How to estimate concrete for a slab
Compute volume from area x thickness, then add a realistic ordering buffer and plan for forms.
Concrete is ordered by volume (cubic yards or cubic meters). For slabs and pads, volume comes from slab area multiplied by slab thickness, with thickness converted into the same unit system (inches to feet, or centimeters to meters). For bag mixes, estimate total cubic feet or liters first, then divide by the yield per bag.
Most real-world errors come from site conditions: uneven subgrade, thickened edges, footings, and waste during placement. Ordering short can create cold joints or force a second delivery that arrives too late to finish properly, so this guide focuses on safe, practical estimating rather than perfect math. Small thickness errors can change the order by a full yard.
The basic formula
Start with finished slab dimensions, convert thickness into the same unit system, compute volume, then convert to the unit your supplier sells. If the slab is sloped, use average thickness (thin edge + thick edge) / 2, and measure thickness at multiple points. If you have thickened edges, steps, or footings, estimate those parts separately and add them for a more reliable order. For multi-part projects (slab + walkway + small pad), estimate each pour as its own line item before you place the order.
- Area = length × width (or break the shape into rectangles and add them).
- Volume = area × thickness (use average thickness if the slab slopes).
- Convert to ordering units: cubic yards (divide cubic feet by 27) or cubic meters (use meters directly).
Ordering tips
- Confirm whether thickness is uniform. If you have thickened edges, grade beams, steps, or footings, estimate those volumes separately and add them.
- Add a realistic buffer (often 5-10%) for uneven subgrade, spillage, and cleanup losses. If you are placing by wheelbarrow or pump, losses can be higher, and if the base is soft/uneven you may lose thickness into low spots unless you grade and compact carefully (project dependent).
- Ask about minimum load and short-load fees, delivery timing, and mix options (PSI, slump, fiber, and any admixtures). Confirm whether you can add water on site and what the supplier allows; water changes workability but can affect strength and finish. Also confirm maximum aggregate size for tight forms or thin sections.
- Plan placement and finishing before the truck arrives: access route, tools, helpers, screeding method, and a curing plan. Also think through joints (control joints and isolation joints) because good joint planning affects durability even though it does not change the yardage. Weather (heat, wind, rain, freezing) can change finishing and curing risk even when yardage is correct, so plan protections in advance.
Concrete ordering checklist
Ready-mix ordering is more than volume. Suppliers will ask about strength (PSI/MPa), slump, aggregate size, and whether you want fiber or admixtures. These choices can affect both cost and placement.
If you’re doing a continuous pour, it’s usually safer to order a little extra than to risk coming up short. Finishing and curing conditions matter too—plan for timing, temperature, and access.
If your pour includes thickened edges, steps, or separate footings, estimate those parts separately and add them. Small geometry omissions are a common reason people come up short.
- Confirm minimum load/delivery fees and how partial yards are billed.
- If you have footings/thickened edges, estimate them separately and add them in.
- Have a placement plan (wheelbarrow, chute access, or pump).
- Confirm how the truck can access the forms and where washout will go.
Concrete buffers & footings (quick)
Thickness and a small buffer are the two biggest levers. If your pour includes thickened edges or footings, estimate them separately.
- Use average thickness across the slab; don’t measure only the lowest spot.
- Add 5–10% buffer for uneven subgrade, spillage, and rounding.
- Confirm delivery minimums; rounding up is usually cheaper than a second short load.
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