How this calculator works
This page turns the dimensions of a pour into the quantity you order. It works out the volume of the shape from its measurements, adds your overage allowance, and then expresses the result three ways: cubic yards for a ready-mix order, cubic feet for the arithmetic, and a bag count for each of the four common bag sizes.
Volume is the part that is genuinely exact. A rectangular slab is length times width times thickness, and no judgement is involved. Weight and bag counts are different: both depend on the specific mix and the specific product, so they are shown as a range or as a per-bag-size count rather than one confident number.
What this page does not do is decide how thick your slab should be, or whether it needs reinforcement. Those are design questions that depend on loads, ground conditions and local requirements. Bring the dimensions from your drawings, or from whoever specified the job.
The formula
For a rectangular slab, every calculator on the internet is doing the same four steps. They are worth seeing, because they let you sanity-check any result you get anywhere.
- Convert the thickness to feet
thickness in inches ÷ 12 - Volume in cubic feet
length (ft) × width (ft) × thickness (ft) - Volume in cubic yards
cubic feet ÷ 27 - Add the overage allowance
cubic yards × (1 + overage ÷ 100)
Cubic yards per 100 square feet, by thickness
Pure arithmetic, no material assumptions involved. Useful for a quick check, or for scaling a number you already have.
| Thickness | Cubic feet per 100 ft² | Cubic yards per 100 ft² |
|---|---|---|
| 2 in | 16.67 | 0.62 |
| 3 in | 25.00 | 0.93 |
| 4 in | 33.33 | 1.23 |
| 5 in | 41.67 | 1.54 |
| 6 in | 50.00 | 1.85 |
| 8 in | 66.67 | 2.47 |
Figures exclude any overage allowance. Multiply by 1.1 for a 10% allowance.
Frequently asked questions
Should I order ready-mix or use bags?
The honest dividing line is how much you need and how fast you can place it. Bags stop making sense somewhere around a cubic yard, because a cubic yard is roughly forty-five 80 lb bags. That is a lot of mixing before the first batch starts going off. Below that, bags avoid delivery minimums and short-load fees. Get a quote both ways for anything near the boundary.
Why does the calculator add overage by default?
Because the failure modes are not symmetrical. Slightly too much concrete costs you the price of the excess. Slightly too little means a cold joint in the middle of your slab, or a second delivery charge, or a frantic trip for bags while the first pour stiffens. The default allowance is there to make the cheap mistake instead of the expensive one.
Do I measure the hole or the finished slab?
The finished slab. If your excavation is deeper than the slab because there is a compacted sub-base underneath, the concrete only fills the top part. Measure what the concrete actually occupies. If the subgrade is uneven, that is what the overage allowance is for.
Why is the weight shown as a range?
Concrete density varies with the mix design, the aggregate and the air content. Quoting one figure to four digits would imply a precision that does not exist. The range covers normal-weight mixes; if you need the actual figure, your supplier can give you the density for the mix they are batching.
Variables and units
For the default “Square or rectangular slab” mode. Other modes label their own fields in the calculator.
length- Length: ft, in, yd, cm, m.
width- Width: ft, in, yd, cm, m.
thickness- Thickness: in, ft, yd, cm, m.
quantity- How many of these: unitless.
waste- Overage allowance: %; optional.
price- Price per cubic yard: per yd³; optional.
Worked example: a 12 ft by 10 ft shed base
You are pouring a rectangular base for a garden shed, 12 ft long by 10 ft wide, at the common 4 in thickness, and you want the usual 10% overage so you are not caught short halfway through.
| Field | Value |
|---|---|
| Length | 12 ft |
| Width | 10 ft |
| Thickness | 4 in |
| How many of these | 1 |
| Overage allowance | 10 % |
| Price per cubic yard | 0 per yd³ |
| Result | Value |
|---|---|
| Concrete to order | 1.63 yd³ |
| Same volume in cubic feet | 44 ft³ |
| 80 lb bags | 74 bags |
| Approximate weight | 6,160–6,600 lb |
These figures are generated by the same calculation the calculator above uses, so they update automatically if the method changes.
Where these numbers come from
Draft: not yet published or indexed
| Assumption | Value used | Status |
|---|---|---|
| Normal-weight concrete density.Fresh normal-weight concrete varies with mix design, aggregate and air content. Confirm against your supplier’s mix data before relying on the weight figure. Applicability: Launch-market starting value only. A future market must supply reviewed regional or product-specific provenance before overriding it. You can replace this with your own figure. | 140–150 lb/ft³ | Not yet verified |
| Default concrete overage allowance.Allowance for spillage, uneven subgrade and form deflection. Short-ordering concrete costs far more than over-ordering slightly. Applicability: Launch-market starting value only. A future market must supply reviewed regional or product-specific provenance before overriding it. You can replace this with your own figure. | 10% | Not yet verified |
| Yield of a 40 lb bag of concrete mix.Bag yield differs between products. Read the yield printed on the bag you are actually buying and enter it here. Applicability: Launch-market starting value only. A future market must supply reviewed regional or product-specific provenance before overriding it. You can replace this with your own figure. | 0.3 ft³ | Not yet verified |
| Yield of a 50 lb bag of concrete mix.Bag yield differs between products. Read the yield printed on the bag you are actually buying and enter it here. Applicability: Launch-market starting value only. A future market must supply reviewed regional or product-specific provenance before overriding it. You can replace this with your own figure. | 0.375 ft³ | Not yet verified |
| Yield of a 60 lb bag of concrete mix.Bag yield differs between products. Read the yield printed on the bag you are actually buying and enter it here. Applicability: Launch-market starting value only. A future market must supply reviewed regional or product-specific provenance before overriding it. You can replace this with your own figure. | 0.45 ft³ | Not yet verified |
| Yield of an 80 lb bag of concrete mix.Bag yield differs between products. Read the yield printed on the bag you are actually buying and enter it here. Applicability: Launch-market starting value only. A future market must supply reviewed regional or product-specific provenance before overriding it. You can replace this with your own figure. | 0.6 ft³ | Not yet verified |
This figure is a working placeholder. It has not yet been checked against the source listed, so treat it as indicative and confirm with your supplier.
Sources
- ASTM C138/C138M, Standard Test Method for Density (Unit Weight), Yield, and Air Content of Concrete: ASTM International Not yet checked
- ASTM C94/C94M, Standard Specification for Ready-Mixed Concrete: ASTM International Not yet checked
- Manufacturer published bag yield (product data sheets): Individual bagged-concrete manufacturers Not yet checked
Published by Material Totals. Review the calculation methodology and source registry.
Suggested citation: Material Totals, “Concrete Calculator,” https://materialtotals.com/concrete-calculator/
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