

Measure the inside faces of the forms, not the outside of the boards. Form lumber is usually 1.5 inches thick, so measuring across the outside of two boards adds 3 inches to every dimension. On a long slab that adds up to real money.

Our example is an L shaped patio. The main section is 16 ft by 12 ft, and a smaller section 8 ft wide sticks out 6 ft from one long side to make room for a grill. Split it into two rectangles and add them:

• Main section: 16 × 12 = 192 square feet
• Grill extension: 8 × 6 = 48 square feet
• Total area: 240 square feet

For curves, circles or angled corners, the Square Feet Calculator can find the area of each piece.

Step 2: Choose the Slab Thickness

Thickness depends on what the slab has to support. These are figures commonly used for residential work, but local codes and the advice of a qualified professional take priority, especially for anything carrying vehicles or attached to a building.

Why the ground underneath matters

A slab is only as thin as the shallowest part of the base. If the gravel underneath dips half an inch in places, those spots become 4.5 inch slab instead of 4. Across our whole 240 square foot patio, an extra half inch would add 240 × 0.5 ÷ 324 = 0.37 cubic yards. Before you calculate, run a string line across the top of the forms and check the depth down to the base at several points so you know what you are really filling.

Step 3: Work Out the Volume in Cubic Yards

Ready mix is ordered in cubic yards. With the area in square feet and thickness in inches, one division gets you there:

For the patio at 4 inches thick: 240 × 4 ÷ 324 = 2.96 cubic yards, which is 80 cubic feet. If you want the reasoning behind the formula, our cubic yard calculation guide covers it step by step.

Step 4: Add Thickened Edges

Many slabs are poured with a thickened edge, also called a turned down edge, where the concrete gets deeper around the perimeter to stiffen the border. It is easy to leave out of an estimate, and it can add a surprising amount.

Work it out as a long, narrow strip running around the outside. The L shaped patio has a perimeter of 68 ft. If the edge is 12 inches wide and goes 4 inches deeper than the rest of the slab, the extra volume is 68 × 1 × (4 ÷ 12) = 22.7 cubic feet, or 0.84 cubic yards. That pushes the total from 2.96 to 3.80 cubic yards, nearly 30 percent more than the flat slab alone.

The same approach works for any extra concrete attached to the slab, such as a step, a footing under a post or a thicker strip where a wall will sit. Calculate each piece separately and add them.

Step 5: Decide on an Allowance

Forms flex slightly under the weight of wet concrete, some mix stays in the wheelbarrow or chute, and bases are never perfectly flat. Contractors commonly allow 5 to 10 percent on top of the measured volume. Since running short is far worse than having a little left over, err toward the higher end on a slab you are pouring yourself.

Many concrete plants batch in quarter- or half yard steps, so 4.10 rounds up to 4.25 cubic yards. Ask your supplier how they measure and whether there is a minimum load.

Step 6: Convert to Bags If You Are Mixing Yourself

Premixed bags list their yield in cubic feet on the label. Divide your total cubic feet by that yield and round up. The patio's 4.10 cubic yards is about 110.8 cubic feet, which works out to:

• 185 bags of 80 lb (about 0.6 cubic feet each)
• 247 bags of 60 lb (about 0.45 cubic feet each)
• 370 bags of 40 lb (about 0.3 cubic feet each)

Those 185 bags of 80 lb weigh 14,800 pounds, more than seven tons to lift, mix and place in one session. That is why a slab this size is almost always poured from a truck. Bags shine on small jobs instead. A pad for an air conditioning unit, 3 ft by 4 ft and 4 inches thick, holds 4 cubic feet: seven 80 lb bags or nine 60 lb bags. The Concrete Calculator does these bag counts for any size.

A quick per square foot check

At 4 inches thick, each square foot of slab needs one third of a cubic foot of concrete. So one 80 lb bag covers about 1.8 square feet, a 60 lb bag about 1.35 square feet and a 40 lb bag about 0.9 square feet. It is a handy sense check when you are standing in the store.

Concrete for Common Slab Sizes

These are the slab sizes people ask about most. Each figure is the exact volume with no allowance, so add your own margin from Step 5 before you order.

The bag column shows why bigger slabs are poured from a truck. For a size that is not listed, the Concrete Calculator works it out for any shape and thickness.

Bags or Ready Mix: Where the Line Falls

There is no fixed cutoff, but these questions usually make the decision obvious:

• How many bags would it take? Mixing more than a few dozen by hand or with a small rental mixer is a long, hard day, and it is difficult to keep every batch consistent.
• Can a truck reach the forms? If not, you will need wheelbarrows or a concrete pump, which adds cost and crew.
• Is the order below the plant's minimum? Small ready mix deliveries often carry a short load fee, which can make bags cheaper for pours of around a yard or less.
• How many helpers do you have? A truck delivers concrete quickly, and it needs to be spread, leveled and finished before it sets.

What Your Concrete Estimate Does Not Include

The volume above covers concrete only, and the same goes for any sand you lay to smooth the base, which our sand guide can help you estimate. A slab usually sits on a compacted gravel base, commonly around 4 inches deep, which is ordered and calculated separately. For the 240 square foot patio, a 4 inch base works out to roughly 3 cubic yards once compacted, and more loose stone than that at delivery. Our guide to how much gravel you need explains how compaction changes that figure, and the Gravel Calculator handles the numbers.

Reinforcement such as rebar or wire mesh takes up so little space that it does not meaningfully change the volume. Whether you need it, and how much, depends on the slab's use and local requirements.

Common Slab Estimating Mistakes

• Trusting nominal lumber sizes. A 2×4 is actually 3.5 inches tall. Set on edge as a form for a "4 inch" slab, it leaves you half an inch short unless the base is set lower.
• Measuring outside the forms. Always measure the inside faces, where the concrete goes.
• Leaving out thickened edges or footings. As the example shows, they can add more than a quarter to the total.
• Mixing up bag yields. Bag sizes and yields vary by brand and product type, so check the label rather than assuming.
• Ordering the bare minimum. A small surplus can fill a few post holes or a stepping stone form. A shortfall can ruin the slab.

Frequently Asked Questions

How much concrete do I need for a 20 × 24 slab?

A 20 ft by 24 ft slab is 480 square feet. At 5 inches thick, that is 480 × 5 ÷ 324 = 7.41 cubic yards before any thickened edges or allowance.

Can I pour a slab in two batches?

It is possible, but the second batch needs to go in while the first is still workable, or you end up with a weak seam known as a cold joint. If a slab must be poured in stages, plan the break at a control joint and get advice for your specific project.

What is a short load fee?

It is an extra charge many ready mix suppliers add when an order falls below their minimum delivery size. Ask about it when you request a quote, because it can change whether bags or a truck is cheaper.

Does rebar or mesh change how much concrete I need?

Not in any practical sense. Reinforcement occupies a tiny fraction of the slab's volume, so calculate the concrete as if it were not there.

How many square feet does an 80 lb bag of concrete cover?

At 4 inches thick, an 80 lb bag yielding about 0.6 cubic feet covers roughly 1.8 square feet. At 6 inches thick, it covers about 1.2 square feet.

Final Thoughts

A dependable slab estimate comes from measuring inside the forms, checking the real depth of the base, adding every thickened edge and footing, and then allowing a modest surplus. Do that and the truck should empty right as the last corner fills. The Concrete Calculator handles slabs, footings and round piers in yards and bags, and the cubic yard calculator works out the gravel base under them. For post holes, steps and walls, see how much concrete you need for other projects. If you are curious how much your pour will weigh, see our cubic yards to tons guide.