Concrete Slab Calculator with Cost & Bag Counts

The formula

• Volume (cu ft) = length × width × (thickness ÷ 12)
• Volume with waste = volume × (1 + waste%)
• Cubic yards = volume with waste ÷ 27
• 80 lb bags = volume with waste ÷ 0.6 cu ft per bag, rounded up
• 60 lb bags = volume with waste ÷ 0.45 cu ft per bag, rounded up

A worked example

A 10 ft × 10 ft patio at 4 in thick needs 33.3 cu ft, or 36.7 cu ft with 10% waste. That is 1.36 cubic yards, 82 bags at 60 lb, or 62 bags at 80 lb. At $6.50 an 80 lb bag the bagged material comes to roughly $403; a ready mix truck at $150 a yard would be about $204 plus any minimum load fee.

More about the Concrete Calculator

Volume first: the 27 cubic feet rule

The full example, end to end. A 10 ft × 10 ft patio at 4 in thick: 10 × 10 × (4/12) = 33.3 cu ft, or 36.7 cu ft with 10% waste. That is 1.36 cubic yards, or 82 bags at 60 lb, or 62 bags at 80 lb. Priced: at $6.50 per 80 lb bag the bagged route is about $403 of material plus a long day of mixing; a ready mix truck at $150 a yard is about $204 plus any short load fee. Same patio, half the material cost, none of the labour, which is why the truck wins past a yard.

The decision tree most DIYers actually face: footings and small repairs (bags, obviously), a shed base or small patio around a yard (the awkward middle, price both), anything bigger (truck, no contest). The hidden costs people forget: bags need mixing (rent a mixer past ~30 bags, your back will thank you), trucks need access (can it reach your forms?), and both need the forms, gravel base and reinforcement ready before the concrete arrives. Concrete waits for no one.

Waste margins and ordering ready mix

Tips from people who have poured. Order the waste allowance, every short pour in history started with "the measurements were exact". Have help lined up before the truck is booked; a 1.5 yard pour is a race against setting time. In hot weather, pour early morning. And call the supplier about minimums and short load fees before you fall in love with the per yard price, the fee can double a small order.

Assumptions to know: the calculator treats your slab as a perfect rectangular prism and prices materials only, no delivery fees, no reinforcement mesh, no formwork lumber, no labour. Irregular shapes need segment by segment calculation (or generous waste allowance). And the bag yields assume standard 60/80 lb premix bags; specialty mixes vary, so check the bag's stated yield for anything exotic.

A worked example: an 18 by 12 foot driveway at 6 inches thick

Suppose you are pouring a driveway 18 feet long, 12 feet wide and 6 inches thick. Six inches is half a foot, so the volume is 18 times 12 times 0.5, which is 108 cubic feet. Divide by 27 to convert to cubic yards and you get exactly 4 cubic yards. Add the standard 10 percent waste allowance and the order becomes 4.4 cubic yards, the figure you actually give the ready mix supplier.

Now price the same job in bags to see why driveways belong to trucks. An 80 pound bag yields about 0.6 cubic feet, so 108 cubic feet needs 180 bags before waste, and 198 bags with the allowance included. At a hypothetical 7 dollars a bag that is 1,386 dollars of bagged concrete, against 4.4 yards of ready mix at a hypothetical 155 dollars a yard, which is 682 dollars, and the truck also saves you a full day of mixing.

The bag count exposes the second hidden cost: weight. Those 198 bags total 15,840 pounds, nearly 8 tons of material to unload, move and feed into a mixer a bag at a time, for a pour that must still be finished and levelled in one session. Bag arithmetic that looks merely expensive on paper becomes physically unreasonable at driveway scale, and the calculator’s dual output exists to show you that tipping point before you commit.

Footings, post holes and trenches: volume that is not a slab

Not every pour is a flat rectangle. A strip footing is still a box, length times width times depth, so a footing trench 20 feet long, 1 foot wide and 1 foot deep holds 20 cubic feet, which is about 0.74 cubic yards, or roughly 34 bags at 80 pounds once you divide 20 by the 0.6 cubic feet each bag yields. The calculator’s footing mode handles the shape; the habit to build is converting every shape into cubic feet first.

