Bagged concrete turns a small slab project into a trip to the home center instead of a phone call to a ready-mix plant. A 5×10 slab is a standard footprint for a shed base, a landing pad, a small patio or a garden path, and the arithmetic behind it comes down to bag yields and thickness. The same volume logic that governs slab-on-grade systems in larger foundations applies here, scaled down to a weekend pour.
Bagged mixes are sold in 40, 50, 60 and 80 pound bags, and each size produces a different volume of finished concrete. Before loading the cart, you need three numbers: the yield of one bag, the volume of the slab at your chosen thickness, and the bag count that follows. The steps below work through each one with figures you can verify on site, then add a cost estimate so the project fits the budget.
Getting the count right matters twice: under-ordering stalls the pour mid-slab, and over-ordering leaves open bags that harden in the garage. The difference between 27 and 29 bags rarely matters; the difference between 28 and 14 does.
What One Bag of Concrete Yields
Every bag of concrete carries a printed yield, the wet volume the dry mix produces once water is added. Yield is stated in cubic feet, usually with the cubic yard equivalent underneath, and the numbers hold across most brands. A concrete calculator applies these same figures to slabs, beams, columns and footings, which makes it a fast cross-check for hand math.
Yield by bag size
The yield scales almost linearly with bag weight. An 80 pound bag produces twice the concrete of a 40 pound bag, and a 60 pound bag sits between them. The table lists the common sizes with their volume and the coverage at a 3 inch pour depth.
| Bag size | Cubic feet per bag | Cubic yards per bag | Coverage at 3 in depth |
|---|---|---|---|
| 40 lb | 0.30 | 0.011 | 1.2 sq ft |
| 50 lb | 0.375 | 0.0138 | 1.5 sq ft |
| 60 lb | 0.45 | 0.017 | 1.8 sq ft |
| 80 lb | 0.60 | 0.022 | 2.4 sq ft |
Cubic feet versus cubic yards
Two units do most of the work in concrete math. Cubic feet suit a small slab, while cubic yards are what suppliers quote for trucked deliveries. One cubic yard equals 27 cubic feet, so converting is a single division. The 5×10 project stays in cubic feet and only moves to yards when comparing against ready-mix minimums.
The yield printed on the bag assumes the recommended water is added and the mix is fully blended. Add more water and the paste thins; add less and the mix turns stiff. Either way the volume barely changes, so treat the printed number as the working figure and keep the water ratio consistent.
Bag Counts for a 5×10 Slab at Each Thickness
A 5×10 slab covers 50 square feet. Multiply that footprint by the thickness in feet to get the volume, then divide by the yield per bag. At 3 inches the slab needs 12.5 cubic feet, which works out to about 21 eighty-pound bags or 28 sixty-pound bags. At 4 inches the volume climbs to 16.7 cubic feet, or roughly 28 eighty-pound and 38 sixty-pound bags. A 6 inch slab uses 25 cubic feet and needs about 42 eighty-pound bags or 56 sixty-pound bags.
Bag count reference table
| Thickness | Slab volume | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|
| 3 in | 12.5 cu ft | 21 | 28 | 42 |
| 4 in | 16.7 cu ft | 28 | 38 | 56 |
| 6 in | 25.0 cu ft | 42 | 56 | 84 |
Why bag size changes the count
The total volume stays the same; only the number of bags changes. Eighty-pound bags mean fewer trips from the car to the mixer, which is why crews prefer them. Sixty-pound bags are easier to lift and knead by hand. The choice is mostly about your back and your schedule.
Thickness is a design choice as much as a math input. A 3 inch pad suits a garden path or temporary platform. A 4 inch slab handles a shed base or small patio when reinforced with wire mesh. Go to 6 inches for parked vehicles or heavy equipment, and add a compacted gravel base beneath any slab that carries load.
Before any mixing starts, the edges of the pour need to be defined. Forming a concrete slab with straight, level forms sets the final grade and keeps the wet mix inside the footprint, and it is the step most likely to be rushed on a small job.
How to Calculate Concrete Volume for Any Slab
The same five steps handle any slab, pad or footing. Keep the units consistent and the result is repeatable.
- Measure the length and width in feet and multiply them for the square footage.
- Convert the planned thickness from inches to feet by dividing by 12.
- Multiply the square footage by the thickness in feet to get cubic feet.
- Divide the cubic feet by 27 to get cubic yards.
- Divide the total volume by the yield per bag and round up to the next full bag.
Worked example: the 5×10 slab at 4 inches
Run the numbers for the standard patio thickness. Five feet times ten feet gives 50 square feet. Four inches is 0.333 feet, so the volume is 50 x 0.333 = 16.7 cubic feet. Multiplying by 0.037 converts that to 0.62 cubic yards. Dividing 16.7 cubic feet by the 0.60 cubic foot yield of an 80 pound bag gives 27.8 bags, so buy 28.
