Slab Concrete Volume
Use this form for flat horizontal pours — patios, driveways, garage floors, sidewalks. Length × width × thickness, all in feet, gives cubic feet.
V = L × W × T
Footing Concrete Volume
Use this form for foundation footings and strip footings — the rectangular trench-fill that supports a wall or column. Same math as a slab; depth replaces thickness.
V = L × W × D
Column Concrete Volume
Use this form for cylindrical pours — Sonotube columns, deck piers, fence-post footings, lally columns. Enter the column diameter (not radius) and height.
V = π × r² × h
Wall Concrete Volume
Use this form for vertical pours — foundation walls, retaining walls, stem walls. Length × height × thickness gives the cubic feet of concrete the form will hold.
V = L × H × T
How It Works
This concrete volume calculator computes the cubic feet, cubic yards, and cubic meters needed for slabs, footings, cylindrical columns, and walls — and then converts the cubic-foot total into the number of 40 lb, 60 lb, or 80 lb premix bags you need to buy. Pick the pour type, enter the dimensions in your preferred length unit, and optionally apply a waste factor (10% is the standard contractor recommendation). Cubic yards is the primary readout because that's how ready-mix trucks bill; bag counts are rounded up because partial bags cannot be returned.
Example Problem
A homeowner wants to pour a 10 ft × 12 ft patio slab, 4 inches (4/12 ft = 0.333 ft) thick. How much concrete is needed, and how many 80 lb bags?
- Knowns: L = 10 ft, W = 12 ft, T = 4 in = 0.333 ft, pour type = slab, waste factor = 10%.
- Volume in cubic feet: V = L × W × T = 10 × 12 × 0.333 = 40 ft³.
- Cubic yards (the ready-mix billing unit): V_yd³ = 40 / 27 ≈ 1.481 yd³.
- Apply 10% waste factor: 40 × 1.10 = 44 ft³ of concrete to actually buy.
- 80 lb bag yields 0.60 ft³, so bags needed = ceil(44 / 0.60) = ceil(73.33) = 74 bags.
- Sanity check: 74 bags × 0.60 ft³/bag = 44.4 ft³, slightly more than 44 ft³ — the ceil is correct.
Pouring 1.5 yd³ from bags is the practical upper limit — beyond that, ordering a ready-mix truck (typically 1 yd³ minimum, 10 yd³ full load) is cheaper than buying and mixing dozens of bags.
When to Use Each Variable
- Pour a Slab — for patios, driveways, sidewalks, garage floors, shed pads — any flat horizontal surface where thickness is the small dimension.
- Pour a Footing — for foundation footings and strip footings — the trench-fill below a wall or column. Code typically requires footings 1.5× to 2× the wall thickness, set below the local frost line.
- Pour a Column — for Sonotube columns, deck piers, fence-post footings, lally columns — any cylindrical pour. Enter the column diameter (not radius).
- Pour a Wall — for foundation walls, retaining walls, stem walls — vertical pours where height replaces the thickness slot of a slab.
Key Concepts
Concrete is priced and ordered in cubic yards (1 yd³ = 27 ft³), but bag yields are quoted in cubic feet (40 lb → 0.30 ft³, 60 lb → 0.45 ft³, 80 lb → 0.60 ft³ for standard ready-mix premix). Bag counts are always rounded up — partial bags can't be returned, and a short-pour means stopping mid-job to drive to the hardware store. A waste factor (typically 10%) accounts for over-excavation, spillage, uneven subgrade, and material left in mixer drums; over-ordering by 10% is far cheaper than under-ordering by even 1 bag. For larger pours (>1.5 yd³ from bags, or any pour with a tight finish deadline) ready-mix delivery is more economical and produces a stronger, more uniform pour because the entire pour cures as a single monolith.
Applications
- Residential patios and driveways (slab pours, typically 4 in thick)
- Foundation footings and stem walls (footing + wall pours)
- Deck piers and Sonotube columns (column pours, 8 in to 12 in diameter)
- Retaining walls and landscape walls (wall pours, 6 in to 12 in thick)
- Fence-post bases and mailbox bases (small column pours)
- Garage floors, shed pads, equipment pads (slab pours)
Common Mistakes
- Entering thickness in inches while length and width are in feet — convert everything to the same unit first (this calculator handles the conversion via the unit dropdown).
- Forgetting the waste factor — 0% waste sounds efficient but produces short-pours when subgrade is uneven or formwork bulges. 10% is the contractor default.
