How Much Concrete Do I Need? The Slab, Footing, and Post-Hole Math That Gets the Order Right

Concrete is the one material where an estimating mistake announces itself in public. Run short in the middle of a pour and the job does not politely pause like a painting project; the mix already placed starts setting while you scramble, the joint between old and new becomes a permanent weak line, and a second delivery costs a fresh fee measured in hundreds. Over-order, and you literally watch money slide back down the chute and drive away, because concrete cannot go back on the shelf.

The good news: the volume math is genuinely simple, three dimensions and one conversion, and this manual works it end to end for the three shapes that cover nearly every residential job, slabs, footings, and post holes. Then it covers what the formula alone cannot tell you: the waste allowance, the bags-versus-truck decision with the short-load economics that drive it, and the ordering-day details that separate a smooth pour from an expensive lesson. This is the deep-dive companion to our general estimating manual; for a first pass on your numbers, our estimator handles the arithmetic in seconds.

The formula, and the trap inside it

Every concrete estimate is the same calculation: the volume of the shape you are filling, converted into the unit concrete is sold in. For the rectangular shapes that dominate residential work, that is length times width times thickness, and the trap is units. Concrete dimensions arrive in a mix of feet and inches, a 12 by 20 foot patio, 4 inches thick, and the formula demands one unit throughout. Convert thickness to feet first: 4 inches is 4/12, or 0.333 feet. The patio’s volume is then 12 × 20 × 0.333, about 80 cubic feet.

The second conversion is the one newcomers miss entirely: concrete is sold by the cubic yard, and a cubic yard is 27 cubic feet, three feet in each of three dimensions. Divide 80 by 27 and the patio needs just under 3 cubic yards. Skip the inches-to-feet conversion and the same arithmetic produces an answer twelve times too large; forget the divide-by-27 and you will order in a unit no supplier recognizes. Written as one line: (length ft × width ft × thickness in ÷ 12) ÷ 27 = cubic yards. Every slab, sidewalk, pad, and stair landing runs through this line, and the whole rest of the craft of ordering is adjustments to its output.

Slabs: working a real example end to end

Walk the patio through completely, because the worked version surfaces every decision the formula hides. The plan: a 12 by 20 foot backyard patio, and the first real decision is thickness, settled before any math. Typical residential practice puts patios, walkways, and shed pads at 4 inches, and driveways or anything bearing vehicles at 5 to 6 inches, sometimes with thickened edges; local code and soil have the final word. Thickness is the estimate’s most sensitive input: the same footprint at 6 inches instead of 4 needs half again more concrete, so a casual “make it a bit thicker” remark changes the order by a full yard.

At 4 inches: 12 × 20 × 0.333 = 80 cubic feet, ÷ 27 = 2.96 yards. Now the corrections. Measure the actual formed excavation at several points rather than trusting the plan, subgrades are never perfectly level, and every low spot is extra volume; if your depth readings average 4.25 inches, calculate with that. Add the waste allowance, 5 to 10 percent, covered fully below, bringing the patio to roughly 3.2 yards. Then round to the supplier’s ordering increment, commonly quarter or half yards: order 3.25 or 3.5. That final number, derived in ten minutes with a tape measure, is the difference between a continuous pour and a mid-slab crisis.

Footings and post holes: the other two shapes

Footings are slabs turned on edge and the math is identical: length of the run times width times depth, in feet, over 27. An illustrative 40 foot footing run at 16 inches wide (1.33 ft) and 12 inches deep (1 ft) is 53 cubic feet, just under 2 yards. The footing-specific caution is depth honesty: trenches are dug, not formed, and real trench bottoms undulate, so measure depth every few feet and calculate from the average of the deeper readings, because a trench never errs shallow where it counts.

Post holes are cylinders: radius squared times pi times depth, minus the volume of the post itself. The standard case, a 12 inch diameter hole, 2 feet deep, around a 4x4 post: the hole holds about 1.57 cubic feet, the post displaces about 0.17, leaving 1.4 cubic feet of concrete, about three 60 pound bags per post at a typical 0.45 cubic foot yield. A 20 post fence run therefore wants around 60 bags, a number that surprises everyone the first time and explains why the fence crew’s truck rides low.

Hand-dug holes are never textbook cylinders, they belly out at the bottom exactly where the auger wandered, so per-post margin matters more here than anywhere: an extra bag per five posts is the practical allowance, and unmixed bags return to the store, which makes generous the cheap direction for bagged work.

