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

At the common 4 in thickness, a 10 × 10 ft slab needs 62 bags of 80 lb premix, counting a 10% waste allowance and the representative 0.017 m³ (about 0.60 ft³) finished yield per bag. As a bulk order the same slab is 1.36 yd³. Pour it 6 in thick and the quantity grows by half again — thickness scales the whole estimate linearly.

How many 80 lb bags of concrete make a cubic yard?

A cubic yard is 27 ft³ and an 80 lb bag yields about 0.60 ft³ of placed concrete, so a yard takes 27 / 0.60 = 45 bags. The 60 lb bag yields about 0.45 ft³, which works out to 60 bags per yard. Yields vary by product, so check the figure printed on the bag you actually buy and use that in the math.

What waste percentage should I add when ordering concrete?

Five to ten percent is the typical range for slabs and footings — it covers over-excavation, uneven subgrade, spillage, and what stays behind in the mixer or pump. Use the high end for rough ground and for small pours, where fixed losses are a bigger share of the total. Running short mid-pour creates a cold joint, so the allowance is cheap insurance, not padding.

At what quantity should I order ready-mix instead of bags?

Somewhere between half a cubic yard and a full yard, price both routes. Bags carry no delivery fee and suit small pours, but a yard means mixing 45 eighty-pound bags by hand, and bagged material costs more per yard than truck-delivered concrete. Large pours are not a choice at all: a 24 × 24 ft garage slab at 6 in is 11.73 yd³ — 528 bags — which only makes sense off a truck.

How much concrete do solid porch steps take?

Count the stepped mass: a flight of n steps cast against grade holds n(n+1)/2 riser-by-tread cells, each riser × tread × width. A typical 3-step flight — 21 in total rise, 11 in treads, 4 ft wide — is 12.833 ft³ net and 14.117 ft³ with 10% waste, which is about half a cubic yard: 24 bags of 80 lb premix or 32 of the 60 lb size. Add the landing slab separately if there is one.

How Much Concrete Do I Need? Slabs, Footings, and the Bag Math

Concrete is bought by volume, and volume is short arithmetic

Every concrete question — bags or truck, one weekend or three, hundreds of dollars or thousands — starts with a volume, and the volume is nothing more exotic than the shape of the hole. A slab or footing is length × width × thickness; a rectangular pier is width × depth × height; a round sonotube is π/4 × diameter² × height. The only trap in the geometry is units: thickness is usually quoted in inches while the plan is in feet, and suppliers price in cubic yards while bags state their yield in cubic feet or cubic metres. Getting one conversion wrong is how a 4 in slab gets ordered as if it were 4 ft thick.

Two conversion facts anchor everything else. A cubic yard is 27 ft³, and a cubic yard is about 0.765 m³ — so a metric volume divides by about 0.765 to become the yards a ready-mix dispatcher expects to hear. Keep the thickness in feet (4 in is 4/12 = 0.333 ft), multiply the three dimensions, and the rest of the estimate is bookkeeping on top of that one number.

The 10 × 10 slab: what the classic project really takes

A 10 × 10 ft slab poured 4 in thick is the benchmark project, and its numbers are worth memorizing as a sanity check for everything else. With the 10% waste allowance that a slab on grade deserves, the order comes to 1.36 yd³ — that is 1.04 m³, from an in-place volume of 0.944 m³. Mixed from 80 lb premix bags — each yielding roughly 0.60 ft³ of placed concrete (about 0.017 m³) — the count rounds up to 62 bags. That’s nearly two and a half tons of material moved twice: once from the store to the site, once from the mixer into the forms.

Scale surprises people in both directions. A 12 × 12 patio at the same thickness needs 1.96 yd³ — 88 bags — while a 20 ft strip footing 16 in wide and 8 in deep is only 0.72 yd³, 33 bags, and six 12-in sonotube piers dug 4 ft deep come to just 0.77 yd³ together. Thickness is the multiplier to respect: a 24 × 24 ft garage slab at 6 in is 11.73 yd³ with waste, the equivalent of 528 bags. Run your exact dimensions through the concrete volume calculator and the m³, yd³, and bag figures come out together, with the waste allowance applied consistently.

