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.
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-calculator 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?
1.75 cubic yards if your supplier delivers in quarter-yard increments. 2.0 if they only do half-yards. 1.48 is the exact math. The math: 10 × 12 × (4/12) = 40 cu ft. Divided by 27 = 1.48 yd³ exact. With the 5% waste cushion, the practical order rounds to the next supplier increment. Bagged alternative for the exact volume: About 67 of the 80 lb bags. Bag yield comes from the manufacturer data sheet (0.60 ft³ per 80 lb bag mixed). Which way to go: At 1.5 yd³, you're at the awkward edge — bagged is 67 bags (a lot of mixing) but you may be below your supplier's ready-mix minimum. The calculator flags the minimum-order warning when relevant.
How many bags of concrete make a yard?
60 bags of the 60 lb mix, or 45 bags of the 80 lb mix. The math: A cubic yard is 27 cubic feet. A 60 lb bag yields about 0.45 cu ft mixed (27 ÷ 0.45 = 60). An 80 lb bag yields about 0.60 cu ft (27 ÷ 0.60 = 45). Which bag to pick: The 80 lb bag is cheaper per cubic foot but a heavier carry. The 60 lb bag is easier on the back but you'll lift more total bags. When ready-mix beats bagged: Solo work or anything past about 30 bags. At that point, you're spending a half day mixing instead of a half hour finishing. Where the yield numbers come from: Bag manufacturer data sheets (Quikrete, Sakrete).
What's the residential ready-mix delivery minimum?
1 to 3 cubic yards is typical for Chicago-suburb suppliers. Below the minimum, expect a short-load fee of $40-$60 per yard under the threshold. Which suppliers in the Chicago suburbs: Ozinga, Prairie, Vulcan — each sets their own minimum. The industry baseline for short-load fees: Roughly $40-$60 per yard under threshold. The calculator flags a warning when the practical order falls below 3 yd³, which is the conservative ceiling that catches most supplier policies. What to do first: Call the dispatcher before placing the order. Minimums and short-load fees vary by supplier and by season — confirm both.
Why does the practical order round up to a quarter-yard?
Because most Chicago-suburb residential dispatchers honor quarter-yard increments at the customer counter, even though their internal truck dispatch is half-yard. The quarter-yard supplier path: A 1.48 yd³ order returns a 1.75 yd³ ticket. The strict half-yard path: The same call returns 2.0 yd³. Why round to a quarter by default: It matches the more common Chicago-area practice and gets you a tighter cushion. If your supplier only takes half-yard, bump the practical order up one more step at the dispatcher's request.
Should I add waste to the calculation when ordering bagged concrete?
Yes, but at the bag-count layer — not at the cubic-yard layer. What the calculator already does: Bag counts are derived from the exact yardage. The practical-order cushion (5%) is for the ready-mix path only. What to do for bagged work: Buy bags for the exact yardage. Add one or two by hand if the subgrade looks dicey or you're working solo and might mis-measure. What NOT to do: Double-cushion — adding 5% to cubic yards AND ceiling the bag count — which 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. What it does not decide: Rebar layout, PSI mix selection, structural detailing. Those are scope for a structural engineer or your local building inspector — residential concrete code is the authority on what's required. Typical Chicago-suburb specs (sanity check, not a substitute for a permit set): • Interior 4″ slab on compacted subgrade — 3,500-4,000 psi mix with welded-wire reinforcement. • Footings in a freeze-thaw zone — minimum depth and reinforcement set by residential code. Where structural decisions belong: The supporting guides cover scope, and the engineer / inspector approves it for your specific site.
What I'd do next
- Slab-only calculator
Garage floor, foundation slab-on-grade, driveway, or patio. Weather-window, vapor-barrier, and control-joint breakdown built in.
- Figure concrete yardage for a slab
Most pours start with a slab. The rectangle math, the cushion, the bag-vs-truck breakpoint.
- Bag count vs ready-mix economics
Once you have the yardage, decide bagged or ready-mix. The economics flip around 1 yd³.
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.