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Concrete bag calculator
Put your slab length, width, depth and bag size in: a 10 ft by 10 ft slab at 4 in deep is 100 sq ft, 33.33 cu ft — 74.07 bags exact, 75 bags at 60 lb.
★ CONSTRUCTION · CONCRETE · BAGS
Concrete Bag Calculator: 75 Bags for a 10x10 ft Slab
On the defaults — a 10 ft by 10 ft slab, 4 in deep, 60 lb bags — you need 75 bags of concrete. This is the concrete bag tool: measure the slab once, allow for the ground, and buy the bags once — a half-poured slab waits for no second trip.
What does concrete bags needed 75 bags mean?
It means seventy-five bags of mix pour a 10 ft by 10 ft slab at 4 inches deep. The slab itself is 100 square feet, since 10 times 10 is 100; at 4 inches deep that is 33.33 cubic feet, and since one cubic yard is 27 cubic feet, the volume is 1.23 cubic yards. One 60 lb bag holds 0.45 cubic feet of mix, so the exact need is 74.07 bags, rounded up to 75 whole bags because bags are sold by the bag, not by the fraction. The spare bag is not spare stock exactly — it is the mix the pour consumes in spillage and uneven ground, so a slab poured to the exact bag runs short before the last corner.
Output · formula · meaning
| Output | Formula | What it means |
|---|---|---|
| Slab area | length × width | 100 sq ft — 10 × 10. |
| Volume | area × (depth ÷ 12) ÷ 27 | 1.23 cu yd — 33.33 cu ft ÷ 27. |
| Bags | volume ÷ mix per bag, rounded up | 75 bags — 33.33 ÷ 0.45, rounded up. |
Three things to check before you trust the number
First, check the bag size you are actually buying: 60 lb is the common bag, but an 80 lb bag holds 0.6 cu ft and cuts this slab to 56 bags, and a 40 lb bag holds 0.3 cu ft and raises it to 112 bags. Second, remember the volume follows the area times the depth, not the length alone: a 10 ft by 10 ft slab is 100 sq ft, and a slab measured by one side under-orders. Third, judge the depth honestly: concrete follows the ground, so 4 in fits a level, well-compacted base, but soft or uneven ground can want a deeper pour, since every dip takes mix. Rebar, mesh and finishing are bought separately — see the driveway calculator for the same pour ordered by the yard.
Worked example
10 ft long, 10 ft wide, 4 in deep, 60 lb bags. Area: 10 × 10 = 100 sq ft. Volume: 100 × (4 ÷ 12) = 33.33 cu ft, and 33.33 ÷ 27 = 1.23 cu yd. Mix per bag: 60 × 0.0075 = 0.45 cu ft. Bags: 33.33 ÷ 0.45 = 74.07, rounded up to 75 bags. In practice that means buying 75 bags, and expecting the last bag to go on spillage and uneven ground. If the same slab used 80 lb bags instead, the need would fall to 56 bags, but the slab poured would be identical.
Every branch, checked
With a 12 ft long slab instead of 10 ft, the area rises to 120 sq ft and the need to 89 bags. With a 12 ft wide slab, the area rises to 120 sq ft and the need to 89 bags, since the slab area is the same either way. With a 6 in slab, the volume rises to 50 cu ft and the need to 112 bags. With 80 lb bags, the volume stays 33.33 cu ft but the need falls to 56 bags, since every bag holds more mix. Slab size, depth and bag size each move the count on their own, so a wrong bag figure is as costly as a wrong measurement.
The reverses, worked backwards
The length behind 75 bags is 10 ft: at 8 ft the need is 60 bags (8 ft × 10 ft slab, 4 in deep, 26.67 cu ft, 0.99 cu yd, 60 lb bags); at 12 ft it is 89 bags (12 ft × 10 ft slab, 4 in deep, 40 cu ft, 1.48 cu yd, 60 lb bags). The depth behind 75 bags is 4 in: at 3 in the need is 56 bags (10 ft × 10 ft slab, 3 in deep, 25 cu ft, 0.93 cu yd, 60 lb bags); at 6 in it is 112 bags (10 ft × 10 ft slab, 6 in deep, 50 cu ft, 1.85 cu yd, 60 lb bags). The bag size behind 75 bags is 60 lb: at 40 lb the need is 112 bags (10 ft × 10 ft slab, 4 in deep, 33.33 cu ft, 1.23 cu yd, 40 lb bags); at 80 lb it is 56 bags (10 ft × 10 ft slab, 4 in deep, 33.33 cu ft, 1.23 cu yd, 80 lb bags). Each reverse moves one input and holds the rest, so the step is exactly one dimension, depth or bag change.
What the defaults assume
The defaults assume a 10 ft by 10 ft rectangular slab, a 4 in pour, and 60 lb bags at 0.45 cu ft of mix each. They do not assume rebar, mesh, edging or finishing — a slab that needs reinforcement counts its steel separately, and the finishing is bought by the job, not by the bag.
Who uses this
- A homeowner pouring a slab: the 75-bag order settles the supply call before the first bag is opened.
- A DIY builder patching a patio section: the same maths on the section alone gives the bags, not a whole yard.
- A contractor quoting a job: slab area, depth, bag size — the quote follows the same three steps, and the bag line shows the client why.
- A landlord budgeting a slab refit: the slab size off the plans, multiplied out, gives the bag line on the refit budget.
Accuracy & limitations
The arithmetic is exact for the inputs entered; the judgement is the depth and the bag size. Real ground is uneven, and the slab on the ground overrides any planning figure; a slab measured gross over-orders, and a bag figure read off a different product under-orders. Treat 75 bags as the planning order for a 10 ft by 10 ft slab at 4 in deep with 60 lb bags, and allow a deeper pour for soft or uneven ground.
Cite this calculator
Bright Side Kit, “Concrete Bag Calculator: 75 Bags for a 10x10 ft Slab”, https://brightoolbox.com/concrete-bag-calculator/, accessed 3 November 2026. Students and researchers: cite the calculator name, the page URL and the date you used it, and quote the defaults (10 ft × 10 ft slab, 4 in deep, 60 lb bags) so the 75-bag result can be checked.
Frequently asked questions
How many concrete bags do I need for a 10x10 ft slab?
75 bags at 4 in deep with 60 lb bags. The volume is 33.33 cu ft, or 1.23 cu yd; at 0.45 cu ft per bag that is 74.07 bags, rounded up to 75.
How do I work out concrete bags from a slab size?
Multiply length by width for the area, multiply by depth in feet for cubic feet, divide by the mix per bag and round up. A 10 ft by 10 ft slab at 4 in is 100 sq ft, or 75 bags at 60 lb.
How much mix is in a 60 lb bag of concrete?
About 0.45 cubic feet. An 80 lb bag holds about 0.6 cu ft and a 40 lb bag about 0.3 cu ft, so the bag size moves the count directly.
Does this count rebar too?
No — this calculator counts bags of mix only. Rebar and mesh are counted separately, by the slab.
Should I buy the exact bags or round up?
Round up: bags are sold by the bag, and the spare bag is the spillage stock. A slab poured to the exact bag runs short before the last corner.
What if my slab is not a rectangle?
Split it into rectangles, work out each area, and add them before multiplying by the depth. The square footage calculator handles the area step section by section.