Concrete Calculator

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Concrete to order —

Cubic metres: —
Cubic feet: —
Cubic yards: —
80 lb premix bags: —
60 lb premix bags: —
Approx. wet weight (kg): —

Multiply length by width by thickness, all in feet, and divide by 27 to get cubic yards. A 10 × 10 ft slab 4 inches thick is 10 × 10 × 0.333 = 33.3 ft³, or 1.23 yd³. That is about 56 bags of 80 lb premix, before you add 5–10% for waste and uneven ground.

About this tool

Ordering concrete is a one-shot job: short by a quarter yard and a pour stalls with a cold joint, over by a yard and you pay for a truck to take it away. This calculator gives the volume of a rectangular slab or footing, or of round columns and sonotube piers, in US or metric units. Set how many identical pours you have and a waste allowance, and it returns the total in cubic yards, cubic feet and cubic metres, the number of 60 lb and 80 lb premix bags, and an approximate wet weight at 2,400 kg/m³. It assumes flat, uniform dimensions; sloped sites, thickened slab edges and stepped footings need to be split into simpler pieces and added up.

How to use it

Pick the shape and units
Choose a slab or footing for rectangular pours, or a column for tubes and piers, each in US or metric units.
Enter the dimensions
Slab thickness goes in inches or centimetres; length, width and height in feet or metres.
Set count and waste
Use the stepper for repeated identical pours and the slider for a waste allowance, typically 5–10%.
Read the order
The headline is the volume to order; the stats list bag counts and the working shows each conversion.

Examples

10 × 10 ft patio, 4 in thick

Length: 10
Width: 10
Thickness: 4
Number of identical pours: 1
Waste allowance: 0
Shape and units: Slab, US (ft, in)

Result Concrete to order: 1.23 yd³
Cubic metres: 0.944
Cubic feet: 33.33
Cubic yards: 1.23
80 lb premix bags: 56
60 lb premix bags: 75
Approx. wet weight (kg): 2,265

One slab = 3.048 m × 3.048 m × 0.102 m = 0.9439 m³
× 1 piece(s) = 0.9439 m³; + 0% waste = 0.9439 m³
0.9439 m³ = 33.33 ft³ = 33.33 ÷ 27 = 1.23 yd³
80 lb bags = 33.33 ÷ 0.60 → 56; 60 lb bags = 33.33 ÷ 0.45 → 75

A typical 100 sq ft patio slab needs a little over 1.2 cubic yards before any waste allowance.

Four 30 cm columns, 3 m tall

Diameter: 30
Height: 3
Number of identical pours: 4
Waste allowance: 10
Shape and units: Column, metric (m, cm)

Result Concrete to order: 0.93 m³
Cubic metres: 0.933
Cubic feet: 32.95
Cubic yards: 1.22
80 lb premix bags: 55
60 lb premix bags: 74
Approx. wet weight (kg): 2,239

One column = π × (0.3 m ÷ 2)² × 3 m = 0.2121 m³
× 4 piece(s) = 0.8482 m³; + 10% waste = 0.9331 m³
0.9331 m³ = 32.95 ft³ = 32.95 ÷ 27 = 1.22 yd³
80 lb bags = 32.95 ÷ 0.60 → 55; 60 lb bags = 32.95 ÷ 0.45 → 74

Each column holds π × 0.15² × 3 = 0.212 m³; four of them plus 10% waste come to about 0.93 m³.

How it is calculated

V = L × W × T (slab) or V = π × (D ÷ 2)² × H (column); total = V × N × (1 + waste ÷ 100); yd³ = ft³ ÷ 27

L, W, T: Slab length, width and thickness
D, H: Column diameter and height
N: Number of identical pours
waste: Extra allowance in percent

All dimensions are converted to metres first (1 ft = 0.3048 m, 1 in = 0.0254 m, both exact), the volume is computed, multiplied by the count and the waste factor, then converted back to cubic feet and cubic yards (27 ft³ per yd³). Bag counts use the manufacturer yields of 0.60 ft³ for an 80 lb bag and 0.45 ft³ for a 60 lb bag, rounded up to whole bags. Weight assumes normal-weight concrete at about 2,400 kg/m³ (150 lb/ft³).

When not to use it

Do not use it for reinforced design or load capacity; it measures volume only.
Sloped or irregular excavations need to be broken into pieces before using it.
Concrete block walls are counted in blocks, not poured volume.

Common mistakes

Entering slab thickness in feet instead of inches gives a volume twelve times too large.
Ordering the exact net volume leaves no margin for spillage or a soft subgrade.
Forgetting that ready-mix trucks often charge a short-load fee under a minimum order.

Frequently asked questions

How many cubic yards are in a 10 × 10 slab?

At 4 inches thick, 10 × 10 × 0.333 ft = 33.3 ft³, which divided by 27 is 1.23 cubic yards. At 6 inches it rises to 50 ft³, or 1.85 cubic yards.

How many 80 lb bags make a cubic yard?

An 80 lb bag of premix yields about 0.60 ft³. A cubic yard is 27 ft³, so it takes 27 ÷ 0.60 = 45 bags. With 60 lb bags at 0.45 ft³ each, it takes 60.

How much waste allowance is normal?

Most contractors add 5 to 10 percent for spillage, form bulge and uneven subgrade. Use the higher end on excavated ground or when the base has not been compacted and screeded flat.

When is ready-mix better than bags?

Past roughly one cubic yard, mixing bags by hand becomes slow and the first batches start to set before the last are placed. Many builders switch to a ready-mix truck or a trailer load at around 1 to 2 yards.

How heavy is wet concrete?

Normal-weight concrete is about 2,400 kg per cubic metre, or roughly 4,000 lb per cubic yard. That matters for trailer loads, wheelbarrow trips and temporary formwork.

How do I measure a sonotube pier?

Choose Column, US, enter the inside diameter of the tube and the depth of the hole plus any height above grade. A 12 inch tube 4 ft long holds about 3.14 ft³.