This concrete bag calculator converts a slab, footing or post hole into bags of premixed concrete at the four standard bag weights — because “how many bags” is the actual question standing in the aisle of the home center, and the answer swings from 56 to 112 bags for the exact same slab depending on which bag you grab. It also shows the cubic-yard total, because past a certain bag count, ordering a ready-mix truck stops being extravagance and starts being arithmetic.

Concrete volume –
Bags needed (with extra) –
Total bag weight to haul –
Ready-mix comparison –

Bag yields are the manufacturers’ standard figures (an 80 lb bag makes 0.60 cu ft of placed concrete). Fast-setting post-hole mixes have slightly different yields — check the bag — but the standard-mix numbers here plan 95% of home projects honestly.

The Only Number on the Bag That Matters: Yield

Bagged concrete is sold by weight and used by volume, and the bridge between them is the yield printed on the bag: a 40 lb bag yields about 0.30 cu ft of concrete, a 50 lb bag 0.375, a 60 lb bag 0.45, and an 80 lb bag 0.60 cu ft. Those yields assume the correct water addition — drowning the mix to make it flow adds volume on the day and subtracts strength forever, the worst trade in concrete. The calculator divides your project’s true volume by the yield of the bag you picked, always rounding the bag count up (a pour that stops 90% through is not 90% successful) and padding with the waste percentage you set.

The shape selector matters more than it looks. Rectangular volumes are length × width × thickness, but round post holes are cylinders — π r² × depth — and they fool intuition: a 12-inch-diameter, 3 ft-deep hole is 2.36 cu ft, about 4 bags of 80 lb mix, not the “bag and a half” most people guess. For fence posts, entering the hole diameter in inches and the depth, then the post count, turns a 12-post fence line into one accurate purchase instead of three trips.

Worked Example (Verified)

  • 10 ft × 10 ft shed pad, 4 inches thick (defaults): volume 33.3 cu ft = 1.235 cu yd. Bag counts: 56 bags at 80 lb, 75 at 60 lb, 112 at 40 lb (with the 5% extra included). That’s 4,480 lb of dry mix in 80 lb bags — the calculator’s haul-weight line exists because this surprises everyone exactly once.
  • Twelve fence-post holes, 10 in diameter × 36 in deep: shape = round, count 12 → each hole 1.64 cu ft, total 19.6 cu ft → about 35 bags of 80 lb standard mix (fewer with fast-set post mix per its own yield). The post itself displaces about 0.25 cu ft per hole and a gravel base a little more, so the real need lands a few bags lower — the calculator stays on the conservative cylinder volume so you never run short mid-pour.
  • Bag-versus-truck breakeven: ready-mix trucks carry 8–10 cu yd and often impose short-load fees under 3–4 yards; bag mixing stays competitive to roughly 1–1.5 cu yd for most people (that’s ~100+ eighty-pound bags), after which delivery wins on cost, consistency and your spine. The comparison line in the results flags the crossover zones.

Mixing and Placing Discipline

  • Water discipline is strength discipline. An 80 lb bag wants roughly 3–3.5 quarts of water — a slump like thick oatmeal, not soup. Every extra splash can cost hundreds of psi of the 3,500–4,000 psi the mix was designed for.
  • Two-bag batches in a drum mixer is the practical rhythm for 80 lb bags; a wheelbarrow and hoe is honest work up to about 20 bags and penance past 50.
  • Stage everything before the first bag opens. Mixed concrete gives you 60–90 workable minutes, less in heat. Bags staged, water measured, forms oiled and screed boards ready — the pour clock starts at wet, not at ready.
  • Thickness honesty: a “4-inch slab” formed with 2×4 lumber is 3.5 inches actual. Enter the true thickness (3.5) — the 14% volume difference is several bags, and form lumber’s nominal sizes have been lying since before you were born.
  • Cure it. Keep the surface moist or covered for at least 3–7 days; most “weak bag concrete” stories are actually “dried out on day one” stories.

