Concrete Calculator

How to use this calculator

Measure the length and width of the area you're pouring and decide how thick the concrete needs to be. Enter those three numbers and you'll get the volume in cubic yards — the unit ready-mix is ordered in — plus cubic feet, cubic meters, and the number of pre-mix bags at the bag size you choose. Pouring more than one slab or footing? Add a row for each and they're totaled together.

The formula, explained

Concrete volume is just length × width × thickness, with all three in the same unit. The only step people trip on is the thickness, which you measure in inches but must convert to feet before multiplying:

Length (ft) × Width (ft) × (Thickness in ÷ 12) = cubic feet
Cubic feet ÷ 27 = cubic yards

The 27 comes from the fact that a cubic yard is 3 ft × 3 ft × 3 ft. To convert to bags, divide the cubic feet by the yield printed on the bag — an 80 lb bag of concrete mix yields about 0.6 cubic feet.

Worked example: a 10 ft × 10 ft patio at 4 inches

1. Area: 10 × 10 = 100 square feet
2. Convert the thickness: 4 ÷ 12 = 0.333 ft
3. Volume: 100 × 0.333 = 33.3 cubic feet
4. Cubic yards: 33.3 ÷ 27 = 1.23 cubic yards
5. In bags: 33.3 ÷ 0.6 = 56 bags of 80 lb mix
6. Add 10% waste: about 1.36 cubic yards, or 62 bags

Fifty-six bags is roughly two pallets and a full day of mixing — which is exactly why most people order ready-mix for a pour this size.

How thick should a concrete slab be?

Thickness is driven by the load the slab carries, not by the size of the slab. Going thinner than 4 inches is the most common way to end up with cracks.

Project	Typical thickness	Base required
Patios, walkways, shed floors	4 inches	4 in compacted gravel
Garage floors	4–6 inches	4 in compacted gravel
Driveways (cars)	4–5 inches	4–6 in compacted gravel
Driveways (RVs, trucks)	5–6 inches	6–8 in compacted gravel
Footings & post bases	8–12 inches	Undisturbed soil

Our guide on how thick a concrete slab should be covers reinforcement and freeze-depth considerations in more detail.

How much concrete for common project sizes

Volumes below include no waste allowance — add 5–10% to whatever you order.

Project	Size & thickness	Cubic yards	80 lb bags
Shed pad	4 × 4 ft @ 4 in	0.20	9
Walkway	3 × 20 ft @ 4 in	0.74	34
Small patio	8 × 8 ft @ 4 in	0.79	36
Patio	10 × 10 ft @ 4 in	1.23	56
Large patio	12 × 20 ft @ 4 in	2.96	134
Driveway (1 car)	10 × 20 ft @ 5 in	3.09	139
Driveway (2 car)	12 × 24 ft @ 5 in	4.44	200
Garage floor	20 × 20 ft @ 4 in	4.94	223

Step by step: how to measure your pour

1. Measure length and width at the forms, not the string lines — forms are what the concrete actually fills. Measure in two or three places and use the largest figure if they differ.
2. Split odd shapes into rectangles. An L-shaped patio is two rectangles. Enter each as its own row rather than averaging.
3. Check your depth in several spots. Subgrade is never perfectly flat. Drop a tape from a straightedge laid across the forms at four or five points and average the readings — if your form is 4 inches but the low corner is 5, you need concrete for the 5.
4. Account for edge thickening. Many slabs are thickened to 8–12 inches around the perimeter. That thickened edge is a separate volume; add it as an extra row.
5. Add your waste allowance last, once the total volume is known.

Choosing the right concrete

Ready-mix or bags?

The crossover point matters for both your back and your wallet.

Volume	Best option
Under ¼ cu yd (posts, small footings)	Bags, mixed by hand or in a small mixer
¼–½ cu yd (small pad, steps)	Bags if you have help; ready-mix if available
½–1 cu yd	Usually ready-mix — bagging gets impractical fast
Over 1 cu yd	Ready-mix delivered by truck

The hidden factor is time. Concrete starts setting as soon as it's wet, so a big bagged pour has to be mixed and placed in continuous batches. Fall behind and you get a cold joint — a weak seam where fresh concrete meets concrete that has already begun to stiffen.

What psi do you need?

Compressive strength is measured in psi and is printed on every bag and quoted on every ready-mix order.

Strength	Use it for
2,500 psi	Non-structural fill, setting posts
3,000 psi	Patios, walkways, shed floors — the residential default
3,500–4,000 psi	Driveways, garage floors, freeze–thaw climates
4,000+ psi	Heavy vehicles, workshop floors, structural work

If you're in a climate with hard freezes, ask for air-entrained concrete. The microscopic air bubbles give freezing water somewhere to expand, and it makes a real difference to how the slab survives its first few winters.

What will it cost?

Prices vary by region, but these ranges are a reasonable planning figure:

• Ready-mix: roughly $130–$170 per cubic yard delivered
• Short-load fee: $60–$150 on orders under about 3–4 cubic yards
• 80 lb bag of mix: roughly $5–$7

Worked cost: the 10 × 10 patio

That slab needs 1.23 cu yd, or about 1.36 cu yd with 10% waste.

• Ready-mix: 1.36 × $130–$170 = $177–$231, plus a short-load fee of $60–$150 → roughly $240–$380 all in
• Bags: 62 bags × $5–$7 = $310–$430, plus a day of mixing

At this size the two routes cost about the same, and ready-mix wins on labor. Below roughly half a cubic yard, the short-load fee flips the maths and bags win. That's the real reason to know your volume before you phone anyone.

