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Concrete & Aggregate

Concrete Calculator

Concrete is the one material you cannot go back for. The truck leaves, the pour goes off, and a cold joint is permanent.

Concrete needed
2.72 yd³
Cost
$500 – $800

Measure these

For a round column this is the diameter.

4 in residential slab, 5–6 in driveway, 6 in+ for heavy vehicles.

Sensible defaults are already applied — open only if you need to change them.

4″20′10′VOLUME · LENGTH × WIDTH × DEPTH
Concrete needed
2.72 yd³
2.47 yd³ measured + 10% · 66.7 ft³
Material cost
$500 – $800
Bags of 80 lb
123

each yields 0.6 ft³ · 2.73 yd³ equivalent

Order this much
2.75 yd³

Plants sell in quarter-yard increments

Placed weight
5.5 tons

≈ 4,050 lb per yd³

Bagged cost
$800

123 bags — compare against the truck price below

Ready-mix $140–200/yd³ plus short-load fee, or bags at retail. The lower of the two is shown as the floor.

At 2.72 yd³ you are a short load. Plants charge a surcharge under 8–10 yd³ — usually $100–200 — so budget for it and ask what their minimum is before you book.

123 bags is roughly 2.7 yards mixed by hand. Past about half a yard a truck is cheaper and vastly faster — and hand-mixing a slab in batches risks a cold joint through the middle of it.

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How much concrete do I need?

Multiply length × width × thickness in feet, then divide by 27 to get cubic yards. A 20 ft × 10 ft slab at 4 inches is 20 × 10 × 0.333 = 66.7 cubic feet = 2.47 cubic yards. Add 10% and you order 2.75 yards.

One cubic yard covers 81 square feet at 4 inches, 65 square feet at 5 inches, and 54 square feet at 6 inches. That is the number worth memorising, because it converts a slab area into a truck order in one step.

Bags are a different unit again. An 80 lb bag yields 0.60 cubic feet, so **45 bags make one cubic yard**. Sixty-pound bags yield 0.45 cubic feet and take 60 to the yard. Once you are past roughly 40 bags, ready-mix is both cheaper and a completely different working day.

How the calculation works

Cubic yards = length × width × (thickness ÷ 12) ÷ 27. For a round column or post hole, area is π × (diameter ÷ 2)², so a 12-inch hole 3 feet deep is 3.14 × 0.25 = 0.785 sq ft × 3 ft = 2.36 cubic feet.

Bag counts come from the manufacturer yield, not the bag weight: 80 lb → 0.60 ft³, 60 lb → 0.45 ft³, 50 lb → 0.375 ft³, 40 lb → 0.30 ft³. Dividing the bag weight by a density is the common mistake and it under-counts, because roughly 6 pints of water per 80 lb bag adds volume.

Placed concrete weighs about 4,050 lb per cubic yard — just over two tons. A single yard is more than most half-ton pickups should carry, and it is why a wheelbarrow route matters more than people expect.

The overage is not padding. Subgrade is never perfectly flat, forms bow outward under the head of wet concrete, and the last barrow is always the one you are short. Ten percent is standard and cheap; being 0.2 yards short is a cold joint.

Typical concrete cost, mid-2026

MaterialUnitTypical range
Ready-mix deliveredper yd³$140 – $200
Short-load surchargeApplies under about 8–10 yd³per load$100 – $200
80 lb bag45 bags = 1 yd³each$5.00 – $8.00
60 lb bag60 bags = 1 yd³each$4.00 – $6.50
Concrete pumpper day$700 – $1,600
Fibre reinforcementper yd³$8.00 – $14.00
Placing and finishingper sq ft$2.00 – $6.00
Saturday / after-hours deliveryper load$75.00 – $200

The crossover is around half a cubic yard. Below it, bags win. Above it, the truck wins on price and wins outright on quality, because one continuous pour has no cold joints.

What goes wrong on this job

  1. Order 10% over and have somewhere to put the extra — a spare form, a splash pad, anything. Sending concrete back costs nothing; being short costs you the slab.
  2. Book the truck for a time, then be ready an hour early. Most plants allow about 5–7 minutes per yard on site before waiting time is charged, and a 10-yard load is not long to place.
  3. Never add water on site to make it easier to work. Every extra gallon per yard drops the strength measurably and drives shrinkage cracking. Ask the plant for a higher slump or a plasticiser instead.
  4. Cut or tool control joints within 6–12 hours, at 24–30 times the slab thickness — that is roughly every 8–10 feet on a 4-inch slab. Concrete cracks; joints are how you choose where.
  5. Keep it wet for a week. Concrete cures by hydration, not by drying, and slabs left to dry in the sun lose a serious fraction of their strength in the top layer that everything actually walks on.

Frequently asked questions

How many bags of concrete are in a cubic yard?

45 bags of 80 lb, 60 bags of 60 lb, 72 bags of 50 lb, or 90 bags of 40 lb. The yields are 0.60, 0.45, 0.375 and 0.30 cubic feet respectively, and 27 cubic feet make a yard.

How many square feet does a yard of concrete cover?

81 square feet at 4 inches thick, 65 at 5 inches, and 54 at 6 inches. Divide 324 by the thickness in inches for any depth.

How much does a yard of concrete cost?

$140–200 per cubic yard delivered in mid-2026, before a short-load surcharge of $100–200 on anything under about 8–10 yards. A single-yard delivery often lands near $300 once the fee is added.

How much does a yard of concrete weigh?

About 4,050 lb, or just over two tons. That is 150 lb per cubic foot for normal-weight concrete.

Is it cheaper to mix your own concrete or buy ready-mix?

Below about half a cubic yard, bags are cheaper. Above it, ready-mix wins on price and decisively on quality — hand-mixing in batches risks cold joints, and 45 bags per yard is a genuinely hard day.