ToolNestFree Concrete Calculator
Last updated: October 26, 2026
Quick answer: Concrete is the one material you cannot return: order too little and you get a cold joint and a short-load fee on the second truck; order too much and you have paid $130–$190 for a wasted yard that sets before you can use it. This free concrete calculator does the ordering math for slabs (patios, driveways, shed bases) and round footings and post holes: enter your dimensions and it returns the volume in cubic feet and cubic yards, the 40, 60, and 80 lb bag counts, and an estimated ready-mix cost when you enter a price per yard. It adds a waste allowance — excavations are never perfectly flat — and it tells you the rule every contractor uses: under about half a yard, bags win; above it, price the truck. Everything runs in your browser; no signup, no app.
Bag yields used: 80 lb ≈ 0.60 cu ft, 60 lb ≈ 0.45 cu ft, 40 lb ≈ 0.30 cu ft (manufacturer-published yields; check your bag's label). This is a quantity estimate, not a structural design — check local code for thickness, reinforcement, and footing depth.
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How to Use the Concrete Calculator
- Choose your shape: slab for patios, driveways, walkways, and shed bases, or round post holes / footings for fence posts, deck footings, and tube forms. The math is different for each — a slab is length × width × thickness, a hole is π × radius² × depth — and the calculator switches the inputs for you.
- For a slab, enter length and width in feet and thickness in inches: 4 inches is standard for patios, walkways, and shed floors; 5–6 inches for driveways and garage floors that carry vehicles. Always check local code — thickness, reinforcement, and base requirements vary by jurisdiction and by load.
- For round holes, enter the diameter in inches, the depth in feet, and how many holes you are pouring. The calculator totals all holes into one order, so a fence line of twelve post holes is one number, not twelve trips to the calculator.
- Set the waste allowance: 10% is the standard for slab pours, because excavations are rarely perfectly flat and some concrete is always lost in the forms and the wheelbarrow. Use 5% only for tight, flat, well-formed work; use 15% for rough excavation. Setting waste to 0% shows the exact geometric volume, which is what you will be short by if you order it.
- Enter a ready-mix price per cubic yard if you have a quote (roughly $130–$190 per yard delivered in most US markets) and press Calculate concrete. Read four things: volume in cubic feet, volume in cubic yards (how the plant bills), bag counts for 40, 60, and 80 lb bags, and the estimated ready-mix cost — plus the bags-vs-truck guidance for your order size.
- Press Copy order to take the order to your notes or to the plant. Order ready-mix by the quarter yard where the plant allows it, and round up, never down: a yard short means a cold joint in the middle of your slab.
Why Use Our Concrete Calculator?
- Slabs and round holes in one page — most thin calculators do slabs only, which leaves fence and deck builders doing hole math by hand. Both shapes, one order total.
- All three bag sizes side by side — 40, 60, and 80 lb counts from the same volume, so you can match whatever your store stocks instead of converting bag yields yourself.
- Waste allowance that means something — 0–15% with honest guidance on when each level fits, because the difference between exact geometry and a real excavation is the difference between finishing the pour and calling for a second truck.
- Ready-mix cost line — enter your plant's price per yard and the cost appears beside the yardage, so the bags-vs-truck comparison is in dollars, not vibes.
- The bags-vs-truck rule, stated plainly — under about half a yard, bags usually win; above about a yard, the truck usually wins; between them, price both. The calculator tells you which side of the line your order falls on.
- Multiple holes, one order — a fence line's twelve post holes total into a single yardage and bag count, which is how you actually order.
Common Use Cases
- A 10×10 ft backyard patio: 4 inches thick, 10% waste: 1.36 cubic yards, 62 bags of 80 lb, or about $238 of ready-mix at $175 per yard. This is the classic crossover order — price both, because bags cost roughly $370–$496 at retail for the same volume, and the truck wins unless the patio is unreachable.
- A fence line of post holes: Twelve 12-inch holes, 3 feet deep, 10% waste: 31.1 cu ft, 1.15 cubic yards, 52 bags of 80 lb. Twelve holes is where people underestimate: each hole looks small, but the total crosses into truck territory, and mixing 52 bags by hand is a full day's work.
- A driveway extension: 20 ft × 12 ft at 5 inches, 10% waste: 100 cu ft exact, 110 cu ft with waste, 4.07 cubic yards — firmly a truck order, roughly $530–$775 of ready-mix before short-load or pump fees. Ordering this in bags would mean 184 bags of 80 lb mix.
- Checking a contractor's concrete quote: A quote that lists 1.0 yard for a 10×10×4-inch patio has forgotten waste (the honest number is 1.36); a quote that lists 2 yards has padded it by nearly half. Run the dimensions here and compare the quote's yardage before you approve it.
Concrete Calculator vs Alternatives
- Thin single-shape concrete calculators (most of the current top 10): They answer the slab question with a clean page, and several do it well. What they do not do is round holes, show all three bag sizes together, or price the truck against the bags, so the moment your job is a fence line instead of a patio you are back to doing cylinder math yourself. This page covers both shapes, all bag sizes, and the cost comparison in one place.
