Concrete Bag Calculator by Project Type

Turning a Shape You Can Measure Into a Bag Count

Most bag-estimating goes wrong before a single division happens. You are standing at a form, a trench, or a row of augered holes with a tape measure and a set of dimensions in mixed units — a slab quoted in feet with a thickness quoted in inches, a hole diameter in inches with a depth in feet — and the volume has to come out of that before the bag count means anything. This calculator is built for that moment. It takes the physical shape as its input, derives the volume itself, and only then converts to bags. If you already hold a cubic-yard figure, you are one step further along and want the 80 lb bags-per-yard calculator instead — that split is explained in full below. For the wider bag-counting reference, our pillar guide on calculating bags of concrete collects the methods and project charts in one place.

Five geometry modes cover almost every job small enough to bag: slab and footing are rectangular prisms (length × width × thickness), post holes and sonotubes are cylinders repeated a set number of times, and custom volume is the escape hatch when the shape does not fit either. Every dimension is entered in a single linear unit — feet, inches, meters, centimeters, or millimeters — converted internally, multiplied, and then divided by the yield of the bag size you picked. The result is the exact count plus ready-to-order +5% and +10% buffers, with the same volume shown in yd³, ft³ and m³ so you can price the bagged route against a truck without re-deriving anything. Because the yields are approximate compacted figures, no output should ever be ordered without a waste allowance — 66.7 bags on paper is 70 bags in the cart.

The Five Shape Modes and What Each One Assumes

How Each Shape Becomes a Volume

• 1) Rectangular shapes — slab and footingVolume = Length × Width × Thickness
  All three dimensions are converted to a common base unit before multiplying, which is why they must be entered in the same linear unit. The thickness term is the sensitive one: doubling a slab from 4 in to 8 in doubles the bag count, while adding a foot to the length of a 10 ft-wide pad adds only about 5.6 bags.
• 2) Cylindrical shapes — post holes and sonotubesVolume = π × (Diameter ÷ 2)² × Depth × Count
  Enter the diameter; the calculator halves it. Because the radius is squared, a measurement error here costs roughly twice what the same proportional error costs on a slab dimension — a hole 10% wider than entered holds about 21% more concrete.
• 3) Bags from volumeBags = Volume ÷ Yield per Bag, rounded up
  Approximate yields, held in the site's shared engine constants: 40 lb → 0.30 ft³; 50 lb → 0.375 ft³; 60 lb → 0.45 ft³; 80 lb → 0.60 ft³. The hero figure rounds up because a fraction of a bag cannot be bought; the Exact tile keeps the unrounded number so you can see how much of the last bag is being used.
• 4) Ordering buffersBags (+5%) = Exact × 1.05  |  Bags (+10%) = Exact × 1.10
  Applied to the unrounded count, then rounded up. The published yields are approximate compacted figures with no allowance for uneven subgrade, over-excavation or spillage, so a buffer is not optional on a real order — 5% for measured formwork, 10% for anything dug by hand.
• 5) Coverage cross-check for flatworkArea per Bag (ft²) = Yield (ft³) ÷ (Thickness in ÷ 12)
  An independent way to verify a slab count without repeating the same multiplication. At 4 in an 80 lb bag covers 1.80 ft², so 56 bags cover 100 ft². Applied to the 120 ft² example below that predicts 68 bags, against 67 from the volume route — agreement to within the rounding step.
• 6) Unit conversions1 yd³ = 27 ft³  |  1 m³ ≈ 35.315 ft³ ≈ 1.308 yd³
  Cubic yards are the US ready-mix ordering unit. The three-way readout exists so that a shape which turns out to be larger than expected can be quoted to a truck without re-measuring anything.

Worked Example 1 — A 12 × 10 ft Shed Pad at 4 Inches

The same three fields carry any rectangular pour. Below, the identical method runs on a 10 × 10 ft pad — a smaller footprint at the same 4 in thickness — costed in 60 lb bags instead, to show where the arithmetic changes and where it does not.