Cubic yards, made physical

The yard is an abstraction until the first time you move one by hand, so calibrate it. One cubic yard is 27 cubic feet, roughly 4,000 pounds of wet mix, two tons, and it fills about 15 to 18 heaped contractor wheelbarrows. It pours a 4 inch slab of about 80 square feet, a modest shed pad; three yards handle the 240 square foot patio of our example; a two-car driveway at 5 to 6 inches runs eight to twelve yards, firmly truck territory.

The physical sense earns its keep twice. At estimate time it is the sanity check: if the formula says a small walkway needs 9 yards, the units went wrong somewhere, because 9 yards is a driveway. At pour time it sets the labor plan: three yards arriving in one truck is twelve thousand pounds of material that must be placed, screeded, and floated inside the mix’s working time, which is why the crew size and wheelbarrow count belong in the estimate conversation, not just the volume.

The waste allowance: why the formula alone under-orders

Our estimating manual made the general case for waste factors; concrete’s version is specific and asymmetric. The additions: subgrade variation, the compacted base sits a half inch low across a corner and silently swallows a quarter yard; form deflection, wet concrete is heavy and bows long form runs outward; residue, some mix always stays behind in the truck’s drum, the pump, and the wheelbarrows; and spillage, which is not clumsiness but the ordinary physics of moving two-ton batches by chute and barrow.

The asymmetry is what sets the percentage. Over-order by a half yard and you lose the price of a half yard, plus the small chore of a planned disposal spot, a pre-built form for a stepping stone or pad puts it to work. Under-order and the failure is structural: the poured section starts setting while the shortfall is arranged, the seam between pours becomes a cold joint, a permanent plane of weakness that telegraphs through as tomorrow’s crack, and the rescue delivery arrives with its own fee, often at short-load rates, the worst pricing in the industry. Five to ten percent extra, leaning toward ten on hand-graded or irregular sites, is not padding; it is the cheapest structural insurance concrete sells.

Bags or the truck: the real crossover math

Bagged mix and ready-mix are the same product at wildly different scales of effort, and the crossover deserves real numbers. An 80 pound bag yields about 0.6 cubic feet: 45 bags make a yard. Sixty pound bags yield about 0.45: call it 60 per yard. Below roughly a yard, bags win, no delivery fee, no minimum, work at your own pace, unmixed bags return to the store. The patio example, at 3.25 yards, would demand nearly 150 large bags, each carried, mixed, and placed inside the working window; that is not a weekend project, it is a punishment.

Ready-mix flips every term above the line: cheaper per yard, factory-consistent strength, and the mixing labor disappears, replaced by delivery economics. Trucks carry an economical minimum, and orders below a few yards draw a short-load fee that can rival the concrete’s own cost, which creates the gray zone between about one and two yards where neither option is clean: there, a towable mixer rental, batching the job with a neighbor’s project onto one truck, or gritting through the bags are all defensible. Past two yards the decision makes itself. The one false economy to name: hand-mixing inconsistency on structural pours. A slab’s strength is its weakest batch, and batch fifty of a long afternoon is nobody’s best work.

Ordering day: the details that make the pour smooth

The order call involves more than a volume, and knowing the vocabulary gets better answers. State the job, not just the yards: slab, footing, or fence, exterior or interior, vehicle load or foot traffic. The supplier will translate that into a mix design, the PSI strength (commonly 2,500 to 4,000 and up, higher for driveways and freeze-thaw exposure), aggregate size, and any admixtures, fiber reinforcement, accelerators for cold weather, retarders for hot. Ask about slump, the wetness of the mix: wetter flows easier and screeds faster but cures weaker, and the crew’s request for “a little more water” at the chute is a strength decision, not a convenience.

Then prepare for the meter you cannot see: trucks price unloading time, with a free window and per-minute charges past it, so everything slow must happen before arrival. Forms braced and oiled, base compacted and damp, reinforcement placed and chaired, wheelbarrow route cleared, enough hands on site, tools staged, screed board cut to width. Confirm truck access while booking, a loaded mixer weighs as much as a small house and treats soft lawns and thin driveways accordingly, and have the washout spot agreed, a lined pit or barrow for the chute rinse, since cement-laden water cannot legally go down storm drains in most places. Ten minutes of ordering vocabulary and an hour of staging buy the calm, continuous pour every other section of this article is trying to protect.

