# How to form concrete steps?

Forms for a concrete slab carry real hydraulic pressure while the concrete is fluid, so stake and brace them for the full head. Strip flatwork forms once the concrete supports its own edge, commonly 24 to 48 hours; vertical forms hold longer. Release agent goes on before placement, never after.

## Short version

- Length × width × thickness, all in feet, divided by 27 gives cubic yards.
- Convert inches to feet first: 4 inches is 0.333 ft, 5 inches is 0.417 ft, 6 inches is 0.5 ft.
- One cubic yard covers 81 ft² at 4″, 65 ft² at 5″, 54 ft² at 6″, and 41 ft² at 8″.
- Add about 10% on top — that is an estimating assumption, not a published standard.
- Split odd shapes into rectangles, calculate each separately, and add the volumes.

## How do you convert slab dimensions into cubic yards?

Concrete volume is calculated in feet and then converted, because a cubic yard is 27 cubic feet. Take the length in feet, multiply by the width in feet, multiply by the thickness in feet, and divide the result by 27.

The step that trips people up is thickness, which is almost always given in inches. Divide the inches by 12 first. A worked example: a 24 ft by 24 ft garage floor at 5 inches thick is 24 × 24 × 0.417 = 240.2 ft³, which divided by 27 is 8.9 cubic yards before waste, or about 9.8 yards with a 10% allowance.

## How many bags of concrete equal a cubic yard?

Bag counts come from the yield printed on the bag, and they are unforgiving at volume. An 80 lb bag yields about 0.60 ft³, so a cubic yard takes roughly 45 bags. A 60 lb bag yields about 0.45 ft³, or about 60 bags per yard. A 50 lb bag is about 0.375 ft³ and a 40 lb bag about 0.30 ft³, which works out to roughly 72 and 90 bags per yard respectively.

Those yields are manufacturer figures from Quikrete and Sakrete technical data sheets, and formulations vary — check the bag you are actually buying. Above about half a cubic yard, ready-mix is almost always cheaper and far less work.

## How much extra concrete should you order?

Ten percent is the working allowance most crews use, and it exists for three specific reasons: forms flex outward under the weight of wet concrete, subgrade is never perfectly level so the real thickness exceeds the nominal, and some material is always lost to spillage and the chute.

That figure is an estimating convention, not a published standard. The asymmetry is what matters — ordering a little extra costs a few dollars, while running short mid-pour forces a cold joint into the slab, which is a permanent plane of weakness you cannot repair out.

## Common mistakes

- **Forgetting to convert thickness from inches to feet** — the answer comes out 12× too large and the order is unusable
- **Ordering the exact calculated volume** — forms flex and subgrade is uneven; running short mid-pour creates a cold joint
- **Folding footings and thickened edges into the slab number** — they are separate volumes and are routinely underestimated