Round post holes use the cylinder formula instead: pi times the radius squared times the depth. A hole 1 foot across has a radius of 0.5 feet, so at 3 feet deep it holds 3.1416 times 0.25 times 3, which is about 2.36 cubic feet. Eight fence post holes therefore need about 18.85 cubic feet of concrete, about 0.7 cubic yards, or roughly 32 bags at 80 pounds with a small margin included.

The common mistake with holes and trenches is measuring the hole you intended rather than the hole you dug. Hand dug holes bell out at the bottom and trenches crumble wider than the spade line, and every extra inch of width across a long trench adds real volume. Measure the finished excavation at a few points, average the widths, and keep the waste allowance at the full 10 percent for anything dug by hand.

The crossover point: when a truck beats bags on cost and sanity

For very small jobs bags win easily: a single post hole or a small equipment pad needs a handful of bags, no minimum load fees, no scheduling, and no pressure to finish before a truck leaves. Below about half a cubic yard, roughly 22 bags at 80 pounds, bagged concrete is usually the sensible route even when the per yard material cost is higher, because the job is finished in an hour with no delivery logistics at all.

The crossover arrives somewhere around one to two cubic yards for most people. A cubic yard is about 45 bags at 80 pounds, which is 3,600 pounds of lifting and mixing for one person, a serious half day of labour before any finishing starts. Once a pour passes that scale, the ready mix premium buys you uniform concrete, placed in minutes, with the whole working time available for screeding and finishing instead of mixing.

Short load fees are the variable that moves the crossover. Many suppliers charge extra when an order falls below their minimum, often a few yards, so a 1.4 yard order can cost more per yard than the headline rate suggests. Get the delivered price for your exact yardage, compare it against the bag total from the calculator at your local bag price, and let those two real numbers, not rules of thumb, make the decision.

How many bags your slab really needs

Bag counting is just volume divided by yield. An 80 lb bag of pre mix yields about 0.60 cubic feet; a 60 lb bag about 0.45. Since a cubic yard is 27 cubic feet, you need roughly 45 eighty pound bags or 60 sixty pound bags per yard. A 10 × 10 ft patio at 4 in thick is 33.3 cu ft. That is 59 eighty lb bags after a 5% waste allowance. The calculator above does this arithmetic and always rounds up, because running short mid pour is far worse than having half a bag left over.

Pros order concrete by the cubic yard; DIYers buy it by the bag. This tool speaks both languages, showing cubic yards, cubic feet and the exact bag count together. If you want the pure volume maths explored further, our <a href="/concrete-calculator/">concrete slab calculator</a> covers slabs, footings and post holes in depth, and the <a href="/home-garden/">Home & Garden</a> hub has estimators for everything around the pour.

Bags vs ready mix: the one yard rule

The rule of thumb is simple: under about one cubic yard, bags are cheaper and practical; above it, a ready mix truck almost always wins. Sixty 80 lb bags at $6.50 each costs $390 before tax, while 1.3 yards of ready mix typically lands around $250, $300 including delivery and fees. Watch for short load fees ($75, $150) on small truck orders, which can flip the maths on borderline jobs.

Then there is the back breaking factor: 45 eighty lb bags means lifting and mixing 3,600 lb of concrete by hand, while one truck pours it in minutes with perfectly consistent strength. The comparison row above prices both options with your local numbers so the decision is arithmetic, not guesswork.

60 lb vs 80 lb bags: which should you buy?

It is the same concrete in a different sack. The 80 lb bag means fewer bags to buy, carry and open, but each one is a serious lift. The 60 lb bag is far friendlier to backs, knees and solo DIYers, at the cost of more bags and more openings. Cost per finished yard usually works out close between the two, so choose by your crew, not your wallet.

Working alone with a wheelbarrow? Take the 60 lb bags. Got a helper and a mixer? The 80 lb bags save trips. One more choice: fast setting mix for posts and small repairs (sets in about 20 to 40 minutes), standard mix for slabs and anything structural. It gives you working time to screed and finish properly.

