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Concrete Bags Calculator: 45 Bags for a 10 ft Slab

On the defaults — a 10 ft by 8 ft slab 4 inches deep, with bags yielding 0.6 cubic feet — concrete bags are 45 bags to buy, 44.45 bags exact, over 26.67 cubic feet. This is the concrete bags tool: multiply length by width by depth for the volume, divide by the bag yield for the exact count, and round up for the bags to buy — a slab guessed from the area alone tells you nothing until the depth behind it is known.

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Concrete bags calculator

Put your slab length, width, depth and bag volume in: a 10 ft by 8 ft slab 4 inches deep needs 26.67 cubic feet — 44.45 bags exact, so buy 45 bags.

How long the slab runs in feet — 10 on the defaults. Length sets the volume directly: every extra foot at the same width and depth adds the same slice again.

How wide the slab runs in feet — 8 on the defaults. Width moves only the volume, never the depth: a wider slab at the same depth needs more bags, never deeper ones.

How thick the slab is in inches — 4 on the defaults, a standard patio depth. Depth moves the volume directly, and it is the input most often guessed wrong: an inch out on depth is a quarter out on bags.

How many cubic feet one bag of mix yields — 0.6 on the defaults, an 80-lb bag. The bag size moves only the count, never the volume: smaller bags mean more bags over the same slab.

Concrete bags (10 ft long, 8 ft wide, 4 in deep, 0.6 cu ft a bag)Concrete bags 45 bags (10 ft long, 8 ft wide, 4 in deep, 26.67 cu ft, 44.45 bags exact, 0.6 cu ft a bag).Volume 10 × 8 × 4 ÷ 12 = 26.67 cu ft; bags 26.67 ÷ 0.6 = 44.45 exact, rounded up to 45 bags. Keep the depth, since the slab on the day settles the order, not the plan.

Slab length: 10 ft

Slab width: 8 ft

Slab depth: 4 in

Concrete volume: 26.67 cu ft

Bags exact: 44.45 bags exact

Bags to buy: 45 bags to buy

Bag volume: 0.6 cu ft a bag

12 ft long, same width, depth and bag: Concrete bags 54 bags (12 ft long, 8 ft wide, 4 in deep, 32 cu ft, 53.33 bags exact, 0.6 cu ft a bag).

6 in deep, same length, width and bag: Concrete bags 67 bags (10 ft long, 8 ft wide, 6 in deep, 40 cu ft, 66.67 bags exact, 0.6 cu ft a bag).

0.5 cu ft a bag, same slab: Concrete bags 54 bags (10 ft long, 8 ft wide, 4 in deep, 26.67 cu ft, 53.34 bags exact, 0.5 cu ft a bag).

10 ft wide, same length, depth and bag: Concrete bags 56 bags (10 ft long, 10 ft wide, 4 in deep, 33.33 cu ft, 55.55 bags exact, 0.6 cu ft a bag).

Length behind 45: 10 ft

At the star the figures are: Concrete bags 45 bags (10 ft long, 8 ft wide, 4 in deep, 26.67 cu ft, 44.45 bags exact, 0.6 cu ft a bag).

At the lower neighbor the figures are: Concrete bags 36 bags (8 ft long, 8 ft wide, 4 in deep, 21.33 cu ft, 35.55 bags exact, 0.6 cu ft a bag).

At the upper neighbor the figures are: Concrete bags 54 bags (12 ft long, 8 ft wide, 4 in deep, 32 cu ft, 53.33 bags exact, 0.6 cu ft a bag).

Reverse verdict: STEP

Depth behind 45: 4 in

At the star the figures are: Concrete bags 45 bags (10 ft long, 8 ft wide, 4 in deep, 26.67 cu ft, 44.45 bags exact, 0.6 cu ft a bag).

At the lower neighbor the figures are: Concrete bags 34 bags (10 ft long, 8 ft wide, 3 in deep, 20 cu ft, 33.33 bags exact, 0.6 cu ft a bag).

At the upper neighbor the figures are: Concrete bags 67 bags (10 ft long, 8 ft wide, 6 in deep, 40 cu ft, 66.67 bags exact, 0.6 cu ft a bag).

Reverse verdict: STEP

Bag size behind 45: 0.6 cu ft

At the star the figures are: Concrete bags 45 bags (10 ft long, 8 ft wide, 4 in deep, 26.67 cu ft, 44.45 bags exact, 0.6 cu ft a bag).

At the lower neighbor the figures are: Concrete bags 54 bags (10 ft long, 8 ft wide, 4 in deep, 26.67 cu ft, 53.34 bags exact, 0.5 cu ft a bag).

At the upper neighbor the figures are: Concrete bags 36 bags (10 ft long, 8 ft wide, 4 in deep, 26.67 cu ft, 35.56 bags exact, 0.75 cu ft a bag).

