Ordering concrete for a small slab usually means counting bags, and the first question is always the same: how many 80 lb bags of concrete make a yard? The short answer is 45. One cubic yard contains 27 cubic feet, and an 80 lb bag of standard premixed concrete yields about 0.60 cubic feet when mixed, so 27 divided by 0.60 gives 45 bags. The longer answer depends on slab thickness, bag size, and how much waste you plan for. Whether you are pouring a foundation pad or installing decorative concrete floor and wall tiles, the same volume math applies, and getting it right keeps you from ending up with too little mix or a driveway full of leftover bags.
How Many 80 lb Bags Make a Yard
The 45-bag figure starts with the bag yield. An 80 lb bag of premixed concrete produces roughly 0.60 cubic feet of wet concrete, about 17 liters. A cubic yard is 27 cubic feet. Dividing 27 by 0.60 equals 45, so one yard of concrete takes 45 of the most popular bag size on the shelf. The same math scales to every bag size, and each size has its own yield.
Bag Sizes and Their Yields
Bagged concrete is sold in 40, 60, 80, and 90 pound sizes. The smaller bags are easier to handle on ladders and in tight spaces, but they multiply the number of trips to the mixer. The table below lists the common sizes with their yields and the bags needed for one cubic yard.
| Bag Size | Yield per Bag | Bags per Cubic Yard |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.60 cu ft | 45 |
| 90 lb | 0.68 cu ft | 40 |
The 90 lb bag works out to about 40 bags per yard, since 27 divided by 0.68 is 39.7. Bigger bags mean fewer bags to open and mix, which is one reason 80 lb bags dominate home-center shelves.
Why 27 Cubic Feet Is the Base Number
A cubic yard is a fixed volume: 3 feet by 3 feet by 3 feet. Concrete is priced and ordered by the yard in both bagged and ready-mix form, so every estimate starts by converting your project volume into cubic feet and then dividing by 27. Mixing units is the most common source of order errors, so keep everything in feet until the final step.
Volume math only gets you the right number of bags if the mix actually fills the form. Air pockets trapped around reinforcement shrink the effective volume of every bag, and the methods for consolidating concrete in tight members keep the placed volume close to the theoretical yield.
Coverage by Slab Thickness
The area an 80 lb bag covers depends entirely on slab thickness. The thicker the slab, the less area each bag fills. At 2 inches thick, one 80 lb bag fills 3.6 square feet. At 4 inches, it fills 1.8 square feet. At 6 inches, it fills 1.2 square feet. The same relationship works in reverse for a full yard: a yard covers 162 square feet at 2 inches, 81 square feet at 4 inches, and 54 square feet at 6 inches.
| Slab Thickness | Area per 80 lb Bag | Area per Cubic Yard |
|---|---|---|
| 2 in | 3.6 sq ft | 162 sq ft |
| 3 in | 2.4 sq ft | 108 sq ft |
| 4 in | 1.8 sq ft | 81 sq ft |
| 5 in | 1.4 sq ft | 65 sq ft |
| 6 in | 1.2 sq ft | 54 sq ft |
Reading the Coverage Table
Each row is the same bag of concrete spread over a different depth. At 2 inches the mix spreads thin, so one bag covers three times the area it covers at 6 inches. The table is handy for quick estimates on patios, walkways, and pads, where thickness is usually fixed at 4 inches.
Why Thickness Changes Area, Not Bag Count
A cubic yard is a yard at any thickness, so the bag count for a yard never changes; thickness only changes how much surface that yard covers. The common mistake is multiplying bags per yard by slab thickness, which double-counts the volume. Pick the thickness first, compute the area, and the yardage follows.
The Square Footage Shortcut
A faster shortcut skips yards entirely: divide the square footage by the area per bag at your thickness. For a 4-inch slab, 120 square feet divided by 1.8 gives 66.7 bags, the same answer as the five-step method in the next section. Use either path, but use one consistently.
