スラブ、フーチング、柱、階段、ポスト穴に必要なコンクリート容積（yd³／m³）を計算。40lb／60lb／80lb／25kg入り配合袋の必要数も表示。10%の余裕を標準で加算。
| 袋のサイズ | 必要な袋数 |
|---|
スラブ / フーチング: 体積 = 長さ × 幅 × 深さ。 円柱 / 柱穴: 体積 = π × 半径² × 高さ。 階段: 段を積み上げたブロックとして近似計算します — i段目は 蹴上げ×(i×踏み面)×幅 で、全段を合計します。
袋あたりの体積: 40lb → 0.011yd³（0.30ft³）、60lb → 0.017yd³（0.45ft³）、80lb → 0.022yd³（0.60ft³）、25kg → 0.0125m³、30kg → 0.015m³。
10%の余裕率は、下地の凹凸やこぼれ、多少多めに発注することを見込んだもので、住宅用打設のベストプラクティスです。
Three geometries share one output. Slab / footing is length × width × depth. Column / post hole is π × radius² × height, multiplied by your quantity. Stairs are summed as cumulative blocks: step i contributes rise × (i × run) × width, added across every step, which is how a poured monolithic flight actually stacks. Everything is computed in cubic feet, multiplied by 1.10 while the waste toggle is on, then reported as cubic yards (÷ 27), cubic metres (× 0.0283168) and cubic feet.
Watch the unit pairing: in imperial mode the large dimensions are feet while depth, diameter, rise, run and stair width are inches; in metric mode they become metres and centimetres. The default slab is 10 × 10 ft at 4 in deep. The bag table converts your volume against fixed yields — 40 lb at 0.30 ft³, 60 lb at 0.45 ft³, 80 lb at 0.60 ft³, plus 25 kg and 30 kg European sizes — and always rounds up to whole bags.
The 10 percent waste factor is a residential rule of thumb covering spillage and an uneven sub-grade; it is not a substitute for measuring an over-dug footing trench, which can swallow far more. Rebar, mesh and embedded post volume are not deducted, and stair side walls, landings and open risers are not modelled at all. For anything you are ordering ready-mix against, treat the figure as a shopping estimate and confirm with your supplier before the truck is booked.
About 46. The tool uses a yield of 0.022 cubic yards, or 0.60 cubic feet, per 80 lb bag, so a full yard takes 45.5 bags and the table rounds that up. Sixty pound bags need roughly 60, and 40 lb bags about 91.
For most residential pours, yes. It is on by default and multiplies the raw volume by 1.10 to cover spillage, an uneven base and slight over-ordering. Switch it off only when you want the exact geometric volume, for example when checking a supplier's own quote.
Choose the Column / post hole shape. Enter the hole diameter in inches, the depth in feet, and set Quantity to the number of holes. The tool applies π × radius² × height per hole and multiplies through, so all your posts come out in one figure.
No. Stairs are summed purely as stacked rectangular blocks, step by step. Cheek walls, a top landing, a thickened base and any hollow or backfilled void underneath are not included, so a real flight with side walls will need meaningfully more concrete.
Because that is how slabs are specified in practice: a 10 by 10 foot pad at 4 inches deep. The tool converts internally, dividing inch values by 12 and centimetre values by 30.48, so you never have to enter a fractional foot.
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