- Rounding bag counts down instead of up — buying 73 bags when you need 73.3 leaves the last 0.18 ft³ unpoured. Always round up.
- Confusing column diameter with radius — Sonotubes are sold by inside diameter (10 in tube → r = 5 in). This calculator asks for diameter.
- Ordering by cubic feet from a ready-mix supplier — they bill in cubic yards (1 yd³ = 27 ft³) with a typical 1 yd³ minimum and a short-load fee under 3 yd³.
Frequently Asked Questions
How many bags of concrete do I need?
Compute the pour volume in cubic feet, divide by the bag yield, and round up. 40 lb bag → 0.30 ft³, 60 lb → 0.45 ft³, 80 lb → 0.60 ft³. For example, 10 ft³ of concrete needs ceil(10 / 0.60) = 17 bags of 80 lb, or ceil(10 / 0.45) = 23 bags of 60 lb. Add a 10% waste factor and round up again.
What's the difference between a slab and a footing in concrete math?
The math is identical: V = length × width × depth (or thickness). The labels differ because a slab is the flat finished surface (typically 4 in thick), while a footing is the structural trench-fill below a wall (typically 8 in to 12 in deep and 1.5× to 2× the wall thickness). Use the slab form for floors and the footing form for foundation work — the calculator treats them the same mathematically.
How thick should a concrete slab be?
Residential patios and walkways: 4 inches. Driveways for passenger cars: 4 inches. Driveways for trucks or RVs: 5 to 6 inches. Garage floors: 4 to 6 inches with #4 rebar on 18-inch centers. Industrial floors with heavy loads: 6 to 8 inches. Always pour over a compacted gravel base and ensure proper subgrade preparation — slab thickness alone doesn't compensate for soft subgrade.
What is concrete waste factor and why use 10%?
The waste factor adds a buffer to your concrete order to cover over-excavation, formwork bulging, spillage, mixer drum residue, and minor measurement errors. 10% is the contractor default for typical residential pours. For complex shapes, sloped subgrade, or first-time pours, use 15%. Skipping the waste factor risks a short-pour, which means stopping mid-job to mix more — and a cold joint that compromises the slab's strength.
How much area does 1 cubic yard of concrete cover?
At 4 inches thick (0.333 ft), 1 yd³ = 27 ft³ covers 27 / 0.333 ≈ 81 ft². At 6 inches thick (0.5 ft), 1 yd³ covers 54 ft². At 3 inches thick (0.25 ft, used for thin overlays), 1 yd³ covers 108 ft². The rule of thumb: coverage in ft² = 27 / thickness in ft.
Should I use 60 lb or 80 lb concrete bags?
80 lb bags yield 0.60 ft³ each, 60 lb bags yield 0.45 ft³ each. 80 lb bags are more cost-effective per cubic foot but heavier to lift and harder to handle alone. 60 lb bags are easier on the back and better for older homeowners or solo pours. For pours under 0.5 yd³, the bag size doesn't change the total cost much; for pours over 1 yd³, you should be ordering ready-mix instead of mixing bags.
How do I calculate concrete for a cylindrical column or Sonotube?
Volume = π × (diameter / 2)² × height. For an 8-inch (0.667 ft) Sonotube that's 4 ft tall: V = π × (0.333)² × 4 ≈ 1.395 ft³ ≈ 0.052 yd³. That's about 3 bags of 80 lb premix. Always enter the column diameter, not the radius — Sonotubes are sold by inside diameter.
When should I switch from bagged premix to ready-mix delivery?
Roughly at 1 to 1.5 cubic yards. A 1 yd³ pour from 80 lb bags is 27 / 0.60 = 45 bags — about 3,600 lb of material to lift, mix, and place before the first batch sets. Ready-mix trucks deliver in cubic yards with a 1 yd³ minimum (short-load fees apply under 3 yd³), and the entire pour cures as one monolithic slab with no cold joints. For driveways, foundations, and any pour over 1.5 yd³, ready-mix is faster, stronger, and usually cheaper.
Reference: Quikrete product data — 'Concrete Mix Yield Reference.' Standard ready-mix premix bag yields (40/60/80 lb) and waste-factor guidance from the Portland Cement Association (PCA) Concrete Technology Today.
Worked Examples
Patio
How much concrete for a 10 × 12 ft patio slab, 4 in thick?