Common concrete mistakes

• Skipping the gravel base. Most slabs need 4 inches of compacted gravel for drainage and even support. Water trapped under a slab is what heaves and cracks it — see how much gravel you need under a concrete slab.
• Forgetting a waste allowance. Subgrade is never level, concrete sticks to the mixer and barrow, and running short mid-pour is far worse than having a little left over. Add 5–10%.
• Mixing up inches and feet. A 4-inch slab is 0.333 ft thick, not 4. Entering it wrong overestimates by twelve times. The calculator handles the conversion, but check your unit toggle.
• Bagging a big pour. Past roughly half a cubic yard, hand-mixing dozens of bags is slow and the cold joints between batches weaken the slab.
• Pouring thinner than 4 inches. It's the single most common cause of cracking in a residential slab, and it saves very little money.
• Ignoring control joints. Concrete will crack; joints decide where. On a 4-inch slab that means joints roughly every 8–12 feet, cut within 6 to 12 hours in warm weather. See control joint spacing for the table by thickness.

Pro tips for a slab that lasts

• Call the supplier before you order. Ask about the minimum load, the short-load fee, and how long you get to unload before waiting time is charged.
• Have somewhere to put the extra. Order slightly over and keep a small form ready — a step, a splash pad, a post base. It beats a wheelbarrow of setting concrete and no plan.
• Pour early on hot days. Above roughly 85 °F concrete sets fast and you lose finishing time. Early morning gives you the working window back.
• Line up help before the truck arrives. Concrete doesn't wait. For anything over a cubic yard, have at least two people plus someone finishing.
• Cure it, don't just leave it. Keep the surface damp or covered for the first several days. Concrete gains most of its strength in that first week, and curing is free.

A note on accuracy

This calculator gives you a clean volume figure from your measurements. Real pours vary a little with subgrade depth, edge thickening, and how level your forms sit, so treat the result as a close planning number and round up when ordering. Ready-mix suppliers often have a minimum order, so it's always worth a quick call before you book.

Where these figures come from

The slab thickness and base guidance built into this calculator comes from the published sources below. The arithmetic itself — converting measurements to volume, and volume to bags or tons — is straightforward unit conversion, and you can check it in the full working the calculator shows. Material densities and bag yields vary by product and supplier, so confirm the figure printed on the bag or quoted by your yard before you order.

• Concrete in Practice (CIP) series Source: National Ready Mixed Concrete Association. Short technical bulletins on slab thickness, jointing, and curing.
• Working With Concrete Source: Portland Cement Association. Technical guidance on concrete basics, slabs-on-ground, and finishing.
• Subgrades and Subbases for Concrete Pavements (EB204P, PDF) Source: American Concrete Pavement Association / Portland Cement Association. The industry reference on what goes under a slab.
• Concrete Materials and Properties (Chapter 4, PDF) Source: Wisconsin Department of Agriculture, Trade and Consumer Protection. Explains the 4-inch base convention and concrete material properties.

Frequently asked questions

How much concrete do I need for a 10x10 slab?

A 10 ft by 10 ft slab at 4 inches thick needs about 33.3 cubic feet — roughly 1.23 cubic yards, or about 56 bags of 80 lb pre-mix. Add 5–10% for waste, which brings it to about 1.36 cubic yards. For a pour this size, ready-mix is usually the easier choice.

How many 80 lb bags of concrete are in a cubic yard?

About 45. An 80 lb bag yields roughly 0.6 cubic feet, and a cubic yard is 27 cubic feet, so 27 ÷ 0.6 = 45 bags. A 60 lb bag takes about 60 bags per yard, a 50 lb bag about 72, and a 40 lb bag about 90.

Should I order ready-mix or use bags?

Bags suit small jobs up to about half a cubic yard — fence posts, small footings, a little pad. Past that, ready-mix delivered by truck is usually cheaper once you count your time, and it avoids the cold joints you get when hand-mixed batches fall behind the pour.

How thick should a concrete slab be?

Four inches is standard for patios, walkways, and shed floors. Garage floors run 4–6 inches, car driveways 4–5 inches, and driveways carrying RVs or trucks 5–6 inches — all over a compacted gravel base. Below 4 inches, cracking risk rises sharply for very little saving.

How much extra concrete should I order?

Add 5–10%. Use the lower end for a small, tidy pour on level, well-prepared subgrade, and the higher end for larger slabs, irregular shapes, or ground you know isn't flat. Running short mid-pour costs you far more than a little leftover does.

What psi concrete do I need?

3,000 psi is the residential default and is fine for patios, walkways, and shed floors. Step up to 3,500–4,000 psi for driveways, garage floors, and anywhere with freeze–thaw cycles. In cold climates, also ask for air-entrained concrete — it handles freezing far better.

How much does a concrete slab cost?

Ready-mix runs roughly $130–$170 per cubic yard delivered, plus a $60–$150 short-load fee on orders under about 3–4 cubic yards. Bagged mix is roughly $5–$7 per 80 lb bag. A 10 × 10 ft patio at 4 inches works out around $240–$380 in ready-mix, or $310–$430 in bags plus a day of labor. Prices vary by region.

Do I need gravel under a concrete slab?

For a slab on ground, yes — almost always. Four inches of compacted gravel drains water away from the underside, spreads the load evenly, and gives you a flat surface to pour on. Skipping it is one of the most common causes of settling and cracking.