Worked Example 2 — 8 Fence Post Holes at 12 × 30 Inches

Worked Example 3 — 6 Sonotubes at 12 in × 48 in

What One Bag Does on Each Shape

Yield alone is an abstraction. What decides a bag size on a real job is how many sacks a single element consumes — one hole, one pier, one hundred square feet of pad. The table reads the shared engine yields against the three shapes worked above, so you can see the bag-size trade in the units the job is actually counted in.

Yields are approximate compacted figures held in the site's shared engine constants and sourced from Quikrete and Sakrete data sheets; coverage and per-element counts are derived from them and rounded up to the whole bag, before any waste allowance.

Reading the Bag Size Off the Shape

Two patterns fall out of the table. On flatwork the bag size barely matters — the same 120 ft² pad is the same total weight of material whichever column you buy, so choose by who is lifting. On discrete elements it matters more than it looks: a 12 in × 30 in hole takes 4 × 80 lb bags but 7 × 40 lb bags, and each of those counts is rounded up per hole in practice, because you mix and place one hole at a time. The smaller the bag, the finer the rounding step and the less of the last sack you throw away — which is the one real argument for a light bag beyond handling.

Product line matters more than brand. Quikrete and Sakrete agree on the standard concrete mix yields above, but fast-setting products commonly ship as 50 lb bags with their own yield — grab fast-set for a post job costed on 60 lb math and the count is wrong before you start. High-strength and crack-resistant blends use heavier aggregate and can run a little under. Region changes what is even on the shelf: the US and Canada stock 40/60/80 lb, the UK and EU sell 20 and 25 kg, Australia and New Zealand standardise on 20 kg. One trap worth naming — in India and much of South Asia the ubiquitous 50 kg bag is cement, not pre-mixed concrete, so a bag count from this tool does not apply at all; use the nominal mix (M5–M25) calculator instead. Whatever you buy, read the yield printed on the bag and re-run the count through the Volume → Bags utility if it differs from the table.

Five Mistakes Specific to Deriving Volume From Dimensions

• Leaving the thickness in inches on a form set to feet. This is the single most expensive input error on the page and it does not look wrong on screen. A 4 in slab entered as "4" with units on feet returns 800 bags instead of 67. Either enter 0.33 ft, or switch the whole form to inches — one unit for every field, every time.
• Entering a radius where the calculator asks for diameter. Both cylinder modes take the full diameter and halve it internally. Feeding a radius in quarters the volume, because the term is squared — six 12 in tubes drop from 32 bags to 8. The same squaring is why sloppy hole measurement costs more than sloppy slab measurement.
• Ignoring post displacement — in the direction that hurts. Post hole mode returns the gross hole volume, which over-states the fill by roughly 3 bags across the 8-hole example. That surplus is doing real work as an over-dig allowance, so leave it. The mistake is stacking a +10% buffer on top of it and paying twice for margin the gross-hole figure already gave you.
• Estimating a trenched footing from the drawing rather than the excavation. The drawn footing is a clean rectangle; the dug one has sloughed sidewalls and a bucket-wide over-cut, and the concrete fills the hole rather than the drawing. Measure the trench you have, then carry the +10% figure — under-ordering a continuous footing means a cold joint through the element the whole structure sits on.
• Averaging the dimensions of an L-shaped or stepped pour. There is no single length and width that represents a compound shape, and splitting the difference always under-counts the larger rectangle. Run each rectangle and cylinder separately, add the bag counts, and buffer the total once — or sum the pieces by hand and enter the result through Custom Volume.

Standards Behind the Shapes and the Yields

Measurement-risk note for this calculator: every figure on this page is only as good as the dimensions entered, and dimension error does not stay proportional. A 10% error on a slab thickness moves the bag count 10%; the same 10% error on a hole or tube diameter moves it about 21%, because the radius is squared. Excavated shapes compound this — trenches over-cut, augered holes bell out, and sub-grade rarely sits at a uniform depth — so a measured formwork pour and a dug one deserve different buffers, not the same one. Measure the excavation you actually have rather than the detail on the drawing, keep every field in one linear unit, and treat the +5% and +10% tiles as the decision they are.