How to Calculate Concrete for Slabs, Footings and Pads
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.
The concrete volume formula
cubic yards = cubic feet ÷ 27
- L, W — slab length and width in feet.
- T — thickness in inches (the ÷12 converts to feet).
- ÷27 — one cubic yard = 3×3×3 = 27 cubic feet.
For footings, same formula with the trench's width and depth. For a round pad, use π × radius² × thickness.
Worked example: 20×20 patio, 4 inches thick
Step by step:
- Cubic feet: 20 × 20 × (4 ÷ 12) = 133.3 cu ft
- Cubic yards: 133.3 ÷ 27 = 4.94 cu yd
- Waste allowance at 10%: 4.94 × 1.10 = 5.4 cu yd to order
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).
How many bags per cubic yard?
| 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.
Standard thicknesses (avoid the classic mistake)
| 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.
Waste, subgrade and real-world buffers
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.
Mix ratios and the water trap
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.
Worked example: Concrete Calculator at its default inputs
Open Concrete Calculator and press calculate without editing a field. What comes back is the table and result below, so the concrete numbers here are checkable in one click.
| Input | Default |
|---|---|
| Length (m) | 5 |
| Width (m) | 3 |
| Depth (m) | 0.1 |
Result: Volume: 1.50 m³
The steps behind that answer
- Formula: Volume = Length × Width × Depth
- Step 1: V = 5 × 3 × 0.1
- Step 2: V = 1.50 m³
Length (m): half, as given, and half again
The table below varies length (m) alone — half, the default, and half again.
| Length (m) | Result |
|---|---|
| 2.5 | Volume: 0.75 m³ |
| 5 | Volume: 1.50 m³ |
| 7.5 | Volume: 2.25 m³ |
The middle row is the default. The outer two show how far the answer drifts when length (m) is wrong by half or by a half in the other direction.
Related guides
Order by volume, then convert carefully
Ready-mix is sold by cubic metres or cubic yards, bags by weight. An 80 lb bag yields about 0.6 cubic feet, a 50 lb bag about 0.375 — the yields differ enough that mixing up the two sizes is a common way to come up short mid-pour.
Calculate the geometric volume first, then add waste. Five to ten percent covers spillage, uneven subgrade and the little that stays in the mixer; a slab on a rough base needs the upper end.
Thickness is the biggest lever on volume
Volume scales linearly with slab thickness, so a 4-inch slab and a 5-inch slab of the same footprint differ by 25%. A patio that looks close in size can be a full truck load apart depending on the thickness specification.
Footings and pads need their own cross-section. A trapezoidal footing uses the average of the two parallel widths times depth times length, not the top width alone.
Short loads are the expensive mistake
Running out mid-pour means a cold joint and a second delivery, both of which cost more than the surplus you would have thrown away. For anything structural, order slightly over and have a place to dump the remainder.
Reinforcement, formwork and labour are separate line items and often exceed the concrete itself. The calculator tells you the material volume, not the project cost.
Frequently Asked Questions
How much does a cubic yard of concrete cover?
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.
How do I calculate concrete for footings?
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.
Is bagged concrete as strong as readymix?
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.
Does rebar change the volume?
Negligibly. Steel displaces some concrete, but the standard practice is to ignore it and let the waste allowance absorb the difference.
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