Concrete Calculator
Estimate cubic yards for a slab. Add a waste factor and optional price per cubic yard.
Quick guide
- Measure slab length x width and confirm the thickness (inches or cm).
- Use finished (installed) dimensions, not forms that will be trimmed.
- Add a buffer for over-excavation, spillage, and uneven subgrade (often 5-10%).
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- If your pour includes thickened edges or footings, estimate those separately and add them.
- Decide whether the slab needs reinforcement (rebar/mesh/fiber) and a vapor barrier—these don’t change volume but change the material list and cost.
- If the slab will drain, use average thickness across the slope; slopes change volume more than most people expect.
Why thickness drives cost
Concrete volume is area x thickness, so small thickness changes can add up fast on larger slabs. When you're near a delivery minimum, rounding and buffers can affect price more than the raw math.
If your pour includes thickened edges, grade beams, or footings, estimate those separately and add them to the slab volume for a more reliable order.
Ready-mix is typically ordered in cubic yards or cubic meters, and trucks may have minimums. Rounding up slightly is usually cheaper than a second short load.
Reinforcement is about performance, not quantity. Rebar, wire mesh, or fiber can help with crack control and load distribution, but it must be placed correctly (cover, spacing, chairs) and paired with good subgrade and joints.
Control joints are a planning step. They don't change yardage, but poor joint planning can make a slab look “failed” even when the concrete quantity was perfect.
Subgrade preparation often decides whether your thickness is real. If the base is soft or uneven, the slab can end up thicker in low areas and thinner in high areas unless you compact and grade carefully.
Worked example: a 12' x 16' slab at 4" thickness is 192 sq ft x (4/12 ft) = 64 cu ft = 2.37 yd³. Adding a 7% buffer is ~2.54 yd³. If your supplier has a 3 yd³ minimum, you may order 3 yd³ anyway and use the extra for small pads or fill.
If you’re placing by wheelbarrow or pump, line loss and cleanup waste can be real. A small overage is usually cheaper than trying to stretch the last few feet of a pour.
Rebar or fiber notes: fiber can help with plastic shrinkage cracking, but it is not automatically a replacement for rebar or mesh. Reinforcement choice depends on loads, joint layout, and local practice (project dependent).
Wire mesh needs correct placement. Mesh lying on the subgrade does little—plan chairs or a placement method so reinforcement ends up in the slab where it is intended to work.
Joint planning is part of concrete planning. Control joint spacing and layout often matter more to crack appearance than small changes in PSI or water content.
Curing is not optional if you want a durable surface. Wind and sun can dry the top fast; a curing plan can reduce dusting, curling, and early cracking (project dependent).
Second worked example: a 10' x 30' driveway strip at 5" thickness is 300 x (5/12) = 125 cu ft = 4.63 yd³. With a buffer and any thickened apron/edge details, you may plan closer to 5.0 yd³ and then confirm minimums and delivery schedule.
Concrete Calculator
Estimate cubic yards for a slab. Add a waste factor and optional price per cubic yard.
Results
- Base volume (cubic yards)
- 1.23
- Waste buffer (cubic yards)
- 0.12
- Estimated cubic yards
- 1.36
How to measure your slab
- Split complex shapes into rectangles and add their areas.
- Convert thickness to feet (in / 12) or meters (cm / 100) before volume math.
- If edges are thickened or there are footings, estimate those separately.
- Subtract voids you won’t fill (large drains, openings, or cutouts) if you want a tighter estimate.
- If the slab is sloped, estimate average thickness (thin edge + thick edge) / 2 and use that thickness for volume math.
- If you have multiple pours (slab + steps + thickened apron), estimate each pour as a separate line item and sum volumes at the end.
Assumptions to double-check
- Thickness is average thickness across the whole pour (high/low spots change volume).
- Concrete is ordered by volume (cubic yards/meters); suppliers may have minimums.
- Ordering short can stop a pour; buffers are usually cheaper than a second delivery.
- Finish requirements (reinforcement, vapor barrier, finish type) don’t change volume math, but they do change your full material list and cost.
- If you are using a thicker edge, the edge volume can be a meaningful percentage of total on small slabs; don’t ignore it for small pads.
- Higher slump or adding extra water can make placement easier but can reduce strength and increase cracking risk—confirm your target mix with the supplier/contractor.
- Hot or windy weather can increase finishing and curing risk; curing method matters even if the yardage is correct.
- If reinforcement is required, it does not change yardage, but it can change slab thickness, joint plan, and finishing workflow (project dependent).
- If you are ordering ready-mix, the supplier’s minimum load and short-load fees can influence your practical order size more than “perfect math.”
Ordering tips
- Confirm PSI, slump, fiber, and admixtures with your supplier before ordering.
- Plan a little extra for waste and wheelbarrow/pump line loss if applicable.
- Round up to delivery minimums and consider timing so you can place it continuously.
- Confirm access for the truck or pump and plan a clear placement path before the concrete arrives.
- Ask whether your supplier charges a short-load fee and what the minimum yardage is; it can change whether ordering “exact” is worth it.
- Plan your full bill of materials: vapor barrier, base stone, reinforcement, chairs, forms, stakes, joint tool/saw, curing plastic or curing compound.
- If you need a broom finish vs smooth trowel, plan the right finishing tools and timing; finish choice affects labor more than yardage.
- If you’re pouring against existing concrete, plan a joint/isolation detail (foam expansion joint) where required; it doesn’t change volume but affects durability.
- If you need reinforcement, plan the full kit (rebar/mesh, tie wire, chairs, cutter) so it doesn’t become an install-day delay.
- If you are pouring in hot/cold weather, ask about admixtures and curing protection before the truck arrives (project dependent).
Common mistakes to avoid
- Using form dimensions instead of finished dimensions (or forgetting to subtract voids).
- Forgetting thickened edges/footings when they're part of the same pour.
- Ordering with no buffer for subgrade variation; ordering short is usually the most expensive outcome.
- Mixing up units (feet vs inches, yards vs cubic feet) and getting a volume off by a factor of 12 or 27.
- Skipping control joints (or placing them poorly) and then blaming the mix when cracks show up.
- Over-watering to make finishing easier, then ending up with weaker concrete and surface dusting.
- Not planning placement speed (help, tools, access) and letting the pour get away from you before you can finish it.
- Placing reinforcement on the ground and never lifting it into the slab (it won’t work as intended).
- Skipping curing/protection because the surface “looks dry,” then getting dusting or surface issues later.
Concrete slab ordering notes (quick)
Concrete volume is area × thickness. Small thickness changes scale fast on big slabs, so verify thickness first.
| Thickness | Rule of thumb | Reminder |
|---|---|---|
| 1 inch | ≈ 0.083 cu ft per sq ft | Use average thickness across the slab |
| 4 inches | ≈ 0.333 cu ft per sq ft | Common for pads/driveways |
| 6 inches | ≈ 0.500 cu ft per sq ft | Heavier duty slabs |
- Add a buffer (often 5–10%) for uneven subgrade and rounding.
- Estimate thickened edges and footings separately when applicable.
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Results are estimates for informational purposes only. Always verify measurements and product specifications.