Bags or ready-mix: where the break-even sits

Bagged premix wins on small pours because there is no delivery fee, no minimum order, and no clock running on a truck while you screed. The arithmetic of a bag is fixed: at 0.60 ft³ of finished concrete per 80 lb bag, a cubic yard takes 27 / 0.60 = 45 bags; the smaller 60 lb bag yields about 0.45 ft³, so a yard takes 60 of them. Metric products tell the same story — a 25 kg bag yields roughly 0.0125 to 0.0166 m³, so a cubic metre swallows 60 to 80 bags. Below about half a cubic yard, bags are hard to beat.

Somewhere between half a yard and a yard, the comparison flips. Mixing 62 bags by hand is a full day of physical work, and the per-yard cost of bagged material runs well above ready-mix once the volume justifies a short-load delivery. The garage-slab quantity — 528 bags — isn’t a bagging job at all; it’s ready-mix territory, ordered in yards with the waste already included. The honest move for any pour near the boundary is to price both routes against the same with-waste volume before committing a weekend to the mixer.

The waste allowance is not padding

Concrete estimates carry a waste allowance because the hole never matches the drawing. Subgrades are excavated slightly deep, forms bow a little, wheelbarrows and pumps hold back their share, and spillage is real. A 5 to 10% allowance is the standard range for slabs and footings, with the high end earned by rough ground and by small pours where the fixed losses loom larger against the total. The allowance multiplies the net volume before anything is converted to bags or yards, which is why every figure in this guide already includes it.

Skipping the allowance fails asymmetrically. Ordering 10% extra costs a few bags or a fraction of a yard; running short mid-pour means a cold joint in the middle of a slab while someone drives for more material — a defect that lasts as long as the concrete does. Round buy quantities up, always: an estimate of 61.3 bags is 62 bags at the register.

Steps are a stepped mass, not a staircase-shaped box

Solid poured entry steps confuse volume intuition because the shape is neither a box nor a wedge. Cast against grade, each step is a prism that runs all the way back: counting from the top, step k is k treads deep, so a flight of n steps contains n(n+1)/2 riser-by-tread cells across its width. The layout comes first, though, and it is governed by code: the IRC caps risers at 7¾ in (R311.7.5), so the step count is the total rise divided by 7.75 and rounded up, which by construction gives equal risers at or under the limit. A 21 in porch rise becomes 3 steps at exactly 7 in each, and the steps calculator flags any tread under the 10 in IRC minimum.

The classic front-porch flight — 21 in rise, 11 in treads, 4 ft wide — contains 6 cells and works out to 12.833 ft³ net, or 14.117 ft³ with 10% waste. That is 0.523 yd³: 24 eighty-pound bags, or 32 sixty-pound ones — a legitimate bagging job, sitting right at the edge of the ready-mix comparison. The concrete steps calculator lays out the risers against the code limits and returns the volume, bag counts, and the formwork quantities in the same pass.

Ratio mixes and designed mixes are different instruments

For DIY quantities, concrete is described by a nominal volume ratio such as 1:2:4 — one part cement, two parts sand, four parts coarse aggregate — which lands near C20 strength and is plenty for slabs, footings, and steps. Splitting an order into materials this way has one wrinkle: the combined dry ingredients compact when mixed with water, so the dry volume is the wet volume times a bulking factor of about 1.54 before it divides into parts. With cement at a bulk density around 1440 kg/m³, the parts convert to the bags of cement and tonnes of aggregate to buy.

Structural work runs the problem in the other direction. The ACI 211.1 absolute-volume method starts from a target 28-day strength and derives the water/cement ratio, then the water, cement, and aggregate masses that fill exactly one cubic metre — for a 30 MPa non-air-entrained mix at 75–100 mm slump with 20 mm aggregate, a w/c of 0.50 and 193 kg/m³ of water give a cement content of 386 kg/m³. That’s proportioning, not quantity takeoff: the concrete mix design calculator handles it when the strength is specified, while the volume tool answers how much of the finished mix the shape needs. Most projects need the second question answered far more urgently than the first.