How I'd actually use this on a flip
The first time the calculator earned its keep was a basement slab pour at the suburban gut rehab. The math came back as 1.91 cubic yards. I called Ozinga and asked for 2.25 to leave a working cushion. The truck arrived with 2.25 on the ticket, the south corner of the basement turned out to have a half-inch sag in the old subfloor that nobody had measured, and we used every cubic foot of the cushion on that corner. The math was right; the order was right BECAUSE it was bigger than the math.
That gap is what the Material Order Cushion is for. The exact math is a starting point, not an order. Practical order is what you tell the dispatcher. Why the cushion is there — that's the experience layer that no AI calculator is going to write for you. When NOT to over-order is the ceiling: at small slabs and post holes, ordering ready-mix is more expensive than running to Home Depot for an extra bag, and a half-yard truck you only need a quarter of is concrete you're paying to dump somewhere.
On the slab modes, default to 5% cushion on level compacted subgrade and bump to 10% if the subgrade is rough or the forms look like they might bow under the head. On post holes, the cushion covers the bell-out at the bottom of an augered hole — they never come back round. On stairs, the wedge approximation undercuts where the slope meets the back wall, so the cushion takes the form pressure into account. The defaults aren't arbitrary; they're what the operator orders.
Where this number breaks down
A few traps that put the calculator on the wrong side of the order:
- Belled footings.The cylinder mode assumes a clean cylindrical hole. A drilled pier with a bell at the bottom (often spec'd for clay-soil column footings) takes 30-50% more concrete than the cylinder math returns. If the engineer spec'd a bell, compute the bell separately and add.
- Stairs against an open back.The wedge approximation pours the full triangular profile under the steps. If the stairs sit against a back wall and you're only pouring tread blocks, the volume is roughly half what the calculator returns. The notes flag this; the form choice changes the math.
- Pour temperature.The math doesn't change with temperature, but the pour does. Below 40°F the cure window blows out and you need curing blankets (ACI 306). Above 85°F slump loss accelerates and you have less than the 90-minute ASTM C94 discharge window to finish (ACI 305). On a Chicago shoulder season — March or October — the swing between night low and afternoon high can put both ends of the day inside the risk window.
- Half-yard dispatchers.Quarter-yard rounding assumes Ozinga / Prairie / similar Chicago-suburb practice. Stricter dispatchers — including most plant-direct suppliers outside the metro — only honor half-yard tickets. If the calculator says 1.75 and the dispatcher says “we round up to 2,” order 2.
- The 28-day cure clock.If you're backfilling a footing or removing forms, plan for the cure. Concrete reaches roughly 70% strength at 7 days and 100% at 28 days (ACI 318). Backfilling a footing before day 14 risks lateral movement and settlement. The calculator output doesn't schedule anything — your project plan does.
Methodology
Every number on this page traces to one of three layers — site arithmetic for the volume math, manufacturer data sheets for bag yields, and industry guidance for waste and order minimums. The per-cluster sourcing tier in methodology spells out which sources back which kinds of claims.
Show the formulas
- Slab / footing:
(L ft × W ft × T″ / 12) / 27= cubic yards. - Cylinder (post hole):
π × (D″/24)² × (H″/12) / 27= cubic yards. - Stairs (monolithic wedge):
(R″/12) × (T″/12) × W ft × N(N+1)/2 / 27= cubic yards. For tread-block-only pours against a back wall, divide by ~2. - Practical order: exact × (1 + waste%/100), rounded UP to the next 0.25 yd³ for Chicago-suburb residential dispatch. Half-yard dispatchers add one further step.
- Bag count: ceiling of the EXACT yardage at 45 bags / yd³ for 80 lb (Quikrete #1101 yields 0.60 ft³/bag) or 60 bags / yd³ for 60 lb (yields 0.45 ft³/bag). Sakrete High-Strength matches both yields and meets ASTM C387.
Frequently asked
How many cubic yards of concrete do I need for a 10×12 slab?
