Concrete Calculator
Slabs, footings, columns, stairs and post holes — worked out in cubic yards, bags and truck loads, with waste allowance and a cost estimate.
What are you pouring?
Measurements
Order this much
Or mix it yourself
Bag counts already include the waste allowance and are rounded up — you cannot buy part of a bag. Anything over about forty bags is usually cheaper and far easier as ready-mix.
Delivery
Reinforcement and base, at a glance
| Item | Quantity | Note |
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How to use this calculator
Step by step
- Pick the kind of pour at the top. The measurement boxes change to exactly what that job needs.
- Choose feet-and-inches or metres. In imperial, thicknesses and small dimensions are in inches and lengths are in feet — each box says which.
- If you are pouring several identical footings, columns or holes, set the count rather than adding them up by hand.
- Leave the waste allowance at 10% unless the sub-grade is rough, in which case raise it. This is the single most common reason a pour comes up short.
- Read the big number as your order quantity. It is already rounded guidance — ready-mix is sold in quarter-yard steps.
- If the bag counts look manageable — under about forty — mixing on site may be cheaper than a short-load delivery. The delivery panel says which side of that line you are on.
Good to know
- Always order slightly more than you calculate. Coming up short mid-pour creates a cold joint, which is a permanent weak line through the slab.
- One cubic yard is 27 cubic feet, and covers 81 square feet at 4 inches thick. That single figure sizes most residential slabs in your head.
- Concrete weighs roughly 150 lb per cubic foot, or 2400 kg per cubic metre. A single cubic yard is about two tonnes — check what your access can carry.
- You have around 90 minutes from batching to placement before the mix starts to stiffen. Have the crew, tools and forms ready before the truck arrives.
- Post holes should reach below the local frost line. The calculator subtracts the post itself from the fill, which is why the number is smaller than the raw hole volume.
- A 4-inch slab wants a 4-inch compacted gravel base beneath it. The reinforcement table works that quantity out alongside the concrete.
The maths behind it
- Slab volume V = length × width × thickness Convert the thickness from inches to feet by dividing by 12.
- Cubic yards yd³ = ft³ ÷ 27 The unit ready-mix is sold in.
- Round column V = π r² h Radius is half the tube diameter.
- Post hole fill V = πr²h − (post area × h) The post displaces concrete it does not need.
- Steps V = Σ tread × (rise × i) × width Each step sits on the stack below it.
- With waste Order = V × (1 + waste%) 10% is standard, 15% on uneven ground.
- Weight ≈ 150 lb/ft³ ≈ 2400 kg/m³ For access, formwork and shoring decisions.
How much waste should I allow?
Ten percent for a formed pour on a prepared base. Fifteen for footings dug into uneven ground, where the trench walls slump and the actual excavation is wider than the drawing. Below about a cubic yard, the fixed losses — what stays in the chute and the barrow — matter more than the percentage, so round generously.
Bags or ready-mix?
Roughly, under a cubic yard bags win; over three cubic yards ready-mix wins comfortably; in between it depends on the short-load fee your supplier charges and how many hands you have. Mixing forty 80-lb bags by hand is a genuinely hard day's work, and the result is less consistent than a truck.
How thick should my slab be?
Four inches for a patio, shed base or footpath. Five to six inches for a driveway, and six with rebar if trucks or an RV will park on it. Thickness matters far less than the base beneath it — a well-compacted gravel base under four inches outlasts a poor base under six.
Does the calculator account for rebar taking up space?
No, and it should not. Steel displaces well under half a percent of the volume in normal residential reinforcement, which vanishes inside the waste allowance. The post-hole calculation is the one exception, because a 4×4 post displaces a genuinely significant share of the hole.
What does "short load" mean?
Most suppliers charge a surcharge for deliveries under about three cubic yards, because the truck and driver cost the same whether it is full or not. The delivery panel flags when your order falls in that range, so you can weigh it against bags before you phone.