Ordering concrete with confidence comes down to running the same checks in the same order, every time. This checklist compresses the Concrete Calculator Complete Guide into checkable actions for before you calculate, order, and build. Run your numbers in the Concrete Calculator as you go.
Before You Calculate
- Measure the longest and widest points — measure at several points and use the largest values, since slabs are rarely perfectly uniform.
- Decide your thickness reference: centimetres (metric) or inches (imperial). Residential slabs: 10 cm (4 in); driveways: 12–15 cm (5–6 in).
- Use one unit system across all inputs. Mixing units is the most common input error of all.
- Average out irregularities: break curved, notched, or tapered sections into measurable segments (see How to Estimate Concrete for Any Project).
- Match the depth reference to the project type — footings use actual excavation depth, not slab thickness.
Before You Order
- Apply a waste factor of 5–10% (calculator default: 10%). Spillage, over-excavation, and uneven subgrade consume extra material; an estimate with no waste built in comes up short.
- Round bag counts UP, never down. A result of 647.1 bags means ordering 648 — running short mid-pour costs far more than one spare bag.
- Double-check the yd³ ↔ m³ conversion if your supplier quotes in different units: 1 m³ = 1.31 yd³.
- Confirm delivery access: truck clearance (gates, soft ground, overhead wires) and any short-load fee below the supplier’s full-truck minimum.
- Add 2–3 contingency bags beyond the rounded-up count for touch-ups and small repairs.
Before You Build
- Check permits with your local authority (AHJ). Footings, structural slabs, and pours near property lines often need permits and inspections — see the code compliance guide.
- Safety gear ready: chemical-resistant gloves, eye protection, boots. Wet concrete is caustic and burns skin on prolonged contact.
- Weather window suitable: no freezing temperatures, heavy rain, or extreme heat during the first 24–48 hours, when fresh concrete is most vulnerable.
- Forms and reinforcement set: formwork level and braced, rebar or mesh positioned before the truck or mixer arrives.
- Curing plan confirmed: how you will keep the concrete moist and protected for the first week (see pro tips for better pours).
Common Last-Minute Catches
Catch these now, not at the supplier’s desk:
- The thickness unit trap. Entering “10” as inches when centimetres are expected inflates volume 2.5×; “4” as centimetres when inches are expected shrinks it. Verify the field’s unit label — more in common concrete calculation mistakes.
- Forgetting the waste factor. A “perfect” 10 m³ with no waste orders 10 m³ for a job that will consume 11 m³. Apply waste last, on the final volume.
- Rounding the bag count down. Calculators return decimals; suppliers sell whole bags. Round up — every time.
- Using plan dimensions instead of actuals. Re-measure the real excavation or form, not the drawing. Forms shift and excavations widen.
- Ordering for yesterday’s scope. If anything changed since you calculated — an added footing, a re-measured section — re-run the calculator before confirming the order.
Quick Reference
Formula: V = L × W × T × (1 + waste)
Worked example: 10 m × 10 m slab, 0.1 m thick → 10 m³; +10% waste → 11 m³ = 14.39 yd³. At 0.017 m³ per 80-lb bag: 11 ÷ 0.017 = 647.06 → 648 bags (see the formula explained guide).
| Item | Metric | Imperial |
|---|---|---|
| Volume (no waste) | 10 m³ | 13.08 yd³ |
| Volume (+10% waste) | 11 m³ | 14.39 yd³ |
| 80-lb bags | 648 (rounded up) | |
FAQ
How many 80-lb bags of concrete are in a cubic yard?
About 45. Run your project through the Concrete Calculator for the exact count with waste applied.
What waste percentage should I add when ordering concrete?
5–10%: 5% for simple, well-formed pours; 10% for irregular shapes, deep excavations, or DIY pours. The calculator defaults to 10%.
Should I round my concrete bag count up or down?
Always round up to the next whole bag: a calculated 647.1 bags means ordering 648. A mid-pour shortfall is far costlier than one spare bag.
How do I convert cubic metres to cubic yards?
Multiply cubic metres by 1.31 (1 m³ = 1.31 yd³). To go the other way, multiply cubic yards by 0.765.
Do I need a permit for a concrete slab or driveway?
Often yes — footings, structural slabs, and work near property lines commonly need permits. Check with your local AHJ first (see the code compliance guide).
Can I use this checklist for footings and columns too?
Yes — only the depth reference changes: footings use actual pour depth rather than slab thickness.
Disclaimer: results are estimates for planning purposes. Actual material needs vary with site conditions, subgrade preparation, and supplier yields — confirm final quantities with your supplier before ordering.
Run your numbers through the Concrete Calculator, then work this checklist top to bottom before you order.
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