Bag yield also assumes the mix inside the bag. Premixed bags carry a fixed proportion of cement, sand, and aggregate, and the concrete mix ratio sets how much paste each bag produces. Field-mixed concrete with a different ratio yields different volumes per bag, so read the bag label before assuming 0.60 cubic feet.
How to Estimate Bags for Your Project
Estimating bags for a real project takes five steps, and each one is a simple calculation.
The Five-Step Bag Estimate
- Measure the slab area in square feet, length times width.
- Choose the slab thickness in feet; 4 inches equals 0.333 feet.
- Multiply area by thickness to get the volume in cubic feet.
- Divide the cubic feet by 27 to get cubic yards.
- Multiply the yards by 45 for 80 lb bags, round up, and add 10 percent for waste.
Worked Example: A 10 by 12 Foot Pad
A 10 by 12 foot pad at 4 inches thick has 120 square feet of area. At 0.333 feet thick, the volume is 40 cubic feet, and 40 divided by 27 is 1.48 yards. At 45 bags per yard, that is 66.6 bags, so plan on 67 bags before waste. Adding 10 percent for spillage and uneven subgrade brings the order to about 74 bags.
Surface preparation can change the thickness you need. When a new slab goes over an existing one, the topping thickness depends on bonding and leveling, and the steps for pouring new concrete over an old surface explain how to size a bonded overlay without guessing at the depth.
Ordering, Waste, and Delivery Choices
Once the bag count is set, ordering decisions come down to waste, cost, and delivery. A 10 percent waste allowance covers spillage, uneven ground, and the partial bag that never quite empties. On small jobs, buying one extra bag beats running out mid-pour, because a cold joint in a slab is a permanent weak line.
The Ten Percent Waste Rule
The 10 percent rule is a starting point, not a ceiling. Rocky or soft subgrade eats mix as it levels the base, and forms that flex during the pour increase waste. For a 100-bag order, plan for 110; for a 10-bag order, the extra bag is the allowance.
Bags vs Ready-Mix on Cost
Cost changes the decision at larger volumes. An 80 lb bag runs about 4 to 6 dollars at most home centers, so a yard of bagged concrete costs roughly 180 to 270 dollars. Delivered ready-mix runs about 100 to 150 dollars per yard, and the gap widens with every yard. Most contractors switch to ready-mix somewhere between 1 and 2 yards, where the mixing labor and bag count stop making sense.
After the pour, verification matters as much as the estimate. A post-concrete inspection and testing routine checks the cured slab for strength, cracks, and honeycombing, and it tells you whether the thickness you ordered actually arrived in the form.
When Bagged Concrete Stops Being Practical
Bagged concrete is the right tool for pads, footings, and small repairs, but it has limits. Beyond a yard or two, the bag count, mixing time, and labor cost climb fast, and the quality of hand-mixed batches varies from one load to the next.
Frequent Errors on Small Pours
- Mixing units, such as ordering inches of thickness as feet
- Forgetting the slab depth in the volume calculation
- Rounding the bag count down instead of up
- Skipping the waste allowance on uneven subgrade
- Adding water beyond the label limit, which weakens the mix
Each error shifts the order by a few bags, and a few bags is exactly the margin that decides whether the pour finishes or stops short.
Structural Concrete Is Not a Bag Product
Engineered elements do not come from bags. Beams, columns, and precast members are batched to a specific design strength under controlled conditions, and the analysis that justifies prestressed concrete over reinforced alternatives assumes consistent, tested material. Bagged mix has its place in footings and flatwork, not in engineered members.
The bag math is straightforward: 27 cubic feet per yard, 0.60 cubic feet per 80 lb bag, 45 bags to the yard, and coverage set by thickness. One more distinction keeps the estimate honest. Plain bagged concrete is normal-strength material, and the difference between lean concrete and normal concrete matters when you need a low-strength bedding layer under footings or a structural slab above it. Measure the pad, fix the thickness, apply the waste rule, and the number of bags on your order will match the number of bags in your yard.