Enrichment · Passion Projects · Building

How many bags
for the patio
and the fence posts.

List each slab, footing and set of post holes. The calculator gives the concrete in cubic feet and yards, the number of bags and what to order from a ready-mix supplier.

Measure the pourWhat you get

The concrete calculator

The worked example is a made-up 12 × 10 ft patio, four fence posts and a step footing. Nothing you type leaves this browser.

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What you get

Enrichment · Passion ProjectsFree

What you finish with
The volume of each part and the whole pour with waste, in cubic feet and yards, the number of premix bags and the ready-mix order. It prints as a summary and downloads as a spreadsheet.
Who it is for
Families planning home projects, construction and trades students, and geometry classes working with volume.
Format
Browser calculator for a phone, tablet or computer, by touch or keyboard. Up to 30 parts. Prints a summary; downloads a spreadsheet (.csv); export and import a file.
Access
Free. No account, no sign-in and nothing to install. Everything you type stays in this browser; nothing is sent to Caveat.
Where it lives
The Passion Projects space in Caveat Enrichment.
Support
Questions, a correction or a request: team@caveatacademic.com.

How to use it

  1. Choose a bag size and a waste allowance.
  2. Add each slab or footing with its length, width and depth.
  3. Add each set of identical post holes with the count, diameter and depth.
  4. Read the cubic yards and bags.
  5. Print the summary for the store or the supplier.

Measure the pour

How the volume is worked out

Prisms and cylinders, in feet.

The worked example: patio and fence postsPatio40.00 cu ft12 × 10 × 4 ÷ 124 post holes6.54 cu ft4 × π × (10 ÷ 24)² × 3+ footing, + 10%55.60 cu ft50.54 × 1.10÷ 272.06 yd³93 bags of 80 lbThe worked example: patio and fence postsPatio40.00 cu ft12 × 10 × 4 ÷ 124 post holes6.54 cu ft4 × π × (10 ÷ 24)² × 3+ footing, + 10%55.60 cu ft50.54 × 1.10÷ 272.06 yd³93 bags of 80 lb
Depth is the number people forget to convert: 4 inches is a third of a foot, which is why slab volumes are smaller than they look.

The arithmetic

A slab or footing is a box: length × width × depth, with depth in inches divided by 12 to make feet. A round hole is a cylinder: π × radius² × depth, with the diameter in inches divided by 24 to give the radius in feet. Parts add up, the waste allowance is added, and 27 cubic feet make a cubic yard. Bags round up to whole bags; truck orders to the next quarter yard.

Before you pour

Bag yields vary by product; read the label. Patio and walk slabs usually sit on 4 inches of compacted gravel and need control joints. Call 811 a few days before digging so buried utility lines are marked. Concrete is caustic while wet: wear gloves, long sleeves and eye protection.

Sources

  1. Portland Cement Association. Concrete basics: estimating volume and ordering ready-mixed concrete (cement.org).
  2. National Institute of Standards and Technology. Handbook 44, appendix C: 27 cubic feet per cubic yard (nist.gov).
  3. Common Ground Alliance. 811, call before you dig (call811.com).
  4. Occupational Safety and Health Administration. Concrete and masonry safety, and wet cement skin hazards (osha.gov).

The worked example is made up. Check the yield on the bags you buy and local building rules. Your figures stay in this browser. Support: team@caveatacademic.com.