The 0.037 shortcut
Once the volume is in cubic feet, multiplying by 0.037 converts directly to cubic yards, since 1/27 equals 0.037. For a quick sanity check on any footprint, remember that a 4 inch slab needs about one 80 pound bag per 1.8 square feet of surface.
Rounding is not waste; it is insurance. 27.8 bags means 28 on the order sheet, and one extra bag per ten covers spillage or a test patch. Leftover dry mix keeps for months if the bag is resealed, so an extra bag beats a second trip to the store.
Soil conditions and edge thickness change the design once a pad carries a structure. Concrete slab foundations with thickened edges and reinforcement follow a different set of rules, and the load path matters more than the bag count.
What a 5×10 Slab Costs
Bagged concrete runs roughly $4 to $8 per cubic foot depending on the mix, the region and the retailer. At 4 inches the 5×10 slab contains 16.7 cubic feet, so the material lands between $67 and $133. A 6 inch slab doubles the volume to 25 cubic feet and runs $100 to $200. Labor, forms, gravel base, reinforcement and delivery push the total higher.
Cost drivers
- Thickness: every extra inch on a 5×10 slab adds about 4.2 cubic feet, or seven 80 pound bags.
- Mix type: fiber-reinforced and higher-strength blends cost more per bag than standard mix.
- Site prep: a gravel base, vapor barrier and stakes add $50 to $150 depending on ground conditions.
Bagged versus ready-mix
Ready-mix delivery has a minimum order, often one cubic yard, which is more concrete than a 5×10 slab needs at any common thickness. Bagged concrete lets you buy the exact count and mix it on site, so for this footprint the bags win on price and waste.
The per-bag price varies more than the yield. A store brand can undercut a national brand by 20 to 30 percent for the same weight, and pallet discounts of 5 to 10 percent appear at 25 bags or more. A 5×10 slab at 4 inches is close to a full pallet by itself, so ask about the pallet price before you load the cart.
Forms for one small pad are usually wood stakes and boards. Once you pour several pads, a shed floor or a garage apron, steel formwork systems become economical because they strip, move and reset faster than lumber.
What Is Inside a Bag of Concrete
Bagged concrete is a dry blend of Portland cement, sand, coarse aggregate and sometimes admixtures. Cement is the binder; once water is added it hydrates and locks the particles together. Sand fills the voids between the larger pieces, and the aggregate carries most of the compressive load. The plant sets the proportions; the installer adds only water.
The role of each component
- Portland cement: the reactive binder, roughly 10 to 15 percent of the mix by volume.
- Sand: filler that spaces the aggregate and pushes trapped air out of the paste.
- Gravel or stone: the bulk of the mix and the main source of compressive strength.
- Water: triggers hydration, and too much of it weakens the finished concrete.
Why sand matters
Sand is the filler that seats the larger particles and drives out air pockets. Too little sand leaves honeycombing along the surface; too much weakens the paste. The factory blend balances these ingredients, which is why the instructions on the bag say to add water and nothing else.
Water is the one ingredient you control. A 60 pound bag mixes with about 2.5 quarts, and an 80 pound bag takes about 3.5 quarts, added gradually. The right amount holds a trowel line without slumping; too much produces soupy concrete that cracks as it cures.
Mix designs change every season, and contractors who pour slabs regularly compare new bag technology, admixtures and equipment at the annual concrete industry show before committing to a product line.
Ordering, Mixing and Pouring Tips
How many bags to buy
- Measure the footprint in feet and multiply length by width.
- Multiply by the thickness in feet to get the volume in cubic feet.
- Divide by the bag yield, 0.60 for an 80 pound bag or 0.45 for a 60 pound bag.
- Round up, then add one or two extra bags for spillage and test patches.
Mixing and placement
A 60 pound bag turns cleanly in a wheelbarrow or small electric mixer in about 15 minutes. A 28 bag pour is a half-day job for two people, so stage the bags, water and tools first. Work from one corner, consolidate with a shovel or vibrator, and screed before the mix stiffens.
Where a new slab will sit on an old one, the surface needs cleaning and a bonding treatment first; the sequence for how to pour concrete over an existing concrete slab is different from a fresh pour and worth following exactly.
Curing and timing
Concrete gains strength fastest in the first week. Keep the surface damp or covered for three to seven days and hold heavy loads off for a full week. Design strength arrives around 28 days, so schedule first use accordingly; in hot, dry weather, mist the surface to slow evaporation and reduce cracking.