Running short of concrete halfway through a pour is one of the most expensive arithmetic mistakes in home improvement: the truck leaves, the placed portion starts setting, and the only fixes are a cold joint or a second delivery with its own minimum charge. Ordering far too much carries its own penalty — ready-mix suppliers bill for what you order, not what you place.
The math itself is forgiving: volume is length × width × depth, converted to cubic yards or bags. What trips people up is unit consistency, the volume posts displace, and the waste allowance separating a clean pour from a short one. Every example below matches the math our Concrete Slab & Footing Calculator performs.
Volume is length × width × depth, all in feet, giving cubic feet; divide by 27 to get cubic yards (1 yd3 = 27 cu ft = 0.7646 m3). A 10 × 10 ft slab at 4 in thick is 33.3 cu ft = 1.23 yd3; with a 10% waste allowance that is 1.36 yd3 — about 62 eighty-pound bags or 82 sixty-pound bags. Allow 5% waste on flatwork, 10% on footings and irregular shapes, 15% on small pours under 1 yd3. Run exact dimensions through the Concrete Slab & Footing Calculator — it applies these same constants and rounds bags up for you.
The Volume Formula, and the Two Conversions That Matter
Every concrete estimate starts as a rectangular box: length × width × depth, all three in the same unit. The classic error is depth — forms are built in inches, but the formula wants feet: 4 in becomes 0.333 ft, 5 in becomes 0.417 ft, 6 in becomes 0.5 ft. Feet × feet × feet gives cubic feet, the unit bag yields are quoted in.
Two conversions carry cubic feet into the real world. Dividing by 27 gives cubic yards, because 1 yd3 = 27 cu ft — the unit ready-mix plants sell by. For metric suppliers, 1 yd3 = 0.7646 m3, the conversion this site's calculators use. The working constants this article and the calculator share:
| Quantity | Working value |
|---|---|
| 1 cubic yard | = 27 cubic feet = 0.7646 m3 |
| 60-lb bag | yields ≈ 0.45 cu ft of mixed concrete |
| 80-lb bag | yields ≈ 0.60 cu ft of mixed concrete |
| 1 cubic yard in bags | ≈ 60 sixty-lb bags, or 45 eighty-lb bags |
Worked Example: A 10 × 10 Slab at 4 Inches
The standard example — the same one the calculator uses — is a 10 × 10 ft slab, 4 in thick. Length and width are already feet; depth converts as 4 ÷ 12 = 0.333 ft. Volume is 10 × 10 × 0.333 = 33.3 cu ft, and 33.3 ÷ 27 = 1.23 yd3 (about 0.94 m3).
Now add waste. For a small hand-poured slab, 10% is a safe planning allowance: 33.3 × 1.10 = 36.7 cu ft = 1.36 yd3. In bags: 36.7 ÷ 0.60 = 61.1, rounding up to 62 eighty-pound bags; in sixty-pound bags, 36.7 ÷ 0.45 = 81.5, rounding up to 82 bags.
That bag count is the honest part of the arithmetic: 62 eighty-pound bags is more than two tons to mix, haul, and place in one session. Somewhere between 1 and 1.5 yd3, ready-mix delivery becomes cheaper and saner than bags.
Footings: Continuous Trenches and Pad Footings
A continuous footing is a long, narrow box — trench length × width × depth, all in feet. A perimeter footing running 60 ft at 12 in wide and 8 in deep is 60 × 1 × 0.667 = 40 cu ft = 1.48 yd3; with 10% waste, order about 1.75 yd3 rounded to the supplier's convenience. If the footing steps down a slope, calculate each run separately — depth changes along the trench.
Pad footings — the isolated squares under posts and columns — are a counting exercise: calculate one pad's volume (length × width × depth) and multiply by the number of pads. Six pads at 2 × 2 ft and 12 in deep are 4 cu ft each, 24 cu ft total, roughly 0.9 yd3 before waste. Footings are structural elements with code-required dimensions — sizes come from your local building department or a design; this section is only the arithmetic for whatever they specify.
Post Holes: Cylinders, Minus the Post
A post hole is a cylinder: π × radius2 × depth, radius and depth in feet, times the number of holes. The step everyone forgets is subtracting the volume the post displaces — concrete fills the annular gap, not the whole hole.
One common case: a 12-inch-diameter hole, 36 in deep. Radius = 0.5 ft, so the hole is π × 0.52 × 3 = 2.36 cu ft. A nominal 4-in post (3.5 in actual) displaces roughly 0.25 cu ft over that depth, leaving about 2.1 cu ft of concrete — 2.1 ÷ 0.60 ≈ 3.5, so budget four 80-lb bags per hole. A smaller 8-inch-diameter, 24-inch-deep hole with the same post holds roughly 0.5 cu ft net — about one 80-lb bag, the classic fence-post rule of thumb. Twenty holes at the bigger size is 42 cu ft; multiply before you buy, not after you start.
Irregular Shapes: Divide and Average
Odd footprints surrender to two tactics. Division: split an L-shaped patio into two rectangles, estimate each, add them. Averaging: for shapes that taper or curve gently, average the widest and narrowest widths and multiply by length — a walk running 3 ft to 5 ft wide over 24 ft is treated as 4 × 24 = 96 sq ft. Both trade a little precision for speed, and the waste allowance covers the trade. True curves use the circle formulas — π r2 for area, times depth.