A 10 ft × 12 ft backyard patio poured 4 inches thick. Standard 4-inch residential slab spec — apply a 10% waste factor and report both yards and 80 lb bag count.
- Knowns: L = 10 ft, W = 12 ft, T = 4 in = 0.333 ft
- Formula (slab): V = L × W × T
- V = 10 × 12 × 0.333 = 40 ft³ = 1.481 yd³
- With 10% waste: 40 × 1.10 = 44 ft³
- 80 lb bags @ 0.60 ft³/bag: ceil(44 / 0.60) = 74 bags
Volume = 1.48 yd³ (40 ft³). 80 lb bags = 74 with 10% waste
At 74 bags (~5,920 lb), ordering 1.5 yd³ of ready-mix is usually cheaper than mixing bags and produces a stronger monolithic pour.
Sonotube
How much concrete for a 6 in × 8 ft cylindrical column?
A 6-inch (0.5 ft) diameter Sonotube column, 8 feet tall — typical deck pier or fence-post footing.
- Knowns: diameter = 6 in = 0.5 ft → r = 0.25 ft, h = 8 ft
- Formula (column): V = π × r² × h
- V = π × (0.25)² × 8 = π × 0.0625 × 8 ≈ 1.571 ft³
- With 10% waste: 1.571 × 1.10 ≈ 1.728 ft³
- 60 lb bags @ 0.45 ft³/bag: ceil(1.728 / 0.45) = 4 bags
Volume ≈ 0.058 yd³ (1.57 ft³). 60 lb bags = 4 with 10% waste
Small column pours are the sweet spot for bagged premix — 4 bags is easy to mix and place inside the working time of the concrete.
Foundation Footing
How much concrete for a 20 ft strip footing, 18 in wide × 12 in deep?
A 20-foot continuous strip footing, 18 inches (1.5 ft) wide and 12 inches (1 ft) deep — typical residential foundation footing under an 8-inch stem wall.
- Knowns: L = 20 ft, W = 1.5 ft, D = 1 ft
- Formula (footing): V = L × W × D
- V = 20 × 1.5 × 1 = 30 ft³ = 1.111 yd³
- With 10% waste: 30 × 1.10 = 33 ft³
- 80 lb bags: ceil(33 / 0.60) = 55 bags — order ready-mix instead
Volume ≈ 1.11 yd³ (30 ft³). Order ready-mix (1.5 yd³ minimum)
Above ~1.5 yd³, ordering a short-load from a ready-mix truck is cheaper and produces a stronger, single-pour footing. Schedule the pour and have all formwork inspected first.
Concrete Volume Formulas
Each pour type uses a simple volume formula in the same length unit for every dimension. Convert to cubic yards by dividing cubic feet by 27 (1 yd³ = 27 ft³); convert to cubic meters by dividing by 35.3147.
Slab / Footing: V = L × W × T
Column: V = π × r² × h (r = diameter / 2)
Wall: V = L × H × T
Bags = ⌈ V_ft³ × (1 + waste%) / yield_per_bag ⌉
Where:
- V — pour volume (ft³, then converted to yd³ and m³)
- L, W, T, D, H — length, width, thickness, depth, height (any matching unit)
- r — column radius (always diameter / 2 for Sonotube pours)
- waste% — buffer percentage (10% contractor default)
- yield_per_bag — 0.30 ft³ (40 lb), 0.45 ft³ (60 lb), 0.60 ft³ (80 lb)
Bag Yield Reference
| Bag Size | Yield (ft³) | Yield (yd³) | Bags per yd³ |
|---|---|---|---|
| 40 lb premix | 0.30 | 0.0111 | 90 |
| 60 lb premix | 0.45 | 0.0167 | 60 |
| 80 lb premix | 0.60 | 0.0222 | 45 |
Yields are nominal — actual yield varies slightly with mix water and aggregate. Round bag counts up: partial bags can't be returned, and a short-pour means stopping mid-job.
Related Calculators
- Cylinder Calculator — the geometry behind Sonotube column pours — solve for radius, height, or volume
- Rectangular Prism Calculator — the underlying L × W × H math for slabs, footings, and walls
- Density Calculator — convert volume to weight using concrete density (≈ 150 lb/ft³)
- Volume Converter — switch between cubic yards, cubic feet, cubic meters, liters, and gallons
- Length Converter — convert inches, feet, meters, and yards before entering dimensions
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