Reverse verdict: STEP

No length at all: Slab length — more than 0 ft.

Negative length: Slab length — more than 0 ft.

No width at all: Slab width — more than 0 ft.

Negative width: Slab width — more than 0 ft.

No depth at all: Slab depth — more than 0 in.

Negative depth: Slab depth — more than 0 in.

No bag volume at all: Bag volume — more than 0 cu ft a bag.

Negative bag volume: Bag volume — more than 0 cu ft a bag.

Concrete bags 45 bags (10 ft long, 8 ft wide, 4 in deep, 26.67 cu ft, 44.45 bags exact, 0.6 cu ft a bag).

12 ft long, same width, depth and bag: Concrete bags 54 bags (12 ft long, 8 ft wide, 4 in deep, 32 cu ft, 53.33 bags exact, 0.6 cu ft a bag)..

6 in deep, same length, width and bag: Concrete bags 67 bags (10 ft long, 8 ft wide, 6 in deep, 40 cu ft, 66.67 bags exact, 0.6 cu ft a bag)..

0.5 cu ft a bag, same slab: Concrete bags 54 bags (10 ft long, 8 ft wide, 4 in deep, 26.67 cu ft, 53.34 bags exact, 0.5 cu ft a bag)..

10 ft wide, same length, depth and bag: Concrete bags 56 bags (10 ft long, 10 ft wide, 4 in deep, 33.33 cu ft, 55.55 bags exact, 0.6 cu ft a bag)..

Reverse: Length behind 45: 10 ft — at 10 ft the figures are Concrete bags 45 bags (10 ft long, 8 ft wide, 4 in deep, 26.67 cu ft, 44.45 bags exact, 0.6 cu ft a bag)..; at the lower neighbor the figures are Concrete bags 36 bags (8 ft long, 8 ft wide, 4 in deep, 21.33 cu ft, 35.55 bags exact, 0.6 cu ft a bag)..; at the upper neighbor the figures are Concrete bags 54 bags (12 ft long, 8 ft wide, 4 in deep, 32 cu ft, 53.33 bags exact, 0.6 cu ft a bag).. — verdict: STEP. Reverse: Depth behind 45: 4 in — at 4 in the figures are Concrete bags 45 bags (10 ft long, 8 ft wide, 4 in deep, 26.67 cu ft, 44.45 bags exact, 0.6 cu ft a bag)..; at the lower neighbor the figures are Concrete bags 34 bags (10 ft long, 8 ft wide, 3 in deep, 20 cu ft, 33.33 bags exact, 0.6 cu ft a bag)..; at the upper neighbor the figures are Concrete bags 67 bags (10 ft long, 8 ft wide, 6 in deep, 40 cu ft, 66.67 bags exact, 0.6 cu ft a bag).. — verdict: STEP. Reverse: Bag size behind 45: 0.6 cu ft — at 0.6 cu ft the figures are Concrete bags 45 bags (10 ft long, 8 ft wide, 4 in deep, 26.67 cu ft, 44.45 bags exact, 0.6 cu ft a bag)..; at the lower neighbor the figures are Concrete bags 54 bags (10 ft long, 8 ft wide, 4 in deep, 26.67 cu ft, 53.34 bags exact, 0.5 cu ft a bag)..; at the upper neighbor the figures are Concrete bags 36 bags (10 ft long, 8 ft wide, 4 in deep, 26.67 cu ft, 35.56 bags exact, 0.75 cu ft a bag).. — verdict: STEP.

Edges: No length at all gives Slab length — more than 0 ft.. Negative length gives Slab length — more than 0 ft.. No width at all gives Slab width — more than 0 ft.. Negative width gives Slab width — more than 0 ft.. No depth at all gives Slab depth — more than 0 in.. Negative depth gives Slab depth — more than 0 in.. No bag volume at all gives Bag volume — more than 0 cu ft a bag.. Negative bag volume gives Bag volume — more than 0 cu ft a bag..

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What does 45 bags to buy, 44.45 bags exact mean?

It means a slab 10 feet long and 8 feet wide poured 4 inches deep holds 26.67 cubic feet of concrete, and at 0.6 cubic feet a bag that volume needs 44.45 bags — which no shop sells, so the order is 45 bags, with a little over half a bag to spare. The length and width set the area, the depth sets the volume, and the bag size sets only the count: a smaller bag never changes the slab, only how many bags carry it. The only judgement is the depth — 4 inches is a standard patio depth, and the depth that settles the order is the one actually formed and levelled, not the one written on the plan.

Output · formula · meaning

OutputFormulaWhat it means
Concrete volumelength × width × depth ÷ 1226.67 cu ft — 10 × 8 × 4 ÷ 12.
Bags exactvolume ÷ bag volume44.45 bags — 26.67 ÷ 0.6.
Bags to buyexact bags, rounded up45 bags — 44.45 rounded up.

