You need approximately 4 to 5 bags of 90 lb concrete mix to create one cubic foot of concrete. To calculate how many 90 lb bags of concrete you’ll need for your project, you first need to determine the total volume of concrete required in cubic yards.
When you’re planning a concrete project, whether it’s a small patio, a sidewalk, or a larger slab, knowing the right amount of material to buy is crucial. Overbuying can lead to wasted money and excess product, while underbuying can halt your progress and lead to costly delays. A common question for DIYers and contractors alike is: How many 90 lb bags of concrete in a yard? Let’s break down the math and provide you with the tools to accurately estimate your concrete needs.
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Figuring Out Your Concrete Volume
Before we can get to the bags, we need to talk about volume. Concrete is typically measured in cubic yards. A “yard” in this context refers to a cubic yard, which is a measure of volume.
What is a cubic yard? A cubic yard is a unit of volume in the imperial and US customary systems. It’s equivalent to a cube that measures 3 feet on each side (3 feet x 3 feet x 3 feet).
To figure out how much concrete to buy, you must first calculate the volume of concrete your project requires. This usually involves measuring the length, width, and depth (or thickness) of the area you’ll be pouring.
Measuring Your Project Area
Let’s say you’re pouring a concrete slab. You’ll need to measure:
- Length: In feet.
- Width: In feet.
- Thickness (Depth): In inches.
Important Note: You’ll need to convert your thickness measurement from inches to feet for the calculation. To do this, divide the number of inches by 12 (since there are 12 inches in a foot).
Example: If your slab is 10 feet long, 8 feet wide, and 4 inches thick:
- Length = 10 feet
- Width = 8 feet
- Thickness = 4 inches / 12 inches/foot = 0.333 feet (approximately)
Calculating the Volume in Cubic Feet
Once you have all your measurements in feet, you can calculate the volume in cubic feet:
Volume (cubic feet) = Length (feet) x Width (feet) x Thickness (feet)
Using our example:
Volume = 10 ft x 8 ft x 0.333 ft = 26.64 cubic feet
Converting Cubic Feet to Cubic Yards
Most ready-mix concrete is sold by the cubic yard, and bag counts are typically based on this unit. So, you’ll need to convert your cubic feet measurement to cubic yards.
There are 27 cubic feet in 1 cubic yard (3 ft x 3 ft x 3 ft = 27 cu ft).
Volume (cubic yards) = Volume (cubic feet) / 27
Using our example:
Volume = 26.64 cubic feet / 27 = 0.987 cubic yards
This means your project requires just under one cubic yard of concrete.
Figuring Out Bag Counts: The Core Calculation
Now that we know how to calculate the volume of concrete needed, let’s address the main question: How many 90 lb bags of concrete in a yard?
This question is often posed by those who are buying concrete in bags rather than ordering from a ready-mix truck. The yield of a bag of concrete mix is what we need to determine.
Concrete Mix Yield
A standard 90 lb bag of concrete mix typically yields about 0.75 cubic feet of concrete. This means that one bag, when mixed with water according to the manufacturer’s instructions, will produce approximately 0.75 cubic feet of hardened concrete.
Bags Per Cubic Foot
To find out how many bags it takes for one cubic foot, we can do a simple division:
Bags per cubic foot = 1 cubic foot / Yield per bag
Bags per cubic foot = 1 cu ft / 0.75 cu ft/bag = 1.33 bags per cubic foot.
So, roughly 1.33 bags of 90 lb concrete are needed for every cubic foot of concrete.
Bags Per Cubic Yard
Now, let’s scale this up to a cubic yard. Since there are 27 cubic feet in a cubic yard:
Bags per cubic yard = Bags per cubic foot x 27 cubic feet/yard
Bags per cubic yard = 1.33 bags/cu ft x 27 cu ft/yard = 35.91 bags.
Therefore, you need approximately 36 bags of 90 lb concrete mix to make one cubic yard of concrete.
Putting It All Together: Estimating Bags for Your Project
Using our previous example of a slab requiring 0.987 cubic yards:
Total bags needed = Required volume (cubic yards) x Bags per cubic yard
Total bags needed = 0.987 cubic yards x 36 bags/cubic yard = 35.53 bags.
Since you can’t buy half a bag, you’d round up and purchase 36 bags of 90 lb concrete mix for this project.
Factors Affecting Concrete Bag Requirements
While the 36 bags per cubic yard figure is a good starting point, several factors can slightly influence the actual number of bags you’ll need.
Bag Size Variations
It’s important to note that bag sizes can vary slightly by manufacturer. While 90 lb is common, you might also find 80 lb or 60 lb bags. The calculation must be adjusted based on the specific bag size you are purchasing.
- 80 lb bags: These typically yield about 0.60 cubic feet.
- Bags per cubic foot = 1 / 0.60 = 1.67 bags
- Bags per cubic yard = 1.67 x 27 = 45 bags (approx.)
