Last updated: September 2026 · Independently QA-tested formulas
Concrete is the classic order-too-little-and-the-pour-fails, order-too-much-and-it-hardens-in-the-truck material. The math is simple; the expensive mistakes are unit mix-ups and forgetting waste. The concrete calculator handles both — here's the hand method so you can sanity-check any quote.
For footings, same formula with the trench's width and depth. For a round pad, use π × radius² × thickness.
Step by step:
Mixing bags instead? One 80 lb bag yields ≈ 0.022 cu yd → 4.94 ÷ 0.022 ≈ 225 bags (round up to a full pallet — 5 bags of buffer included in the 10% rule).
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 40 lb | ≈ 0.011 cu yd (0.30 cu ft) | ~90 |
| 60 lb | ≈ 0.017 cu yd (0.45 cu ft) | ~60 |
| 80 lb | ≈ 0.022 cu yd (0.60 cu ft) | ~45 |
Yields vary slightly by brand; the bag print is authoritative for its mix.
| Application | Typical thickness |
|---|---|
| Garden walkway | 3 in |
| Patio / shed pad | 4 in |
| Residential driveway (cars) | 4–5 in |
| Driveway with trucks/RV | 5–6 in + rebar |
Entering 4 feet instead of 4 inches inflates your order 12× — always double-check the unit field.
Excavated soil is never perfectly flat, forms flex, and some mix always stays in the wheelbarrow. 5% covers a well-prepared base on a simple rectangle; 10% covers uneven ground, odd shapes and hand mixing. Over-ordering readymix by more than ~1 yard often triggers a short-load fee anyway, so consolidate pours where you can.
Nominal site mixes are specified by volume of cement : sand : gravel. Common grades: M10 ≈ 1:3:6, M15 ≈ 1:2:4, M20 ≈ 1:1.5:3, M25 ≈ 1:1:2 — higher cement share means higher strength. US bagged mixes are labeled in psi instead (e.g. 4,000 psi), so match the label to your project requirement rather than converting ratios on site.
Water is where DIY pours go wrong. A 0.5 water–cement ratio means about 25 litres per 50-kg bag; adding extra water makes the mix easier to pour but weakens the hardened paste. Use just enough for workability and compact instead.
| Bag size | Yield (approx.) | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 cu ft | 45 |
| 60 lb | 0.45 cu ft | 60 |
| 25 kg | ≈11.6 L | ≈87 |
Yields assume typical bagged-mix density; your brand's calculator line on the bag wins over any table. And a 25-kg figure comes out to roughly 87 bags per yard — which is exactly why big pours use readymix.
At 4 inches thick, one cubic yard covers 81 sq ft (27 × 12 ÷ 4). At 3 inches, 108 sq ft; at 6 inches, 54 sq ft.
Trench length × width × depth, all in feet ÷ 27. A 40 ft × 1 ft × 1 ft trench = 1.48 cu yd; order 1.6 with waste.
Quality mixes (e.g. 4000 psi blends) reach comparable strength when cured properly. The practical limits are batch consistency and fatigue — a 5-yard pour by hand is ~225 bags.
Negligibly. Steel displaces some concrete, but the standard practice is to ignore it and let the waste allowance absorb the difference.