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Redmoon Converters
🔨 Construction & DIY

Concrete Volume & Bags Calculator

Calculate concrete volume (yd³ / m³) for slabs, footings, columns, stairs and post holes, plus how many 40lb / 60lb / 80lb / 25kg bags of mix to buy. 10% waste built in.

Built and maintained by Paul Clark, Redmoon Software · Checked against what this tool computes on

ft
ft
in
Concrete needed
0.00 yd³
0.00 m³ · 0.0 ft³
Bag sizeBags needed
Quick reference

Slab / footing: volume = length × width × depth. Column / post hole: volume = π × radius² × height. Stairs: approximated as cumulative blocks — step i has rise×(i×run)×width, summed across all steps.

Bag yields: 40 lb → 0.011 yd³ (0.30 ft³), 60 lb → 0.017 yd³ (0.45 ft³), 80 lb → 0.022 yd³ (0.60 ft³), 25 kg → 0.0125 m³, 30 kg → 0.015 m³.

The 10% waste factor accounts for uneven sub-grade, spillage and slightly over-ordering — best practice for residential pours.

How it works

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.

Frequently asked questions

How many 80 lb bags of concrete make a cubic yard?

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.

Should I leave the +10% waste toggle on?

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.

How do I calculate concrete for a round post hole?

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.

Does the stairs mode include the side walls or a landing?

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.

Why does the depth field use inches when length uses feet?

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.

Volume is easy; ordering is where it goes wrong

Slab volume is length × width × thickness, and the arithmetic is never the problem. What causes short pours is that concrete is ordered in fixed increments and cannot be topped up gracefully — a slab that runs out two barrows short sets with a cold joint through it.

Sub-base irregularity is the usual culprit. A nominal 100 mm slab poured over ground that dips 20 mm in places uses noticeably more than the calculation, because the extra depth is spread across the whole area rather than confined to the dips.

Bag yields, waste and the ready-mix threshold

Bag yields quoted on the packaging are for the mixed volume, not the dry contents — the water becomes part of the finished volume. Working from dry weight rather than stated yield undercounts substantially.

A waste allowance of 5–10% is normal and is not padding: it covers spillage, over-excavation, what stays in the mixer and what stiffens before it is placed.

Somewhere around a cubic metre, hand-mixing bags stops being sensible. Beyond that the labour, the consistency between batches and the working time all argue for ready-mix — and ready-mix arrives with its own constraint, since the truck will not wait indefinitely and the pour has to be organised before it turns up.

Where these numbers come from

The standard or formula behind this tool, so you can check it rather than take it on trust.

  • Bag yields are for the MIXED volume, not the dry contents.

    Order for the void plus a margin. Concrete is the least forgiving material to under-order because a partial pour cannot easily be topped up later.

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