- 60 lb bags: These typically yield about 0.45 cubic feet.
- Bags per cubic foot = 1 / 0.45 = 2.22 bags
- Bags per cubic yard = 2.22 x 27 = 60 bags (approx.)
Water Content and Mixing Consistency
The amount of water you add to the mix can affect the final volume slightly. Using too much water can reduce the strength and density of the concrete, and while it might slightly increase the volume, it’s not recommended for structural integrity. Always follow the manufacturer’s recommended water-to-mix ratio.
Compaction and Surface Finish
The way the concrete is vibrated or tamped can also affect the final density and, thus, the volume. Proper compaction removes air pockets. For most DIY projects like slabs, this is less of a factor than in large commercial pours.
Waste and Spillage
It’s always wise to add a small buffer to your estimate to account for potential waste, spillage, or slight inaccuracies in measurement. A good rule of thumb is to add 5-10% extra.
For our 0.987 cubic yard example:
- 10% buffer = 0.987 cubic yards x 0.10 = 0.0987 cubic yards
- Total volume with buffer = 0.987 + 0.0987 = 1.0857 cubic yards
- Total bags with buffer = 1.0857 cubic yards x 36 bags/cubic yard = 39.08 bags.
- Rounding up, you’d buy 40 bags to be safe.
Using a Concrete Bag Calculator
To simplify the process, many online tools and apps offer a concrete bag calculator. These calculators typically ask for your project’s dimensions (length, width, thickness) and the size of the bags you plan to use, and they automatically provide an estimate of the number of bags required. This is a great way to double-check your own calculations.
When using a calculator, ensure you input your measurements in the correct units (usually feet for length and width, and inches for thickness, with automatic conversion).
Example: Estimating Concrete for a Slab
Let’s walk through another common scenario: pouring concrete for a slab for a small garden shed.
Project Dimensions:
- Slab Length: 12 feet
- Slab Width: 10 feet
- Slab Thickness: 4 inches
Step 1: Convert Thickness to Feet
Thickness = 4 inches / 12 inches/foot = 0.333 feet
Step 2: Calculate Volume in Cubic Feet
Volume (cu ft) = 12 ft x 10 ft x 0.333 ft = 39.96 cubic feet
Step 3: Convert Volume to Cubic Yards
Volume (cu yd) = 39.96 cu ft / 27 cu ft/cu yd = 1.48 cubic yards
Step 4: Calculate Number of 90 lb Bags
Using 36 bags per cubic yard:
Bags needed = 1.48 cu yd x 36 bags/cu yd = 53.28 bags
Step 5: Add a Buffer (e.g., 10%)
Buffer = 1.48 cu yd x 0.10 = 0.148 cu yd
Total volume = 1.48 + 0.148 = 1.628 cu yd
Total bags = 1.628 cu yd x 36 bags/cu yd = 58.6 bags
Conclusion for the Shed Slab: You should purchase approximately 59 bags of 90 lb concrete mix.
Common Concrete Projects and Bag Estimates
Here’s a quick look at common DIY projects and their approximate bag needs (using 90 lb bags):
| Project Type | Common Size Example | Calculated Volume (cu yd) | Estimated 90 lb Bags (approx.) | Estimated 90 lb Bags (with 10% buffer) |
|---|---|---|---|---|
| Small Patio | 10 ft x 10 ft x 4 in | 0.37 cu yd | 13 | 14 |
| Sidewalk (3 ft wide) | 30 ft x 3 ft x 4 in | 0.33 cu yd | 12 | 13 |
| Garden Bed Edging | 20 ft x 6 in x 4 in (per section) | ~0.02 cu yd per section | <1 per section | <1 per section |
| Small Shed Foundation | 12 ft x 10 ft x 4 in | 1.48 cu yd | 53 | 59 |
| Single Car Driveway | 18 ft x 10 ft x 4 in | 2.22 cu yd | 80 | 88 |
| Standard Concrete Steps | 3 ft wide, 1 ft rise, 1 ft tread | ~0.05 cu yd per step | ~2 per step | ~2-3 per step |
These are estimates. Always calculate your specific project dimensions.
When to Consider Ready-Mix Concrete
For larger projects, such as pouring a full driveway or a substantial patio, relying on bagged concrete can become impractical and costly. Hauling dozens or hundreds of bags, mixing them individually, and ensuring consistency can be challenging.
When does ready-mix become a better option?
- Volume: Projects requiring more than 1 cubic yard are often more efficiently handled with ready-mix.
- Consistency: Ready-mix plants have precise control over the mix proportions, ensuring consistent quality and strength.
- Labor Savings: A concrete truck and crew can pour and finish a large area much faster than a DIYer with bags.
- Cost-Effectiveness: For larger volumes, the cost per cubic yard of ready-mix can be less than the equivalent bagged material, especially when you factor in the labor and potential waste with bags.