One more scaling rule: thickness decisions belong to the slab thickness guide, but volume scales linearly with depth — every extra inch on a 10 × 10 slab adds 8.3 cu ft, a third of a yard, fourteen more 80-lb bags.
Waste Allowances: How Much Extra to Figure
Waste is not sloppiness — it is spillage, over-excavation, a subgrade that dips an inch low, and the heel of concrete left in the mixer. The site's planning numbers: 5% for flatwork on well-prepared ground, 10% for footings and anything irregular, and 15% for small pours under 1 yd3, where one spilled batch wrecks the whole percentage. On ready-mix orders, convert the allowance into the advice every supplier gives: order about a quarter yard beyond the calculated total.
Bags or Ready-Mix? Ordering Decisions
Bagged concrete is cheap per bag, needs no delivery, and stops at exactly the count you need — but every bag must be mixed, and 60-plus bags is a full crew day. Ready-mix costs more per yard on paper but arrives mixed, consistent, and in any volume.
Most ready-mix plants have a minimum load, commonly around 1 yd3, and charge a short-load fee below about 3 yd3. That fee is why the break-even against bags lands in the 1–1.5 yd3 range: below it, bags usually win; above it, the truck wins on money, back, and finish quality. When you call, quote yardage plus the quarter-yard safety margin, specify the mix (the supplier can batch to a specification — see our concrete mix ratios guide), and ask about air entrainment for freezing winters.
One planning constraint: concrete sets on its own schedule. A cold joint — the weak plane where fresh concrete meets concrete that has already set — is what you get when a pour stops mid-slab and restarts an hour later. If you must stage a pour, plan the break at a control joint or form edge.
Common Mistakes
- Mixing units at the depth step. Multiplying 10 × 10 × 4 gives a number 12× too big — inches snuck into a feet formula. Convert depth to feet first, every time.
- Ordering the net volume. The net volume assumes perfectly braced forms on a perfectly flat subgrade, neither of which exists.
- Counting bags by weight instead of yield. A 60-lb and an 80-lb bag are not interchangeable — divide volume by yield (0.45 vs 0.60 cu ft), then round up.
- Forgetting the post displaces concrete. Hole volume overstates what you need — subtract the post, especially across dozens of fence holes.
- Stopping a pour mid-slab. The cold joint between this morning's concrete and this afternoon's is a permanent weak plane — stage at joints or edges by plan.
- Estimating a thickened-edge slab as a flat box. The thickened perimeter needs its extra volume counted, or the pour runs short exactly where it matters most.
Plan It With the Concrete Slab & Footing Calculator
The Concrete Slab & Footing Calculator runs this guide's math in one pass: dimensions in; cubic feet, yards, and meters out; bags rounded up; a rebar grid estimate; and your waste factor applied — the same constants, without the long division.
Open the Concrete Slab & Footing CalculatorFrequently Asked Questions
How many 80-lb bags of concrete make a cubic yard?
Forty-five. A cubic yard is 27 cubic feet, an 80-lb bag yields about 0.60 cu ft, and 27 ÷ 0.60 = 45; in sixty-pound bags at 0.45 cu ft, the same yard takes 60. Hand-mixing 45 bags is a full day — which is why the yard, not the bag, is the ordering unit beyond small pours.
How much concrete do I need for a 10 × 10 slab?
At 4 inches thick, 10 × 10 × 0.333 ft = 33.3 cu ft = 1.23 yd3, or 1.36 yd3 with a 10% waste allowance — about 62 eighty-pound bags or 82 sixty-pound bags. Thickness scales the result linearly: at 5 inches the same slab needs 1.54 yd3 net, 1.70 with waste.
Can I pour a slab in two stages?
Not as one monolithic slab. Once concrete sets, the next batch bonds only weakly to it — that interface is a cold joint, a built-in weak plane. If staging is unavoidable, plan the break at a control joint or an edge and finish each stage separately; two separated pours beat one interrupted pour.
How much extra concrete should I order?
Figure 5% waste on flatwork over prepared ground, 10% on footings and irregular shapes, and 15% on small pours under a cubic yard. On ready-mix deliveries, that translates into the standard advice: order about a quarter yard beyond the calculated volume. A little leftover costs a few dollars; a short pour costs a cold joint.
How many bags of concrete per fence post?
It depends on the hole. An 8-inch-diameter, 24-inch-deep hole with a 4×4 post holds roughly half a cubic foot of concrete — one 80-lb bag. A 12-inch-diameter, 36-inch-deep hole holds about 2.1 cu ft net of the post — roughly four bags. Compute the cylinder (π × radius2 × depth), subtract the post, divide by 0.60.
Related MyHouseLogic Guides
Volume is the middle of the concrete plan — thickness and mix come first, curing and steel follow.
- Concrete slab thickness — typical thicknesses for patios, shed pads, and driveways.
- Concrete mix ratios explained — what 1:2:3 means by volume and why the water–cement ratio dominates strength.
- Concrete curing guide — the 28-day strength curve and weather protection for a fresh pour.
- Concrete reinforcement — rebar grids and wire mesh, and what steel does and does not do for a slab.
The bottom line: multiply in feet, divide by 27, subtract what the posts displace, and add waste for the pour you will actually run. The yard is the ordering unit, the bag is the mixing unit, and the quarter-yard margin on every truck order is cheaper than any cold joint.