Concrete bag calculator
This turns a volume into a bag count using the yield you set. Real coverage shifts with the mix and how you finish it, so keep a spare bag or two on hand rather than trusting the tally to the last unit. It does not decide how thick or how strong the pour needs to be — that is your plans' job.
Enter a known volume, or the length, width and thickness to have the tool work the volume out, then set the yield your bag prints. The result is the bag count for the job — the raw figure and the rounded-up number you actually carry to the till.
Exemple détaillé
Say you need half a cubic yard. That is 0.5 × 27 = 13.5 cubic feet. At 0.6 ft³ per 80 lb bag that is 13.5 ÷ 0.6 = 22.5 bags; add 10% for waste and round up and you would buy 25. Switch to 60 lb bags at 0.45 ft³ each and the same volume needs 30 before waste.
In metric, say you need 0.4 m³. At 0.014 m³ per 30 kg bag that is 0.4 ÷ 0.014 = 28.6 bags; add 10% (0.44 m³ ÷ 0.014 = 31.4) and round up and you would buy 32. In 25 kg bags at about 0.0117 m³ the same volume is 34 before waste.
Sur le chantier
Post holes. Eight holes, 10 in across and 30 in deep. Each is π × (5 ÷ 12)² × 2.5 = 1.36 ft³, less the 4 × 4 post inside it (0.21 ft³): 1.15 ft³ net, 9.2 ft³ for the eight. Switch the tool to "known volume", type 9.2 ft³ — or 0.34 yd³ (0.26 m³) — and at 0.6 ft³ a bag with 10% it says 17 bags. Two bags a hole, and one over.
A small pad. 4 ft × 4 ft at 4 in is 5.33 ft³: 8.9 bags, buy 10 with waste. In 60 lb bags (0.45 ft³) it is 13. That is the size of job bags are for — past about 40 or 50 bags you are mixing all day and a short load starts to win. In metric, a 1.2 × 1.2 m pad at 100 mm is 0.144 m³: 10.3 of the 30 kg bags, buy 12 with waste.
How long it takes. A small mixer does two 80 lb bags a batch, and a batch is about ten minutes with the wheelbarrow run. Twenty-five bags is thirteen batches — two hours plus setup, and each bag wants roughly the water the bag says. Plan the pour around that, not around the bag count.
What went wrong?
Quick referencefor bagged mix
- Bags per cubic yard
- 80 lb (0,6 ft³) 45 · 60 lb (0,45) 60 · 40 lb (0,3) 90 · 50 lb (0,375) 72
- Sacs
- ft³ × (1 + waste) ÷ yield, rounded up · yards × 27 first
- Ten 80 lb bags
- 6 ft³ — a 4 × 4 pad at 4½ in, or five 10 in × 30 in post holes
- Where bags stop making sense
- about 40–50 bags; past that price a short load
- Métrique
- 30 kg ≈ 0.014 m³ → 71 bags per m³ · 25 kg ≈ 0.0117 m³ → 85 per m³
- Metric bags
- m³ × (1 + waste) ÷ yield, rounded up · 1 yd³ = 0.765 m³
Waste & overagewhat to add, and why
- Bag jobs are mixed one batch at a time, and every batch leaves a little in the tub and on the hoe; 10% covers that on a slab and more on posts where the hole is never the size you dug it. Type it in the waste field.
- Buy one bag more than the buy line on any job with a dozen bags or fewer — the cost of one bag is small next to a second trip for the last shovelful.
- Above about a yard of concrete, price a short-load delivery before you buy 45 bags; the concrete page has a break-even comparer.
Mistakes to catchbefore you order
- Entering the bag's weight (80) in the yield field, which wants the mixed volume (about 0.6 ft³ for that bag) — the count comes out a hundredth of what it should.
- Reading a bag's printed yield as mixed concrete when it is the dry volume; the mixed yield on the label is the number to use.
- Typing a volume in cubic feet when the field is set to cubic yards, or the reverse — check the suffix on the field before reading the count.
What an inspector looks forthe building inspector, wherever the small pour is a structural one
On this part of the job
- That bagged mix is allowed for this element at all. A small pad or a post base usually is; a footing carrying the house may need certified ready-mix, and that call is in the drawing or the code, not on the bag.
- The strength printed on the bag against the strength the drawing asks for.
- Continuity of the pour. Mixing bags one at a time builds cold joints, so the inspector looks at how the lift was placed as much as at what went in it.
- Consolidation — rodded or vibrated, not just poured and patted, especially around steel and anchors.
Have this ready
- One empty bag kept aside; the printed label is the easiest evidence of what went in.
- The count you actually used, against the count you calculated.
- Clean water and a mixer sized for the pour, so the mix is consistent bag to bag.
Every figure an inspector measures against comes from the code your jurisdiction has adopted, the stamped drawing, or the manufacturer's printed instructions — so none of them are printed here. Look the number up in your own book, type it into the tool, and the arithmetic is yours.
La formule
bags = (volume in ft³ × (1 + waste)) ÷ yield per bag, rounded up
Get the volume into cubic feet — a cubic yard is 27 of them, or measure length × width × thickness directly — then divide by what one bag yields once mixed. The waste margin covers spillage and an uneven base, and rounding up is simply because you buy whole bags. In metric it is the same sum in cubic metres: length × width × thickness (millimetres ÷ 1,000, centimetres ÷ 100), times the waste, over the bag's m³ yield.
What this tool assumes
That a cubic yard is 27 cubic feet — a definition. The yield per bag and the waste are yours to set, because the bag's printed figure and your site conditions are the two things that genuinely move the count.
Questions
- How many bags of concrete make a cubic yard?
- About 45 of the 80 lb size. A mixed 80 lb bag yields roughly 0.6 cubic feet, and a cubic yard is 27 cubic feet, so 27 ÷ 0.6 ≈ 45. A 60 lb bag makes about 0.45 cubic feet, so a yard is closer to 60 of those. In metric, a cubic metre takes about 71 of the 30 kg bags at 0.014 m³ each (1 ÷ 0.014 ≈ 71). The yield is an editable field here — set it to whatever your bag prints.
- Should I enter a volume or the dimensions?
- Whichever you already have. If a supplier or another calculator told you the yardage, type that in the volume box. If all you have is a pad or trench to measure, switch to dimensions and give the length, width and thickness — the tool works out the volume for you and then the bag count.
- How accurate is the bag yield?
- The 0.6 ft³ per 80 lb bag figure is a common, honest average, but brands and mixes differ and a stiffer or wetter mix changes it slightly. The number the bag itself states beats any default, so overwrite it. Because the yield is the biggest lever on the count, getting it right matters more than the last decimal on your measurements.
- When is bagging not worth it?
- Once the tally climbs past roughly ten bags, the mixing time and the cost usually tip toward ready-mix by the yard. The tool will happily count a hundred bags for you, but treat a three-figure result as a nudge to price a truck instead — your back will thank you.
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