Ballast for Concrete Calculator
Calculate how much coarse aggregate (ballast) you need for a concrete slab or footing pour, based on area, thickness, and aggregate size.
Measure straight across the widest point.
Adjust depth below if needed.
Covers spillage and compaction. 5–10% is normal.
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Bags or bulk delivery?
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Coverage & weight
Material profile
For a 100 sq ft slab poured 4 inches thick, you’ll need about 1.36 cubic yards, or roughly 1.97 tons of 20mm coarse aggregate including 10% waste — this is the coarse-aggregate portion of the mix; add sand and cement per your mix ratio.
Coarse aggregate needed for common concrete pours
These estimates use 20mm coarse aggregate at 2,900 lb per cubic yard with a 10% waste allowance built in.
| Project | Depth | Cubic yards | Tons |
|---|---|---|---|
| Shed pad, 64 sq ft | 4 in | 0.87 yd³ | 1.26 tons |
| Single-car garage slab, 240 sq ft | 4 in | 3.26 yd³ | 4.73 tons |
| Patio slab, 225 sq ft | 4 in | 3.06 yd³ | 4.43 tons |
| Driveway slab, 800 sq ft | 6 in | 16.30 yd³ | 23.63 tons |
| Foundation strip, 100 sq ft | 8 in | 2.72 yd³ | 3.94 tons |
| Addition footing, 80 sq ft | 8 in | 2.17 yd³ | 3.15 tons |
Aggregate types, density and mix use
| Type | Weight per yd³ | Best for | Drainage |
|---|---|---|---|
| 20mm coarse aggregate | 2,900 lb | Standard concrete mix (1:2:4) | Low |
| 10mm coarse aggregate | 2,850 lb | Fine finish concrete, thin slabs | Low |
| All-in ballast | 2,850 lb | General-purpose concrete mixing | Moderate |
Figures are for dry material. Wet or freshly delivered material can weigh noticeably more.
How to calculate ballast you need
- Measure the slab or footing length × width in feet; for footings measure trench length × width.
- Set the pour thickness 4in for a standard shed or patio slab, 6in for driveways, 8in for footings.
- Convert to cubic yards multiply area by depth in feet and divide by 27.
- Add 10% waste covers spillage, formwork loss, and uneven subgrade.
- Convert to tons multiply cubic yards by aggregate density (2,850-2,900 lb/yd³) and divide by 2,000 — this is the coarse-aggregate portion of your mix; order cement and sand separately per your mix ratio.
Worked example
Using the calculator’s default 10 ft × 10 ft slab poured 4 inches thick with 20mm coarse aggregate:
- Area10 ft × 10 ft = 100 sq ft
- Depth in feet4 in ÷ 12 = 0.333 ft
- Volume100 sq ft × 0.333 ft = 33.3 cu ft
- Raw cubic yards33.3 ÷ 27 = 1.23 yd³
- Plus 10% waste1.23 × 1.10 = 1.36 yd³
- Weight (20mm coarse aggregate, 2,900 lb/yd³)1.36 × 2,900 ÷ 2,000 = 1.97 tons
- Order amountRound up to 2 tons of coarse aggregate for the mix
Recommended depth by project
| Project | Recommended depth | Notes |
|---|---|---|
| Project type | Depth | Notes |
| Thin slab / patio topping | 4 in | Use 10mm aggregate for a finer finish |
| Standard shed or garage slab | 4-5 in | 20mm aggregate is standard for structural slabs |
| Driveway slab | 6 in | Reinforce with mesh or rebar at this thickness |
| Footings / foundations | 8 in | Deeper for load-bearing walls; check local code minimums |
| Reinforced structural footing | 10-12 in | Confirm depth with a structural engineer |
Bags or bulk delivery?
Bagged coarse aggregate (typically 50 lb bags) costs far more per ton than a bulk load, so it only pays off for tiny pours like fence-post footings. Once a pour needs more than about half a ton of aggregate, bulk delivery plus the flat delivery fee is cheaper — and for anything past a couple of tons, ready-mix concrete is often worth pricing against site-mixing altogether.