The waste allowance nobody should skip

Always order 5 to 10% more than the maths says. Subgrade is never perfectly level, forms flex under the load, a little spills at the wheelbarrow, and the ground drinks the first pour. Running short mid slab forces a cold joint, a visible, weaker seam where new concrete meets concrete that has already started to set.

First pour? Use 10%. Experienced crew on a well prepped, level base? 5% will do. The calculator defaults to 5%, bump it up for rough or uneven ground, and remember that an extra bag or two costs a few dollars while a ruined pour costs a weekend.

What concrete costs in 2026

Bag prices in 2026 run roughly $5 for a 60 lb bag and $6.50 for an 80 lb bag at big box stores. Ready mix delivered runs about $125, $165 per cubic yard depending on mix strength and region, plus short load or delivery fees on small orders. Those are the two numbers the calculator compares.

But concrete is not the whole project budget. Price your forms, the compacted gravel base (usually 4 in), rebar or wire mesh, a concrete sealer, and any tool rental, mixer, screed, float, before you commit. A realistic all in number beats a surprise trip back to the store every time.

A worked patio pour comparing both bag sizes honestly

Suppose your patio measures 12 feet by 8 feet at 4 inches thick. The volume is 12 times 8 times 4 divided by 12, which is 32 cubic feet, or about 1.19 cubic yards. Add 10 percent for uneven ground and spillage and you need 35.2 cubic feet of placed concrete. In 60 lb bags yielding 0.45 cubic feet each, 35.2 divided by 0.45 is 78.22, so you buy 79 bags. At $5.20 a bag, that order costs $410.80 before delivery.

Run the same pour in 80 lb bags before you decide. At 0.60 cubic feet a bag, 35.2 divided by 0.60 is 58.67, so 59 bags do the same job. The bag count falls by twenty, but every lift gets heavier, and twenty fewer bags does not always mean twenty fewer problems if one person is moving, opening, and mixing the whole pile alone. The calculator gives you both counts precisely so this trade off is a decision, not a surprise.

Pour day mistakes that no bag count can rescue

The costliest mistake is forming and levelling carelessly, then expecting the bag count to survive contact with a base that dips and rises. A slab planned at 4 inches that actually averages 5 inches across low spots uses a quarter more concrete than the neat calculation promised. Screed the base flat, check the depth in several places with a marked stick, and only then trust your count, or keep the waste allowance generous enough to forgive the ground you actually have.

The second mistake is mixing at the wrong pace. Bagged concrete waits for nobody: too many helpers mixing ahead creates piles that begin to stiffen before they are placed, while too few leaves an unplaced strip as the first section sets. A cold joint between a set section and a fresh one is a permanent weak line. Plan the crew, the water supply, and the wheelbarrow route before the first bag is opened, not after it.

Bagged concrete versus ready mix on a small to medium pour

Bags win on control. You mix exactly what the next section needs, pause without a truck meter running, work behind a house no truck can reach, and spread a small pour across a weekend. For a few fence posts, a small pad, or a repair, nothing else makes sense. The price you pay is physical: dozens of bags to transport, lift, open, and mix, with quality depending on your consistency from the first batch to the last.

Ready mix wins on effort and uniformity once the volume grows. One delivery arrives mixed to a consistent standard, a crew can place it quickly, and the concrete does not depend on tired arms at bag forty. Its weaknesses are timing and minimums: short load fees, a discharge window, and access for a heavy truck. Use the calculator’s cubic yard figure as the honest comparison point, price your bag total, delivery included, against a real ready mix quote for that same yardage.

How to use the Concrete Calculator

1. Measure the slab, footing or patio length and width in feet.
2. Enter the pour thickness in inches, 4 in is standard for a patio or shed slab, 6 in for a driveway.
3. Leave the waste allowance at 10% unless the shape is simple and the forms are tight.
4. Add your local price per bag or per cubic yard, then press Calculate, you'll get both the bag count and the truck option, priced.