Odd shapes: L-pours, circles, and curves

Real yards refuse to be rectangles, and the estimating move for every irregular footprint is the same: decompose it into shapes you can already solve. The L-shaped patio is two rectangles sharing an edge; calculate each, sum the volumes, done. The wraparound walkway is a chain of rectangles; the notched slab is a big rectangle minus a small one, and subtraction is as legitimate as addition, compute the full footprint, remove the cutout’s volume, and the forms can be as awkward as the deck posts demand. Sketch the decomposition on paper with dimensions before touching a calculator, because the sketch is where missed sections announce themselves; the arithmetic never fails, only the inventory of pieces does.

Circles and curves need one more formula. A round pad is radius squared times pi times thickness, the same cylinder math as the post holes, at patio scale: an illustrative 10 foot diameter pad, 4 inches thick, is 5 × 5 × 3.14 × 0.333, about 26 cubic feet, just under a yard.

Freeform curved borders resist exact formulas, so estimate them as the rectangle that would contain them and let the overage join the waste allowance, or step the curve into short straight segments and sum those; either way, curves earn a point or two more allowance than straight forms, since curved formwork deflects more and the eye-fitted boundary always encloses more area than the plan promised. Complexity in footprint is never a math problem, only a bookkeeping one, and the sketch is the bookkeeping.

Steps and stairs: small pours that punish guesswork

Poured steps look tiny and estimate badly by eye, because a staircase is solid concrete all the way down, not just the treads you see. The clean method treats the staircase as a stack of horizontal slabs, one per step, each running from that step’s nose to the back of the pour. An illustrative three-step entry, each tread 12 inches deep, each riser 7 inches, 4 feet wide: the top step is a slab 1 foot deep, the middle 2 feet, the bottom 3 feet, each 7 inches thick and 4 feet wide. Summed, roughly 14 cubic feet, over half a yard for what looks like an afternoon’s decoration, which is exactly why step pours are the classic bag-count surprise.

Two adjustments refine the number. Fill is legitimate: the interior of a large step mass can carry compacted rubble or gravel displacing concrete, reducing both cost and cure stress, provided enough thickness remains around it per the job’s spec. And landings count as their own slab at full depth, measured like any other. The general lesson steps teach is worth keeping: any pour with vertical faces contains more volume than the visible surfaces suggest, so estimate solids from the section drawing, never from the finished silhouette. Guess a staircase by eye and the shortfall arrives at the worst geometry in concrete, half-filled forms with a setting batch below.

The quantities around the concrete

The volume line covers the mix, but the pour’s shopping list is longer, and the supporting quantities follow the same footprint math. Base material first: slabs sit on compacted gravel, commonly around 4 inches of it, which is the same length-times-width-times-depth calculation and often rivals the concrete’s own volume; it is sold by the yard or the ton, and your supplier converts between them. Reinforcement next: welded wire mesh is sold in sheets covering stated areas, so divide the slab’s footprint by sheet coverage and add overlap; rebar for footings runs by linear feet along the trench, doubled or tripled by the number of runs the spec calls for, plus chairs to hold everything at height in the pour.

Then the consumables the first-timer forgets and the crew never does: form lumber by the perimeter’s linear feet plus stakes every few feet, form-release oil, a vapor barrier under interior slabs by footprint area, expansion-joint material along any edge meeting existing concrete, and curing compound or sheeting by area. None of these is individually expensive; collectively they are a second store trip on pour morning if unlisted. The habit that catches them all is estimating from the section drawing rather than the plan view: draw the slab in cross-section, label every layer from subgrade to finish, and each label becomes a line on the order sheet with its own quantity, computed from the same three dimensions you already measured.

Weather, timing, and the working window

The estimate has a fourth dimension: when the pour happens, because concrete’s chemistry runs on temperature and the working window sets the labor plan. Hot weather accelerates the set, shrinking the time between chute and final finish; big summer pours want early-morning truck slots, extra hands, and sometimes a retarder in the mix, ordered by name when booking. Cold slows curing and, below freezing, threatens it structurally, fresh concrete that freezes loses strength permanently, so cold-season pours want accelerators, insulating blankets on the order list, and honest respect for the forecast. Rain is simpler: fresh concrete and a downpour do not share a day, and rescheduling a truck beats rinsing a ruined finish.

