1473
Cement bag chart
Put your area, depth and bag yield in: 100 square feet at 4 inches needs 81.48 bags, order 82 bags.
★ CONSTRUCTION · BAGS · CEMENT
Cement Bag Chart: 82 Bags for a 100 Sq Ft Slab at 4 In
On the defaults — a 100-square-foot slab, 4 inches deep, 60-lb bags yielding 0.45 cubic feet each, and 10% waste — you need 81.48 bags of concrete mix exact, so order 82 bags. This is the CEMENT version — bags of mix by slab area and depth — the drywall-sheet-chart page is the DRYWALL version — sheets by wall area; the insulation-roll-chart page is the INSULATION version — rolls by area; and the plywood-sheet-chart page is the PLYWOOD version — sheets by area at the same 10% waste rule.
What does 81.48 bags mean?
It means a 100-square-foot slab at 4 inches deep needs eighty-one point four eight bags of concrete mix once 10% waste is added: the slab is 100 × 4 ÷ 12, which is 33.33 cubic feet, waste takes it to 36.67, and 36.67 ÷ 0.45 is 81.48, so the order is 82 bags. The 1.10 is the whole story in miniature — spillage, uneven sub-base, and a form that bows a little all eat concrete, so the bare volume alone understates the order: without waste the same slab needs exactly 74.07 bags, and the waste seven bags are what keep the last corner filled. The part people get wrong is ordering the exact figure when it is not whole: 81.48 bags cannot be bought, and ordering 81 leaves part of the slab short.
Three things to check before you trust the number
Is the area really 100 square feet? Slabs are rarely plain rectangles, so the measured area is where most orders go wrong: the 50-square-foot branch needs 40.74 bags exact, order 41, while the 200-square-foot branch needs 162.96, order 163, for the same depth and bag size. Measure each section separately, length times width, add them, and allow for curves by splitting them into rectangles and triangles — a guessed area is the most expensive guess on the job.
Is the depth really 4 inches? Depth scales the volume directly: the 2-inch branch needs 40.74 bags exact, order 41, while the 6-inch branch needs 122.22, order 123, for the same 100 square feet and bag size. Check the depth at several points, not just at the form edge — a sub-base that dips by half an inch in the middle adds about 4 bags to a 100-square-foot slab, which is exactly what the 10% waste allowance is for.
Is the bag yield really 0.45 cubic feet? Bag sizes vary, and the bag count follows the yield exactly: the 80-lb branch needs 61.11 bags exact, order 62, while the 40-lb branch needs 122.22, order 123, for the same slab. Use the yield printed on your bag, not a remembered figure — a 60-lb bag is 0.45 cubic feet, an 80-lb bag is 0.60, and a bag from a different brand may differ by a few hundredths, which is several bags on a large slab.
Worked example
On the defaults: 100 square feet, 4 inches deep, 60-lb bags at 0.45 cubic feet, 10% waste.
- Volume: 100 × 4 ÷ 12 is 33.33 cubic feet.
- Volume with waste: 33.33 × 1.10 is 36.67 cubic feet.
- Exact bags: 36.67 ÷ 0.45 is 81.48 bags.
- Order: 81.48 rounded up is 82 bags.
And the other way: the area needing exactly 81.48 bags at 0.45 cubic feet a bag is 100 square feet, the bag yield needing exactly 81.48 bags over 100 square feet at 4 inches is 0.45 cubic feet, and the area needing exactly 162.96 bags at 0.45 cubic feet a bag is 200 square feet.