- Manufacturer bag-count charts: QUIKRETE and other makers publish honest yield tables — this page uses their yields (0.60 / 0.45 / 0.30 cu ft). A chart cannot do your arithmetic, though: for a 20×12 driveway at 5 inches, or twelve post holes at mixed depths, you still need a calculator, and this page is that calculator next to the chart.
- Ready-mix plant phone quotes: The plant will quote you a price per yard and a short-load fee, and that quote is the number to enter in the price field — this calculator turns it into a total. What the plant will not do is tell you whether bags would have been cheaper, which is the comparison this page adds.
- Tile calculator (ToolNest's companion tool): That tool answers the material question for tile: joints, boxes, spares. If your question is concrete — yards, bags, truck-vs-bags — this concrete calculator is the right one; if your question is tile, use the tile calculator.
Most concrete calculators give you a yardage and stop. This page gives you the order: cubic feet, cubic yards, all three bag counts, ready-mix cost, and the bags-vs-truck verdict — the numbers you actually hand to the plant or the store.
Frequently Asked Questions
How much concrete do I need for a 10×10 ft slab?
At 4 inches thick with 10% waste: the slab is 10 × 10 × (4 ÷ 12) = 33.3 cu ft exact, 36.7 cu ft with waste, which is 1.36 cubic yards. That is 62 bags of 80 lb mix (36.7 ÷ 0.60, rounded up), 82 bags of 60 lb, or 123 bags of 40 lb. At $175 per yard, ready-mix costs about $238. Enter your own dimensions above — thickness is the input people get wrong most often.
How do you calculate concrete volume?
For a slab: volume (cu ft) = length (ft) × width (ft) × thickness (in) ÷ 12, then multiply by (1 + waste %). Divide by 27 for cubic yards, because a cubic yard holds 27 cubic feet. For a round hole: volume (cu ft) = π × (diameter ÷ 2 ÷ 12)² × depth (ft), times the number of holes, then the same waste and yard conversion. Bag counts are volume ÷ yield per bag, rounded up: 0.60 cu ft for 80 lb, 0.45 for 60 lb, 0.30 for 40 lb.
How many bags of concrete are in a cubic yard?
About 45 bags of 80 lb, 60 bags of 60 lb, or 90 bags of 40 lb — because one cubic yard is 27 cu ft and the yields are 0.60, 0.45, and 0.30 cu ft per bag. Those are manufacturer-published yields for standard mix; check your bag's label, because specialty mixes (high-strength, fast-setting) can differ slightly. The calculator uses these yields and rounds every bag count up.
How thick should a concrete slab be?
Typical residential slabs: 4 inches for patios, walkways, and shed floors; 3–4 inches for light-duty paths; 5–6 inches for driveways and garage floors that carry vehicles. Thickness is a structural and code question, not just a volume input — reinforcement, base preparation, and local frost depth all matter. When in doubt, ask your local building department or a contractor before you pour; adding an inch later is not an option.
Should I buy bags or order ready-mix?
Use the half-yard rule: under about 0.5 cubic yards, bagged mix usually wins — no delivery fee, no short-load fee, no clock ticking on a truck. Above about 1 cubic yard, ready-mix usually wins on both price and labor: a yard is 45 bags of 80 lb mix to haul, open, and mix by hand. Between 0.5 and 1 yard, price both — ready-mix runs roughly $130–$190 per yard delivered, but trucks charge short-load fees (often $50–$150) for orders under about 8 yards. The calculator's guidance line tells you which side your order falls on.
How much does a yard of concrete cost?
Roughly $130–$190 per cubic yard delivered in most US markets for standard 3,000 PSI mix, with the Northeast and West Coast toward the top of the range and the Southeast and Midwest toward the bottom. Higher-strength mixes (4,000–5,000 PSI) add about $10–$25 per yard. Fuel surcharges, weekend delivery, and pump-truck rental (roughly $800–$1,200 per day for backyard pours a truck cannot reach) all stack on top. Bagged mix, by comparison, runs about $270–$360 per yard at $6–$8 per 80 lb bag — plus your labor.
How much concrete do I need for fence post holes?
One 12-inch-diameter hole 3 feet deep is π × (0.5 ft)² × 3 ft = 2.36 cu ft. Four such holes are 9.42 cu ft exact, 10.37 cu ft with 10% waste, or 0.38 cubic yards — 18 bags of 80 lb mix. That is a bags job, not a truck job. Enter your diameter, depth, and hole count above and the calculator totals the whole fence line into one order.
What is a cold joint, and why does ordering short cause one?
A cold joint is the weak seam where fresh concrete meets concrete that has already begun to set — it happens when the first load runs out before the second arrives, or when a pour is interrupted. It is a structural and waterproofing weakness, not just a cosmetic line. That is why the calculator rounds up and why ready-mix orders should be rounded up to the next quarter yard: the cost of a little extra concrete is trivial next to the cost of a joint in the middle of your slab.