Concrete Volume Calculator
Calculate how much concrete your project needs. Enter slab, footing, or round-column dimensions to get cubic yards and the number of 40, 60, or 80-lb bags — with waste included.
Figure out how much concrete your pour needs. Pick the shape, enter the dimensions, and you'll get the volume in cubic yards and the number of premixed bags — plus whether to bag it or order ready-mix.
What are you pouring?
Pick the shape of your pour. A slab is any flat rectangle (patio, floor, driveway); a footing is a long strip; a column is a round pier or post footing.
Slab dimensions
Length and width in feet, thickness in inches.
Bag size
The premixed concrete bag size you’d buy, for the bag count. An 80-lb bag yields about 0.6 cu ft, a 60-lb bag 0.45, a 40-lb bag 0.30.
Waste / overage
Extra concrete for spillage, uneven subgrade, and over-excavation. 10% is the common allowance — never plan a pour with zero margin, since you can't add to concrete that's already setting.
Concrete Needed
1.4 cu yd
≈ 62 × 80-lb bags · 36.7 cu ft (incl. 10% waste)
You're at the bags-vs-truck crossover
Around a cubic yard it's a judgment call. Weigh 62 80-lb bags (plus the labor and a mixer) against a small ready-mix delivery, which is easier but often carries a short-load minimum. Either way, round up — running short mid-pour is the one mistake you can't undo.
Volume includes your waste allowance and rounds up; bag counts assume yields of about 0.6, 0.45, and 0.30 cu ft for 80, 60, and 40-lb bags. This estimates concrete only — not rebar or wire mesh, the compacted gravel base under a slab, or the forms. For structural footings and anything load-bearing, follow local code and your permit for required dimensions and reinforcement. Order in quarter-yard increments for ready-mix, and always round up: you cannot add to a pour once it starts to set.
💡About this calculator▼
Concrete is unforgiving to estimate: order too little and your pour sets before you can finish it, leaving a cold joint; order too much and you've paid for material you're scraping into a wheelbarrow. This calculator gives you a confident number before you mix or call the plant.
Pick what you're pouring — a flat slab, a strip footing, or round columns and piers — enter the dimensions, and you'll get the volume in cubic yards plus the number of premixed bags in your chosen size. A waste allowance is built in, because no real pour comes out to the exact calculated volume.
It also answers the question every concrete project runs into: should you buy bags or order ready-mix? The tool flags the crossover for you — small jobs are cheaper and simpler with bags, while once the volume climbs past a yard or so, having ready-mix delivered by the truck saves money, time, and a very sore back.
The calculator finds the volume of your pour, adds a waste overage, and converts that into both cubic yards and bags.
It computes volume from the shape you choose. A slab is length × width × thickness. A footing is length × width × depth, multiplied by how many you're pouring. A column or pier is a cylinder — π × radius² × height — times the count. All the inch measurements (thickness, footing width and depth, column diameter) are converted to feet first, so the result comes out in cubic feet.
Then it applies your waste allowance (10% by default). This isn't padding for its own sake: real subgrades are uneven, forms bulge slightly, some concrete is lost to spillage and the mixer, and — crucially — you can never add to a pour that has already begun to set. A little overage is cheap insurance against a ruined slab.
Finally it converts. Dividing cubic feet by 27 gives cubic yards, the unit ready-mix plants sell in. Dividing by the yield of your bag size (about 0.6 cu ft for an 80-lb bag, 0.45 for 60-lb, 0.30 for 40-lb) and rounding up gives the number of bags. The exact formulas and a worked example are below.
📐How it's calculated▼
The estimate computes volume, adds waste, then converts to yards and bags.
Step 1 — Volume (cubic feet): Slab: Length × Width × (Thickness ÷ 12) Footing: Length × (Width ÷ 12) × (Depth ÷ 12) × Count Column: π × (Diameter ÷ 24)² × Height × Count (feet for length/height, inches for the rest)
Step 2 — Add waste: Total = Volume × (1 + Waste % ÷ 100)
Step 3 — Convert: Cubic yards = Total ÷ 27 Bags = Total ÷ bag yield, rounded up (bag yield ≈ 0.60 / 0.45 / 0.30 cu ft for 80 / 60 / 40-lb bags)
Example: A 10 × 10 ft slab, 4 inches thick, 10% waste, 80-lb bags
→ Volume: 10 × 10 × (4 ÷ 12) = 33.3 cu ft
→ With waste: 33.3 × 1.10 = 36.7 cu ft
→ Cubic yards: 36.7 ÷ 27 = 1.4 yd
→ Bags: 36.7 ÷ 0.60 = 62 bags
At 62 bags, this 100-square-foot patio is right at the point where many people would order ready-mix instead — mixing 62 bags by hand is a serious day's work.
📎Source: Portland Cement Association — Concrete Basics
🔍Finding your inputs▼
What are you pouring: Choose the shape that matches your project. "Slab" covers any flat rectangle — a patio, shed or garage floor, walkway, or driveway. "Footing" is a long strip, like a footing under a wall or a thickened edge. "Column / pier" is a round form (a cardboard Sonotube) for deck footings, fence-post or mailbox bases, and porch piers.
Slab — length, width, thickness: Length and width in feet, thickness in inches. Four inches is standard for patios, walkways, and shed floors; step up to 5–6 inches for driveways or anywhere vehicles park. For an L-shaped slab, split it into rectangles, run each, and add the results.