Cost Notes

Standard mix pricing is regionally stable: roughly $4–$6 for 60 lb and $5–$8 for 80 lb bags at national chains, with fast-setting post mix at a premium. The example pad at 56 × 80 lb bags lands around $300–$450 in material — against $600–$900 for a short-load delivered yard with fees in many markets, and against a lifetime of Saturdays if you value the mixing honestly. Reinforcing (mesh or rebar), gravel base and form lumber are separate line items the bags don’t include.

Related Tools

Frequently Asked Questions

How many bags of concrete do I need for a 10×10 slab?

At 4 inches thick, a 10 × 10 ft slab is 33.3 cu ft (1.235 cu yd). Divided by bag yields: 56 bags of 80 lb mix (0.60 cu ft each), 75 bags of 60 lb (0.45), or 112 bags of 40 lb (0.30) — including a 5% waste allowance. At 6 inches thick, multiply all of those by 1.5. The calculator above runs any slab size and thickness, and its haul-weight readout tells you what the purchase actually weighs (4,480 lb for the 80 lb option here — bring the truck, not the sedan).

How much concrete does an 80 lb bag make?

About 0.60 cubic feet of placed concrete — the standard manufacturer yield for an 80 lb bag of premix at the correct water content. The full family: 40 lb → 0.30 cu ft, 50 lb → 0.375, 60 lb → 0.45, 80 lb → 0.60. Convenient mental math: a cubic yard (27 cu ft) needs 45 eighty-pound bags, exactly — which is also the fastest way to see why big pours belong in trucks.

How many bags for a fence post hole?

A typical post hole (10–12 inches diameter, 30–36 inches deep) holds roughly 1.6–2.4 cu ft after allowing for the post itself, so plan on 3–4 bags of 80 lb mix per hole — the calculator’s round-hole mode with your post count gives the exact order. Fast-setting post mixes are designed for this job (poured dry or with minimal water, set in 20–40 minutes) and their bags carry their own yield; verify against the printed figure when you switch products.

Is it cheaper to mix bags or order ready-mix?

Bags win on small jobs (under roughly ½–1 cu yd) because trucks add delivery and short-load fees that dwarf material cost at tiny volumes. Ready-mix wins as volume grows: a delivered yard of concrete often costs less than the 45 eighty-pound bags that equal it, arrives perfectly proportioned, and places in minutes instead of hours of mixing. The honest crossover for most DIYers is around 1–1.5 cu yd — this calculator flags bag counts over 50 and 100 so you notice yourself approaching it. Mixer rental (a towable 1-yard unit) is the middle path worth pricing for the 1–3 yard range.

How thick should a bag-mix slab be?

Four inches (3.5 actual over 2×4 forms) for patios, walkways and shed pads on a compacted gravel base; six inches for driveway aprons and anything a vehicle crosses; thicker edges (a turned-down footing of 8–12 inches) where frost heave or loads concentrate. Thickness is the cheapest strength there is — doubling thickness multiplies load capacity far more than upgrading mix strength — but every added inch is 25% more bags at 4-inch baseline, which the calculator makes visible before your back learns it.

Does a stronger bag mix change the bag count?

No — high-strength mixes (5,000 psi “crack-resistant” and similar) yield essentially the same volume per bag weight as standard 3,500–4,000 psi mix; you buy them for strength, fiber reinforcement or finish quality, not coverage. What does change yield is water: over-watered concrete slumps into place easier and covers the same volume, but cures weaker and cracks sooner. Judge mixes by the psi on the bag and mix them at the printed water ratio.

How many bags can I mix in a day by hand?

Realistically 30–50 eighty-pound bags with a drum mixer and a helper, fewer with a wheelbarrow and hoe, and the pour must stay continuous — cold joints between batches mixed hours apart are structural weak lines. If the calculator’s count for your project exceeds your honest daily rate, split the project into separately-formed pours (a patio in two halves with a control joint between them) or switch to delivery. Concrete rewards realistic planning more than almost any other material.

Is this concrete bag calculator free?

Yes — free, instant, no sign-up. Slab or round-hole shapes, all four bag weights, waste included, plus the ready-mix crossover warning when bags stop making sense.