For a 10 ft × 12 ft × 4″ slab, the exact volume is 1.48 cubic yards (10 × 12 × 0.333 = 40 ft³, divided by 27). With a 5% waste cushion that rounds up to 1.75 yd³ as the practical-order amount on a Chicago-suburb supplier that honors quarter-yard residential dispatch. If your supplier only delivers in half-yard increments, order 2.0 yd³. The bagged alternative for the exact volume is 67× 80 lb bags, derived from Quikrete's published 0.60 ft³ yield per 80 lb bag.
How many bags of concrete make a yard?
60 bags of 60 lb concrete mix per cubic yard, or 45 bags of 80 lb. These numbers come from Quikrete and Sakrete data sheets — a 60 lb bag yields 0.45 ft³ once mixed, and an 80 lb bag yields 0.60 ft³, so a cubic yard (27 ft³) takes 27/0.45 = 60 bags or 27/0.60 = 45 bags. The 80 lb bag is the cheaper-per-cubic-foot option but the heavier carry. For solo work or anything past 30 bags, ready-mix is usually the better call.
What's the residential ready-mix delivery minimum?
Most Chicago-suburb suppliers — Ozinga, Prairie, Vulcan — set residential delivery minimums between 1 and 3 cubic yards. Below the minimum, expect a short-load fee of $40-60 per yard under the threshold (NRMCA CIP 31). The calculator flags the warning when the practical-order amount falls below 3 yd³, which is a conservative ceiling that catches most supplier policies. Always confirm the dispatcher's minimum and short-load policy before placing the order — fees vary by supplier and by season.
Why does the practical order round up to a quarter-yard?
Most Chicago-suburb residential dispatchers honor quarter-yard increments even though internal truck dispatch is half-yard. Ordering 1.48 yd³ from a quarter-yard supplier returns a 1.75 yd³ ticket; the same call to a strict half-yard dispatcher returns 2.0 yd³. SiteworkMath rounds to the next quarter to match the more common Chicago-area practice — if your supplier only takes half-yard, bump the practical up one more step.
Should I add waste to the calculation when ordering bagged concrete?
Yes, but at the bag-count layer, not at the cubic-yard layer. The calculator shows bag counts derived from the exact yardage (not the practical, which already includes the ready-mix cushion). For bagged work, buy bags for the exact yardage and add one or two by hand if the subgrade looks dicey or you're working solo and might mis-measure. Double-cushioning at both layers — adding 5% to cubic yards AND ceiling the bag count — over-buys by 10-15% and leaves bags hardening in the garage.
Does this calculator handle structural rebar or PSI specs?
No. The calculator returns volume only. Rebar layout, PSI mix selection, and structural detailing are scope for a structural engineer or your local building inspector — ACI 332-20 covers residential code requirements. For a 4″ residential interior slab on level compacted subgrade, 3500-4000 psi mix with welded wire reinforcement is the typical Chicago-suburb spec; for footings in a freeze-thaw zone, ACI 332 sets the minimum depth and reinforcement. The methodology dropdown on the calc widget shows the volume formula; structural decisions live in the supporting guides.
Related guides
- How to figure concrete yardage for a slab →
- How to figure concrete yardage for steps →
- How to figure concrete yardage for footings →
- How to figure concrete yardage for a circle →
- How many bags of concrete make a yard? →
By James Wu. Volume math is site arithmetic, formulas shown above. Bag yields trace to Quikrete Concrete Mix #1101 data sheet and Sakrete High-Strength Concrete Mix (ASTM C387). Ready-mix minimum and short-load guidance from NRMCA CIP 31 — Ordering Ready Mixed Concrete. Discharge window from ASTM C94/C94M — Standard Specification for Ready-Mixed Concrete. Engine logic in lib/sitework/concrete.ts. Not structural-engineering advice — for residential code requirements see ACI 332-20 or the local building inspector. Full methodology.