Footing — length, width, depth, count: Length in feet (the total run), width and depth in inches. Enter how many separate footings of that size you're pouring. Footing dimensions are usually set by local code and your permit, so use the required numbers rather than guessing.
Column / pier — diameter, height, count: Diameter in inches (this is the Sonotube size you'd buy), height in feet (the depth of the hole plus any above-grade portion), and the number of identical columns. Deck and porch projects often have 4 to 12 of these.
Bag size: The premixed bag you'd buy if bagging the job — 40, 60, or 80 pounds. The calculator uses each bag's typical concrete yield to compute how many you need. 80-lb bags mean fewer bags to haul and open; 40-lb bags are lighter to carry. This only affects the bag count, not the volume.
Waste / overage: The extra you order beyond the bare calculated volume. Ten percent is the standard allowance and a good default. Bump it up for a hand-dug footing with rough sides or a sloped, uneven subgrade where the real volume runs over; you can trim it toward 5% only for tightly formed work on a level base.
⚠️Special situations▼
Should I use bags or order ready-mix concrete?
Volume decides it. Below about a half-yard, bags are clearly easier and cheaper. From roughly half a yard to a yard it's a judgment call — weigh the bag cost plus a rented mixer and a hard day's labor against a small ready-mix delivery. Above one to two yards, ready-mix almost always wins: it's poured in minutes instead of mixed in dozens of batches. One catch with ready-mix is the short-load fee plants add for small orders, so ask about that when you compare prices. This calculator flags which zone your pour falls in.
My slab or footing is an odd shape
Break it into simple pieces. An L-shaped patio is two rectangles; a pour with a round pad and a walkway is a slab plus another slab. Run the calculator once per piece and add the volumes (or the bag counts). For a slab that's thicker in one area — like a thickened edge or a footing integrated into a slab — calculate the main slab and the thicker strip separately, then sum them.
How thick should my slab be?
Four inches is the standard minimum for patios, walkways, shed floors, and other non-vehicle slabs. Go to 5–6 inches for driveways, garage floors, and anywhere a car or truck will sit, and thicker still for heavy equipment. Thickness drives volume directly, so an extra inch on a big slab adds real concrete — a 4-inch to 6-inch bump is 50% more material. Check local code for anything structural, and don't forget a compacted gravel base beneath the slab, which this calculator doesn't include.
Do I need to account for rebar, gravel, or forms?
This calculator covers the concrete itself only. Reinforcement (rebar or welded wire mesh) displaces a negligible amount of concrete, so it doesn't change the volume meaningfully — but it's a separate material to buy. The compacted gravel or crushed-stone base that goes under a slab is also separate and is calculated like its own slab (area × depth). And the forms — lumber or stakes — are their own line item. Budget for all three alongside the concrete.
What if I come up a little short during the pour?
This is exactly what the waste allowance is meant to prevent, because coming up short is costly. Concrete starts to harden within an hour, so if you run out and the first batch has begun to set, adding fresh concrete creates a cold joint — a visible, weaker seam. With bags, keep a few spares on hand and only open them if needed (unopened bags are returnable at most stores). With ready-mix, order a hair over rather than under; paying for a little extra concrete is far cheaper than a compromised slab.
❓Common questions▼
How much concrete do I need?
Multiply the area by the thickness to get the volume, then convert: for a slab, length × width × (thickness in feet) gives cubic feet, and dividing by 27 gives cubic yards. For example, a 10 × 10 ft slab at 4 inches thick is about 1.2 cubic yards before waste, or roughly 56 80-lb bags. Add about 10% for waste. Enter your project's shape and dimensions above and the calculator does the full conversion, including bags, for you.
How many bags of concrete are in a cubic yard?
A cubic yard is 27 cubic feet. Since an 80-lb bag yields about 0.6 cubic feet, a 60-lb bag about 0.45, and a 40-lb bag about 0.30, a full cubic yard takes roughly 45 80-lb bags, 60 60-lb bags, or 90 40-lb bags. That's a lot of mixing — which is why projects approaching a cubic yard are usually cheaper and far easier to pour with ready-mix delivery than with bags.
How many 80-lb bags of concrete make a cubic yard?
About 45. Each 80-lb bag of premixed concrete yields roughly 0.6 cubic feet, and a cubic yard is 27 cubic feet, so 27 ÷ 0.6 ≈ 45 bags. For 60-lb bags it's about 60, and for 40-lb bags about 90. Mixing 45 bags by hand is a strenuous, full-day job, so a cubic yard is right around the point where most people switch to ordering ready-mix.
How much extra concrete should I order for waste?
About 10% is the standard allowance and a safe default. The extra covers uneven subgrade, spillage, slight form movement, and the bit left in the mixer — and it guards against the bigger problem of coming up short, since you can't add to a pour that's already setting. Tighten toward 5% only for precisely formed work on a level base; bump it higher for rough, hand-dug footings. It's always cheaper to have a little left over than to ruin a slab with a cold joint.
Is it cheaper to mix my own concrete or order ready-mix?
For small jobs, bags are cheaper; for larger ones, ready-mix wins on both cost and effort. Bags make sense up to roughly half a cubic yard. Between a half-yard and a yard or so it's a toss-up once you factor in mixer rental and your labor. Past a couple of yards, ready-mix is clearly cheaper per yard and saves hours of back-breaking mixing — just watch for the short-load fee plants charge on small deliveries. The calculator tells you which zone your pour is in.