How much does a bag of concrete cover?
An 80 lb bag covers about 1.8 sq ft at 4 inches. The exact number depends on bag size and slab thickness — here is the full chart.
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Coverage by bag size and thickness
| Bag | Yield | 3 in | 4 in | 5 in | 6 in |
|---|---|---|---|---|---|
| 40 lb | 0.30 cu ft | 1.2 sq ft | 0.9 sq ft | 0.72 sq ft | 0.6 sq ft |
| 50 lb | 0.375 cu ft | 1.5 sq ft | 1.13 sq ft | 0.9 sq ft | 0.75 sq ft |
| 60 lb | 0.45 cu ft | 1.8 sq ft | 1.35 sq ft | 1.08 sq ft | 0.9 sq ft |
| 80 lb | 0.60 cu ft | 2.4 sq ft | 1.8 sq ft | 1.44 sq ft | 1.2 sq ft |
Coverage = yield ÷ thickness in feet. Add 10% for spillage and uneven subgrade.
Bags for common slabs (80 lb, 4 inches)
| Slab | Area | Bags (no waste) | With 10% |
|---|---|---|---|
| 4 × 4 ft | 16 sq ft | 9 | 10 |
| 6 × 6 ft | 36 sq ft | 20 | 22 |
| 8 × 8 ft | 64 sq ft | 36 | 40 |
| 10 × 10 ft | 100 sq ft | 56 | 62 |
| 12 × 12 ft | 144 sq ft | 80 | 88 |
Mixing notes
- An 80 lb bag needs about 3 quarts of water; a 60 lb bag about 2¼ quarts.
- Mix only what you can place in 20 minutes.
- Cold water slows set in hot weather; never add more water to "stretch" a bag.
Where the 0.60 cubic feet actually comes from
Every bagged mix prints a yield — 0.60 cubic feet for an 80 lb bag of standard concrete mix. That figure is not a calculation. It is a measurement: the manufacturer batches a known recipe, adds the specified water, fills a calibrated container, and divides the volume by the number of bags. The method is specified in ASTM C387, and it is run under lab conditions with exact water.
Two things follow. First, yield belongs to the whole mix rather than to the cement in it — sand and gravel are most of the volume, which is why a bag that weighs twice as much does not cover twice the area. Second, the printed figure assumes you add the water stated on the bag and no more. Water is the only ingredient that changes volume without adding solids, so a wet mix appears to stretch further while curing weaker: each extra gallon per cubic yard costs roughly 200 psi of compressive strength.
The three ways a bag count comes up short
The arithmetic is exact, so a short pour is almost always a site problem rather than a math problem.
- Ground that is not flat. Thickness is set by the forms at the edges, not by the soil underneath. A one-inch dip across 100 square feet swallows 8.3 cubic feet — about 14 extra 80 lb bags on a job that supposedly needed 56.
- Pouring thicker than planned. A quarter inch over 200 square feet is 4.2 cubic feet, or seven bags. Nobody notices a quarter inch.
- Spillage and the half-bag at the end. Small, constant, and the reason the usual allowance is 10% rather than 2%.
Look at the first item again. Over-excavation is the most common reason a pour runs short, and it disappears the moment the concrete goes in — there is no way to discover it afterwards except by ordering more.
Opening a bag and using part of it
An opened bag starts absorbing moisture immediately. In a humid garage it stiffens within weeks, and a bag that has gone lumpy will not mix back to its rated strength no matter how hard you beat it. Plan to finish a bag rather than open one more.
If you genuinely need a fraction, weigh it. A bathroom scale is accurate enough to split an 80 lb bag into halves or thirds, and weighing takes less time than guessing. Eyeballing “about a third of a bag” is how a small patch ends up with the wrong water ratio, which is the one error bagged concrete does not forgive.
Unopened bags keep for about a year stored dry and off a concrete floor. Open leftovers are better spent on a paver base or a post collar than kept for a job that may never happen.
Frequently asked questions
How much does an 80 lb bag cover?
About 1.8 sq ft at 4 inches, 2.4 at 3 inches, 1.2 at 6 inches.
How many bags for a 10 × 10 slab?
56 bags of 80 lb at 4 inches; buy 62 with 10% waste.
Does a 60 lb bag cover the same?
No — 0.45 cu ft versus 0.60, so about 25% less area per bag.
How much does a 40 lb bag cover?
1.2 sq ft at 3 inches, 0.9 sq ft at 4 inches, and 0.6 sq ft at 6 inches. A 40 lb bag yields 0.30 cubic feet, half of an 80 lb bag.
Does a wetter mix cover more area?
It appears to, and it cures weaker. Extra water adds volume without adding solids, and each gallon per cubic yard costs about 200 psi. Mix to the water stated on the bag.
How heavy is a mixed 80 lb bag?
About 86 lb. The 80 lb is dry weight; the roughly 3 quarts of water the bag calls for adds a little over 6 lb.
Can I patch a step with one bag?
Yes, if the patch is at least 2 inches thick and the edges are cut square. A patch feathered to zero thickness at the edge will break away within a season.
Note: Yields vary slightly by product. Check the bag you buy; the chart assumes standard US bagged mixes.
Related: Bags per yard · Concrete calculator · All material calculators