Cement bags lookup: area × depth at 60-lb bags (10% waste included)
| Area | Depth | Volume | Volume with waste | Exact bags | Order bags |
|---|---|---|---|---|---|
| 50 sq ft | 2 in | 8.33 cu ft | 9.17 cu ft | 20.37 bags | 21 bags |
| 50 sq ft | 3 in | 12.50 cu ft | 13.75 cu ft | 30.56 bags | 31 bags |
| 50 sq ft | 4 in | 16.67 cu ft | 18.33 cu ft | 40.74 bags | 41 bags |
| 50 sq ft | 6 in | 25.00 cu ft | 27.50 cu ft | 61.11 bags | 62 bags |
| 100 sq ft | 2 in | 16.67 cu ft | 18.33 cu ft | 40.74 bags | 41 bags |
| 100 sq ft | 3 in | 25.00 cu ft | 27.50 cu ft | 61.11 bags | 62 bags |
| 100 sq ft | 4 in | 33.33 cu ft | 36.67 cu ft | 81.48 bags | 82 bags |
| 100 sq ft | 6 in | 50.00 cu ft | 55.00 cu ft | 122.22 bags | 123 bags |
| 200 sq ft | 2 in | 33.33 cu ft | 36.67 cu ft | 81.48 bags | 82 bags |
| 200 sq ft | 3 in | 50.00 cu ft | 55.00 cu ft | 122.22 bags | 123 bags |
| 200 sq ft | 4 in | 66.67 cu ft | 73.33 cu ft | 162.96 bags | 163 bags |
| 200 sq ft | 6 in | 100.00 cu ft | 110.00 cu ft | 244.44 bags | 245 bags |
| 400 sq ft | 2 in | 66.67 cu ft | 73.33 cu ft | 162.96 bags | 163 bags |
| 400 sq ft | 3 in | 100.00 cu ft | 110.00 cu ft | 244.44 bags | 245 bags |
| 400 sq ft | 4 in | 133.33 cu ft | 146.67 cu ft | 325.93 bags | 326 bags |
| 400 sq ft | 6 in | 200.00 cu ft | 220.00 cu ft | 488.89 bags | 489 bags |
Volume is area × depth ÷ 12; volume with waste is volume × 1.10; exact bags are volume with waste ÷ 0.45 cu ft, and order rounds up to whole bags, so an 81.48-bag figure orders 82 bags.
Every branch, checked
- 50 sq ft slab, 4 in deep, 60-lb bags: 50 sq ft slab, 4 in deep, 60-lb bags: 40.74 bags exact — order 41 bags.
- 200 sq ft slab, 4 in deep, 60-lb bags: 200 sq ft slab, 4 in deep, 60-lb bags: 162.96 bags exact — order 163 bags.
- 100 sq ft slab, 2 in deep, 60-lb bags: 100 sq ft slab, 2 in deep, 60-lb bags: 40.74 bags exact — order 41 bags.
- 100 sq ft slab, 6 in deep, 60-lb bags: 100 sq ft slab, 6 in deep, 60-lb bags: 122.22 bags exact — order 123 bags.
- 100 sq ft slab, 4 in deep, 80-lb bags: 100 sq ft slab, 4 in deep, 80-lb bags: 61.11 bags exact — order 62 bags.
- 100 sq ft slab, 4 in deep, 40-lb bags: 100 sq ft slab, 4 in deep, 40-lb bags: 122.22 bags exact — order 123 bags.
The reverses, worked backwards
- Area needing exactly 81.48 bags at 0.45 cu ft a bag: 100 sq ft — at 100 sq ft the figures are 81.48 bags exact — order 82 bags.; at the lower neighbor the figures are 73.33 bags exact — order 74 bags.; at the upper neighbor the figures are 89.63 bags exact — order 90 bags..
- Bag yield needing exactly 81.48 bags over 100 sq ft at 4 in: 0.45 cu ft — at 0.45 cu ft the figures are 81.48 bags exact — order 82 bags.; at the lower neighbor the figures are 91.67 bags exact — order 92 bags.; at the upper neighbor the figures are 73.33 bags exact — order 74 bags..
- Area needing exactly 162.96 bags at 0.45 cu ft a bag: 200 sq ft — at 200 sq ft the figures are 162.96 bags exact — order 163 bags.; at the lower neighbor the figures are 154.81 bags exact — order 155 bags.; at the upper neighbor the figures are 171.11 bags exact — order 172 bags..
What the defaults assume
The defaults are a 100-square-foot slab, 4 inches deep, and 60-lb bags, so the headline is a small patio or path section at the common patio depth. The model is one division, one multiplication and one round-up: area times depth divided by 12 for cubic feet, times 1.10 for waste, divided by bag yield for bags — with a tiny epsilon subtracted before the ceiling so a value landing exactly on a whole bag, like 120 square feet at 5 inches with a 0.55 yield, orders that bag and not one more. The area reverses are checked at 10 square feet either side; the yield reverse is checked at 0.05 cubic feet either side. The model does not include reinforcement, a thicker edge, or waste beyond the 10% allowance — for an uneven sub-base, allow 15% waste instead of 10% by multiplying the volume by 1.15 before dividing.