Timing also gates everything after the screed. The window between placement and each finishing pass, floating, edging, brooming, is set by the mix and the weather, not the crew’s preference, and it shrinks in heat exactly when fatigue peaks. This is why the crew-size question from the yardage section returns here with force: three yards on a mild morning is a different job from the same three yards at noon in July. The scheduling rule that contains all of this: book the truck for the coolest workable slot, stage the site the day before, and let the forecast, not the calendar, pick the date. Concrete never hurries for anyone, and every attempt to hurry it is legible in the surface for the slab’s whole life.

After the truck: curing is part of the estimate

The pour ends the delivery, not the job, and the last quantities on the list serve the week that follows. Concrete does not dry, it cures, a chemical reaction consuming water over weeks, and the surface that dries out early stops strengthening early: the powdery, dusting, hairline-cracked slabs of the world are mostly curing failures, not mix failures. Keeping the surface moist for the first days, commonly about a week for typical residential mixes, is the entire discipline, done with curing compound sprayed after finishing, plastic sheeting, or scheduled misting, each an item and a quantity, by area, on the same order sheet as the mix itself.

Time is the other curing quantity. Typical mixes take foot traffic after a day or two, vehicle loads after about a week, and reach their rated strength around 28 days, the number PSI specs quietly assume; heavy loading before schedule spends strength the slab never recovers. Forms generally strip after a day or two for slabs, longer for structural elements, per the spec. Budget the calendar accordingly: the driveway poured Saturday does not park the car Monday, and the fence posts set this morning want a day or two before the rails load them, details that cost nothing when planned and callbacks when not. A pour is a three-act job, prepare, place, cure, and the estimate that covers all three acts is the one these pages have been building from the first line.

Where the estimate actually goes wrong

The failure patterns repeat so reliably they can be listed in descending order of expense. Unit slips lead: inches used as feet, the divide-by-27 forgotten, a diameter used where the cylinder formula wants radius, each producing answers wrong by multiples, not margins, which is why the physical sanity check, does this number sound like a shed pad or a driveway, catches more disasters than careful re-arithmetic. Thickness assumptions follow: calculating at 4 inches while the form boards say 5.5, or quoting the drawing while the excavation runs deep, both discovered at the bottom of an empty chute.

The subtler failures round out the list. Ordering formula-exact with no allowance, then feeding the shortfall to a short-load fee. Treating the bag count casually, “a few bags” becoming a second store run per fence post. Booking the truck before the site is ready and paying standby minutes while the last form gets braced. And the planning failure that contains all others: running the numbers the night before the pour instead of the week before, leaving no room to re-measure, re-spec, or reschedule when something disagrees. Concrete rewards the boring virtues, early math, checked units, measured reality, and it invoices every shortcut at delivery prices.

The pre-order checklist

The whole manual, compressed to the sheet worth taping to the form boards.

• Confirm the spec first. Thickness and PSI by use: around 4 inches for patios and pads, 5 to 6 for vehicles, strength and details per local code.
• Measure the excavation, not the plan. Depth at several points, average honestly, calculate with reality.
• Run the line. (L ft × W ft × thickness in ÷ 12) ÷ 27 = yards; cylinders for post holes, radius not diameter.
• Sanity-check physically. One yard ≈ 81 square feet at 4 inches; does the answer sound like the job?
• Add 5 to 10 percent. Subgrade, deflection, residue; the shortfall costs a cold joint, the surplus costs a stepping stone form.
• Pick bags or truck by the crossover. Under a yard, bags; past two, truck; between, mind the short-load fee.
• Order in the supplier’s increment, site staged before arrival. Access confirmed, crew ready, washout planned, slump and mix settled on the phone.

Run the list top to bottom and the pour becomes what it should be: heavy work, but no surprises.

The bottom line

Concrete estimating is one line of arithmetic wearing work boots: length by width by thickness in feet, over 27, plus five to ten percent for the ways real sites disagree with drawings. The craft around the line is knowing which inputs dominate, thickness above all, which direction of error is expensive, short, always short, and where the crossover between a pallet of bags and a booked truck actually sits. Work the example for your own slab with our estimator, read the general method if you want the estimating logic that carries to every other material, and give the math its week of lead time. The pour that follows will be the dull, continuous, single-truck kind, which in concrete is the only kind worth having.

Read this as shop-floor education, not a spec sheet. Every volume, coverage figure, bag yield, fee, and mix number above is a worked example, and your product, your supplier, your region, and your site will each move the real figures. Anything structural, anything load-bearing, and anything a permit touches gets specified against the local building code with a qualified professional in the loop, and the final order gets confirmed with your supplier before the truck is ever booked.