- Small poursunder 0.5 ton, such as fence post footings — bagged aggregate keeps a small mix simple.
- Medium pours0.5 to 3 tons, such as sheds or patios — compare bagged cost to a bulk minimum load plus delivery.
- Large poursover 3 tons, such as driveways or slabs — bulk delivery, and consider ready-mix concrete instead of site-mixing.
Common mistakes
- Ordering ballast as if it's the whole mix“ballast for concrete” here sizes the coarse-aggregate component only; a full mix also needs sand and cement in the correct ratio.
- Wrong aggregate size for the job20mm aggregate in a thin 2-3in topping leaves a rough finish and weak edges; use 10mm for thin sections.
- Not accounting for mix ratiothis calculator sizes the aggregate needed to fill the pour volume, not the cement or sand — use a 1:2:4 (cement:sand:aggregate) ratio as a baseline.
- Skipping compaction or vibrationtrapped air pockets around coarse aggregate weaken the cured slab and leave voids at the surface.
- Pouring on unprepared subgradesoft or wet ground under the slab lets the mix segregate and settle unevenly as it cures.
- Underestimating footing depth for frost lineshallow footings in freeze zones crack from frost heave; check local frost depth requirements before finalizing thickness.
What this calculator assumes
Adjust your result if your project differs from these.
- Depth is the finished pour thickness, formwork included.
- Figures are for the coarse-aggregate component only, not finished ready-mix concrete volume.
- Density reflects standard crushed stone aggregate, damp and loose.
- 10% waste covers spillage, over-excavation, and formwork variance.
- Prices are material only; cement, sand, and labor are separate.
- Bag coverage assumes 50 lb bags of coarse aggregate.
Common questions
What size aggregate is used in concrete?
20mm coarse aggregate is the standard for structural slabs and footings; 10mm is used for thin toppings and fine-finish work.
What ratio of ballast to cement do I need?
A common general-purpose mix is 1:2:4 (cement:sand:coarse aggregate) by volume; a stronger structural mix uses 1:1.5:3.
Can I use all-in ballast instead of separate aggregate and sand?
Yes, for general-purpose or non-structural pours all-in ballast simplifies the mix. For structural work, separate coarse aggregate and sand give more control over strength.
How thick should a concrete slab be?
4 inches is standard for a shed, patio, or garage floor. Driveways typically use 6 inches, and footings 8 inches or more.
Do I need rebar or mesh in a 4 inch slab?
Recommended for any slab that will support vehicle weight or is prone to cracking from ground movement; a plain 4in patio slab can often skip it.
How many tons of aggregate are in a cubic yard of concrete?
Coarse aggregate alone runs about 1.425 to 1.45 tons per cubic yard of aggregate volume (2,850-2,900 lb/yd³); the finished concrete volume also includes sand and cement paste.
What's the difference between 10mm and 20mm aggregate?
20mm gives a stronger, more economical mix for standard slabs; 10mm produces a smoother finish and is used for thin sections where large stones won’t fit.
How deep should a footing be?
8 inches is a common minimum, but check your local frost depth and load requirements — deeper footings are often required for load-bearing walls.
Can I mix concrete without coarse aggregate?
No — a cement-sand-water mix without coarse aggregate is mortar, not structural concrete, and lacks the compressive strength for slabs and footings.
How much does concrete aggregate cost per ton?
Prices vary by region and aggregate size; this calculator’s default estimate uses about $44 per ton plus a flat delivery fee.
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Project
| Shape | — |
| Dimensions | — |
| Depth | — |
| Area | — |
| Material | — |
| Waste allowance | — |
Volume & weight (incl. waste)
| Cubic yards | — |
| Cubic feet | — |
| Weight | — |
| Tons (rounded up) | — |
Shopping list
| Bulk order | — |
| Bagged alternative (0.5 cu ft) | — |
| Estimated cost | — |
Notes
Estimate generated by materialcalculate.com. Assumes uniform depth and standard dry material densities. Confirm quantities with your supplier before ordering.