Three things to check before you trust the number

First, check the depth honestly: 4 inches must be the depth actually poured, not the depth of the form boards — a form that sits an inch into soft ground pours five inches, and an inch out on depth is a quarter out on bags. Second, check the length and width against the formed area, not the planned area: a slab that runs long by a foot on each side holds more concrete than the plan, and the calculator trusts the measurements you enter. Third, check the bag yield against the bag itself: 0.6 cubic feet suits an 80-lb bag, while a 60-lb bag yields 0.45 and a 40-lb bag yields 0.3 — a yield borrowed from the wrong bag size orders the wrong count by a third or a half.

Worked example

10 ft long, 8 ft wide, 4 in deep, 0.6 cu ft a bag. Volume: 10 × 8 × 4 ÷ 12 = 26.67 cubic feet. Bags exact: 26.67 ÷ 0.6 = 44.45 bags. Bags to buy: 44.45 rounded up = 45 bags. In practice that means a small patio slab takes forty-five 80-lb bags — the number to order, with the half-bag spare covering spillage, an uneven base, and a form that runs a touch deep.

Every branch, checked

With a 12 ft length instead of 10, the volume rises to 32 cubic feet — 53.33 bags exact, so buy 54 bags. With a 6 in depth instead of 4, the volume rises to 40 cubic feet — 66.67 bags exact, so buy 67 bags. With bags yielding 0.5 cubic feet instead of 0.6, the volume stays 26.67 cubic feet but the count rises to 53.34 bags exact, so buy 54 bags. With a 10 ft width instead of 8, the volume rises to 33.33 cubic feet — 55.55 bags exact, so buy 56 bags.

The reverses, worked backwards

The length behind 45 bags is 10 ft: at 8 ft the volume is 21.33 cubic feet, 35.55 bags exact, so buy 36 bags; at 12 ft it is 32 cubic feet, 53.33 bags exact, so buy 54 bags. The depth behind 45 bags is 4 in: at 3 in the volume is 20 cubic feet, 33.33 bags exact, so buy 34 bags; at 6 in it is 40 cubic feet, 66.67 bags exact, so buy 67 bags. The bag size behind 45 bags is 0.6 cu ft: at 0.5 cu ft the count is 53.34 bags exact, so buy 54 bags; at 0.75 cu ft it is 35.56 bags exact, so buy 36 bags — the bag size never touches the volume, only the count.

What the defaults assume

The defaults assume a level rectangular slab 10 ft by 8 ft poured a true 4 inches deep, with 80-lb bags yielding 0.6 cubic feet each and no waste allowance beyond the round-up. They do not assume an uneven base, footings, thickened edges, or spillage — a slab with a footing trench, a thickened edge, or a soft base that swallows mix all need extra bags beyond the round-up, so treat 45 as the slab count, not the site count.

Who uses this

Accuracy & limitations

The arithmetic is exact for the inputs entered; the judgement is the depth, the formed area, and the bag yield. Real slabs vary with base level, form depth, and spillage — so treat 45 as the planning order for a true 10 ft by 8 ft slab at 4 inches with 80-lb bags, and add a bag or two for an uneven base before quoting it to anyone else.

Cite this calculator

Bright Side Kit, “Concrete Bags Calculator: 45 Bags for a 10 ft Slab”, https://brightoolbox.com/concrete-bags-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 long, 8 ft wide, 4 in deep, 0.6 cu ft a bag) so the result can be checked.

Frequently asked questions

How many bags of concrete for a 10 by 8 slab?

45 bags on the defaults: 10 × 8 × 4 ÷ 12 = 26.67 cubic feet, and 26.67 ÷ 0.6 = 44.45 bags exact, rounded up to 45 bags of 80-lb mix.

Why round up instead of buying the exact count?

Because shops sell whole bags, and a slab poured short cannot be topped up cleanly once the first pour sets. The round-up is the cheapest insurance on the job — half a bag spare beats a cold joint.

How many cubic feet does an 80-lb bag cover?

About 0.6 cubic feet, the figure used here. A 60-lb bag yields about 0.45 cubic feet and a 40-lb bag about 0.3 — check the bag itself, because yields vary by brand and mix.

Does depth really matter that much?

Yes: depth multiplies the whole volume, so an inch out on a 4-inch slab is a quarter out on bags. A slab formed at 5 inches instead of 4 needs 56 bags, not 45, for the same footprint.

What about waste and spillage?

This calculator counts the slab only, plus the round-up. For an uneven base, a long wheelbarrow run, or a first-time pour, add 5–10% — two to five bags on this slab — rather than trusting the round-up alone.

How is this different from the concrete volume calculator?

That page counts cubic feet for the slab; this page counts bags for the store. Use that page for a ready-mix order, this page for bagged mix.

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