How to measure for concrete
Concrete is ordered by volume, so the job is to find the cubic footage of the form you are filling, convert it to cubic yards, and then decide whether to pour ready-mix or mix premix bags. Pick the shape above, enter the dimensions, and the calculator returns cubic yards, the number of bags for your chosen bag size, and an estimated cost.
Measuring a slab or footing
For a rectangular slab, patio, or footing, measure the length and width in feet and the thickness in inches. A typical slab is about 4 inches thick; footings usually run 8–12 inches. For an L-shaped or irregular pour, split it into rectangles, calculate each one, and add the results.
Measuring a round column
For a round column, pier, or tube form, measure the inside diameter in inches and the height in feet. The calculator converts the diameter to a radius and uses the area of a circle, so you do not have to do the geometry yourself.
How the math works
Everything starts from volume in cubic feet, which is then divided by 27 (the number of cubic feet in a cubic yard):
- Slab volume (ft³) = length × width × (thickness ÷ 12)
- Column volume (ft³) = π × (diameter ÷ 12 ÷ 2)² × height
- Cubic yards = volume ÷ 27
Bag yields
Premix bags are labeled by weight, but what matters for counting is the cubic-foot yield of each bag. The calculator uses these standard yields and always rounds the bag count up to a whole bag:
- 80 lb bag ≈ 0.60 ft³
- 60 lb bag ≈ 0.45 ft³
- 40 lb bag ≈ 0.30 ft³
A full cubic yard is 27 cubic feet, which is about 45 of the 80 lb bags — a lot of mixing. Past roughly half a yard, ready-mix delivery is usually faster and cheaper.
Common mistakes to avoid
- Forgetting to convert thickness. Thickness and diameter are entered in inches while length and height are in feet — the tool divides by 12 for you, but keep your tape readings consistent.
- Skipping the waste factor. Add about 10% for spillage, over-excavation, and uneven subgrade so you are not short a few bags mid-pour. The calculator builds this in.
- Counting bags and ready-mix together. The bag count and the ready-mix yardage are two ways to fill the same volume — order one or the other, never both.
Estimating cost
The cost shown is for ready-mix concrete priced by the cubic yard, because that is how most slabs and footings are actually poured. Bags are listed as a quantity for small jobs, but they are not added to the cost so the ready-mix and bag options are not double-counted. Short-load fees, delivery, pump rental, and finishing labor vary widely by region — use the regional adjustment and edit the unit price to match a local quote. The figure is an estimate of material cost only.