If your concrete mix calculation shows you need 1.5 cubic yards or more, it’s worth getting a quote from a ready-mix supplier.
The Process of Pouring Concrete
Whether you use bags or ready-mix, the fundamental process of pouring concrete involves several key steps:
- Preparation: Excavate the area, compact the subgrade, and set up forms to contain the wet concrete. Ensure proper drainage.
- Reinforcement: For slabs or structural elements, install rebar or wire mesh to add tensile strength.
- Mixing (if using bags): This is a crucial step. You’ll need a wheelbarrow or a concrete mixer. Add the dry mix and water according to instructions, mixing thoroughly until you achieve a consistent, workable texture. Avoid adding too much water.
- Placement: Pour or shovel the wet concrete into the forms. Start at one end and work your way across.
- Screeding: Use a straight edge (like a 2×4) to level the concrete, cutting off high spots and filling low spots.
- Floating: After screeding, use a float to further smooth the surface and embed larger aggregate.
- Edging and Grooving: Create rounded edges and control joints to prevent cracking.
- Finishing: Use a trowel for a smooth finish or other tools for textured surfaces.
- Curing: This is the process where concrete hardens and gains strength through a chemical reaction. Keep the concrete moist for several days, especially in hot or windy weather. This is critical for durability.
Frequently Asked Questions (FAQ)
Q1: How many 90 lb bags of concrete do I need for a 4×8 foot slab that is 4 inches thick?
A1: First, convert the thickness to feet: 4 inches / 12 = 0.333 feet. Calculate the volume in cubic feet: 4 ft x 8 ft x 0.333 ft = 10.66 cubic feet. Convert to cubic yards: 10.66 cu ft / 27 cu ft/cu yd = 0.395 cubic yards. Using our estimate of 36 bags per cubic yard, you’ll need approximately 0.395 x 36 = 14.2 bags. It’s best to round up and buy 15 bags of 90 lb concrete mix.
Q2: Can I mix concrete in a wheelbarrow?
A2: Yes, you can mix concrete in a wheelbarrow for small projects that require only one or two bags at a time. For larger quantities, a concrete mixer is highly recommended as it makes the process much easier and ensures a more consistent mix.
Q3: What is the best way to estimate concrete for a complex shape?
A3: For irregularly shaped areas, you’ll need to break the area down into simpler geometric shapes (rectangles, squares, circles, triangles) and calculate the volume of each section separately. Then, add all the volumes together. Alternatively, you can use a concrete bag calculator that allows for more complex shapes or contact a professional.
Q4: How much does a 90 lb bag of concrete cost?
A4: The cost of a 90 lb bag of concrete mix can vary significantly depending on the brand, type of mix (e.g., standard concrete, mortar, topping mix), and your location. Generally, you can expect to pay between $5 to $10 per 90 lb bag. Always check with your local hardware or building supply store for current pricing.
Q5: What happens if I use too much water in my concrete mix?
A5: Using too much water weakens the concrete and can lead to cracking, spalling, and reduced durability. It also increases the amount of shrinkage as the concrete dries. Always follow the manufacturer’s recommended water-to-mix ratio on the bag.
Q6: How do I calculate the volume of concrete for a circular patio?
A6: To calculate the volume of concrete for a circular patio, you’ll need the radius (half the diameter) and the thickness.
First, find the area of the circle: Area = π * radius² (where π is approximately 3.14159).
Then, convert the thickness to feet.
Finally, calculate the volume in cubic feet: Volume (cu ft) = Area (sq ft) * Thickness (ft).
Once you have the volume in cubic feet, convert it to cubic yards by dividing by 27. Then, multiply by the number of bags per cubic yard.
Q7: Is it cheaper to buy bagged concrete or ready-mix for a small project?
A7: For very small projects (e.g., a few cubic feet), bagged concrete is usually more convenient and cost-effective. However, as the volume of concrete increases, the cost per cubic yard of bagged concrete can start to exceed that of ready-mix, not to mention the significant labor involved in mixing many bags.
Q8: What is the difference between concrete and cement?
A8: Cement is a binder, a key ingredient in concrete, but it is not concrete itself. Concrete is a composite material made from cement, aggregate (like sand and gravel), and water. When cement is mixed with water, it hardens and binds the aggregates together.
Q9: How to properly cure concrete?
A9: Proper curing is essential for achieving strong, durable concrete. It involves keeping the concrete moist for an extended period after placement. Methods include covering the surface with plastic sheeting, burlap bags kept constantly wet, or applying a liquid curing compound. The duration of curing depends on the mix, temperature, and humidity, but at least 3-7 days is generally recommended.
By following these calculations and considerations, you can confidently determine how many 90 lb bags of concrete you need in a yard for your next project, ensuring you have the right materials for success.