Output · formula · meaning
| Output | Formula | What it means |
|---|---|---|
| Volume | area × depth ÷ 12 | 33.33 cu ft — 100 × 4 ÷ 12. |
| Volume with waste | volume × 1.10 | 36.67 cu ft — 33.33 × 1.10. |
| Exact bags | volume with waste ÷ bag yield | 81.48 bags — 36.67 ÷ 0.45. |
| Order bags | exact bags rounded up to whole bags | 82 bags — 81.48 rounded up. |
| Area behind a target bag count | target bags × bag yield ÷ 1.10 × 12 ÷ depth | 100 sq ft — 81.48 × 0.45 ÷ 1.10 × 12 ÷ 4. |
| Bag yield behind a target bag count | volume with waste ÷ target bags | 0.45 cu ft — 36.67 ÷ 81.48. |
| 50 sq ft slab, 4 in deep, 60-lb bags | same model | 40.74 bags exact — order 41 bags. |
| 200 sq ft slab, 4 in deep, 60-lb bags | same model | 162.96 bags exact — order 163 bags. |
| 100 sq ft slab, 2 in deep, 60-lb bags | same model | 40.74 bags exact — order 41 bags. |
| 100 sq ft slab, 6 in deep, 60-lb bags | same model | 122.22 bags exact — order 123 bags. |
| 100 sq ft slab, 4 in deep, 80-lb bags | same model | 61.11 bags exact — order 62 bags. |
| 100 sq ft slab, 4 in deep, 40-lb bags | same model | 122.22 bags exact — order 123 bags. |
| Area needing exactly 81.48 bags at 0.45 cu ft a bag | reverse | 100 sq ft: 81.48 bags exact — order 82 bags. |
| Bag yield needing exactly 81.48 bags over 100 sq ft at 4 in | reverse | 0.45 cu ft: 81.48 bags exact — order 82 bags. |
| Area needing exactly 162.96 bags at 0.45 cu ft a bag | reverse | 200 sq ft: 162.96 bags exact — order 163 bags. |
| 0 sq ft area | same model | 0.00 bags exact — order 0 bags. |
| Negative area | same model | Area — 0 or more. |
| Negative bag yield | same model | Bag yield — more than 0. |
| 120 sq ft, 5 in, 0.55 cu ft yield — exactly 100.00 bags | same model | 100.00 bags exact — order 100 bags. |
Who uses this
- A homeowner pouring a patio slab — 100 square feet at 4 inches is 81.48 bags exact, order 82 bags of 60-lb mix.
- A builder checking a supplier quote — the lookup table gives exact bags and order bags for four areas and four depths.
- Someone choosing a bag size — the 80-lb branch needs 62 bags and the 40-lb branch needs 123 for the same slab.
- A buyer checking a delivery quote — the reverses give the area or bag yield behind any quoted bag count.
Accuracy & limitations
Figures are the model’s own arithmetic on the numbers you type, computed from unrounded values and displayed rounded HALF_UP to 2dp, with order bags rounded up to whole bags. The main limitation is the job itself, not the arithmetic: uneven sub-base, thicker edges and spillage all move the real order, and the 10% waste figure is a standard allowance, not a measurement of your slab. This page is a calculation tool, not a materials quote.
Cite this calculator
A 100 sq ft slab at 4 inches deep and 10% waste needs 36.67 cu ft of concrete: 81.48 bags exact at 0.45 cu ft a bag, so order 82 bags. Source: Bright Side Kit, https://brightoolbox.com/cement-bag-chart/, accessed 29 October 2026.
Frequently asked questions
How many bags of concrete do I need for a slab?
Divide the volume with waste by the bag yield, then round up: 100 square feet times 4 inches divided by 12 is 33.33 cubic feet, times 1.10 is 36.67, divided by 0.45 is 81.48 bags exact, so order 82 bags.
Why add 10% waste?
Spillage, an uneven sub-base, and forms that bow a little all consume concrete. Without waste the same slab needs exactly 74.07 bags; the extra seven bags are what keep the last corner filled.
How many cubic feet does a 60-lb bag make?
About 0.45 cubic feet. An 80-lb bag makes about 0.60 cubic feet and a 40-lb bag about 0.30 — use the yield printed on your bag, because brands differ slightly.
How deep should a patio slab be?
Four inches is the common depth for a patio or walkway. A driveway usually needs 5 or 6 inches, and every extra inch adds a quarter to the volume at 4 inches, so depth is worth measuring carefully.
Why does the order round up instead of rounding to nearest?
Bags are sold whole, and a partial bag cannot fill a partial slab. Rounding 81.48 down to 81 would leave part of the slab short; rounding up to 82 covers it with a little spare.
When should I allow more than 10% waste?
For an uneven sub-base, a slab with a thicker edge, or a lot of small sections, allow 15% waste instead of 10% by multiplying the volume by 1.15 before dividing. The extra bags are cheaper than a second delivery.