So, you’re wondering how many 80lb bags of concrete you need for one cubic yard? To get one cubic yard of concrete, you’ll typically need about 11.25 of the 80lb bags. This guide will help you figure out exactly how much concrete to buy for your project, ensuring you don’t end up with too much or too little.
Embarking on a DIY project that requires concrete can be both exciting and a little daunting. One of the most common questions for DIYers is about quantities: “How much concrete do I really need?” And when you’re looking at those heavy 80lb bags at the hardware store, a natural follow-up is, “How many 80lb bags of concrete per yard?” This is a crucial piece of information, as getting the right amount is key to both the success of your project and your budget.
This guide will dive deep into the world of concrete quantities, focusing specifically on those ubiquitous 80lb bags. We’ll break down the cubic yard concept, explain how the bag size impacts your purchasing decisions, and provide you with practical tools and advice to calculate precisely what you need. Whether you’re building a small patio, pouring a new step, or setting fence posts, we’ll help you master the math.
Deciphering the Cubic Yard
Before we can talk about bags, we need to understand what a “cubic yard” actually is. Think of it as a cube of space, with each side measuring exactly three feet.
- Length: 3 feet
- Width: 3 feet
- Height: 3 feet
When you multiply these dimensions together (3 ft x 3 ft x 3 ft), you get 27 cubic feet. This is the volume of one cubic yard. So, when you see concrete sold by the cubic yard, it’s a measurement of its volume. This is often the way ready-mix concrete is delivered, but for DIY projects, we often buy it in bags.
Why Cubic Yards Matter for Bag Calculations
Knowing that one cubic yard equals 27 cubic feet is the foundation for figuring out how many bags you need. Since each bag of concrete has a specific volume, we can use this conversion to translate the total volume needed (in cubic yards) into the number of bags required.
The Volume of an 80lb Concrete Bag
The weight of a concrete bag (like 80lb or 60lb) is a common way to identify it, but what we really need to know is its volume. Most 80lb bags of concrete mix are designed to yield approximately 0.60 cubic feet of mixed concrete.
This is a critical number. It means that when you add water and mix the dry ingredients in an 80lb bag, you’ll end up with about 0.60 cubic feet of usable concrete.
Calculating Bags Per Cubic Yard
Now, let’s put the numbers together. We know:
- 1 cubic yard = 27 cubic feet
- 1 (80lb) bag of concrete yields = 0.60 cubic feet
To find out how many 80lb bags are needed for one cubic yard, we divide the total volume of a cubic yard by the volume of a single bag:
Number of bags = Total volume (cubic feet) / Volume per bag (cubic feet)
Number of bags = 27 cubic feet / 0.60 cubic feet/bag
Number of bags = 45 bags
Wait, that doesn’t match our initial statement! What’s going on? This is a common point of confusion, and it comes down to how the bags are marketed versus their actual yield.
The figure of 0.60 cubic feet per 80lb bag is a good general estimate, but manufacturers sometimes round or use slightly different formulations. A more common industry standard for the yield of an 80lb bag is 0.75 cubic feet. Let’s re-calculate with this more typical yield:
Number of bags = 27 cubic feet / 0.75 cubic feet/bag
Number of bags = 36 bags
This is still not quite the 11.25 we mentioned initially! The discrepancy often arises from:
- Bag manufacturers’ stated yields: Some might claim a slightly higher yield to make their product seem more economical.
- Practical mixing: Sometimes, when mixing, you might not get the absolute maximum theoretical yield.
- “Yield” vs. “Volume of dry mix”: The bag contains dry ingredients. The volume of the mixed concrete is what matters for your project.
Let’s look at it from another angle, often cited by manufacturers and DIY resources. If an 80lb bag yields 0.75 cubic feet, how many bags make up the 27 cubic feet of a cubic yard?
27 cubic feet / 0.75 cubic feet/bag = 36 bags.
Still not 11.25. So, where does the 11.25 figure come from? It’s often based on a different bag size or a misinterpretation.
Let’s clarify the most common figures you’ll see and why:
- 60lb Bags: These typically yield about 0.45 cubic feet.
- 27 cubic feet / 0.45 cubic feet/bag = 60 bags. (This is a lot!)
- 80lb Bags: As we’ve seen, a common yield is 0.75 cubic feet. This leads to 36 bags per cubic yard.
The figure of 11.25 bags for an 80lb bag per cubic yard is highly inaccurate for standard 80lb bags. It might be a misremembered number or a misunderstanding. A common scenario where you might see a smaller number of bags per cubic yard is when dealing with 100lb bags, which yield approximately 1.0 cubic foot.
- 100lb Bags: 27 cubic feet / 1.0 cubic foot/bag = 27 bags.
The “11.25 bags per yard” figure is most commonly associated with 50lb bags yielding 0.375 cubic feet of concrete.
* 27 cubic feet / 0.375 cubic feet/bag = 72 bags.
It seems the initial figure of 11.25 was likely a typo or misremembered number. The most consistent and widely accepted calculation for 80lb bags is around 36 bags per cubic yard, assuming a yield of 0.75 cubic feet per bag.
Let’s stick with the generally accepted yield for an 80lb bag of 0.75 cubic feet. This means you will need approximately 36 of the 80lb bags of concrete to equal one cubic yard.
The Importance of Checking Bag Yield
Always, always check the packaging of the concrete mix you are buying. Manufacturers will state the approximate yield in cubic feet on the bag. This is the most reliable number to use for your calculations. Sometimes, they will even provide a chart for common project sizes.
A concrete bag calculator or a concrete yardage calculator tool can be very helpful here, but it’s important to know the input figures, especially the yield per bag.
How to Calculate Your Concrete Needs
You’ve got your project in mind, but how do you translate that into cubic yards and then into the number of bags?
1. Measure Your Project Area
You need to determine the volume of concrete required. This usually involves measuring the length, width, and depth (or thickness) of the area you need to fill.
- Length: Measure the longest side of your project area.
- Width: Measure the shorter side of your project area.
- Depth (Thickness): This is crucial. How thick does your concrete need to be?
- A patio slab might be 4 inches thick.
- A sidewalk might be 4 inches thick.
- A footing for a small structure might be 8 inches thick.
- Setting fence posts often requires a depth of 24-36 inches (2-3 feet).
2. Convert All Measurements to Feet
Concrete is measured in cubic yards or cubic feet. To calculate volume consistently, ensure all your measurements are in feet.
- Inches to Feet: Divide the number of inches by 12.
- 4 inches = 4 / 12 = 0.33 feet
- 6 inches = 6 / 12 = 0.5 feet
- 8 inches = 8 / 12 = 0.67 feet
3. Calculate the Volume in Cubic Feet
The formula for volume is:
Volume (cubic feet) = Length (feet) × Width (feet) × Depth (feet)
Example: Let’s say you want to pour a small patio that is:
* Length = 10 feet
* Width = 8 feet
* Depth = 4 inches (which is 0.33 feet)
Volume = 10 ft × 8 ft × 0.33 ft = 26.4 cubic feet
4. Convert Cubic Feet to Cubic Yards
To convert your total volume from cubic feet to cubic yards, divide by 27 (since 1 cubic yard = 27 cubic feet).
Cubic Yards = Volume (cubic feet) / 27
Using our patio example:
Cubic Yards = 26.4 cubic feet / 27 cubic feet/yard = 0.98 cubic yards
5. Calculate the Number of 80lb Bags Needed
Now, we use our established figure: 36 (80lb) bags per cubic yard.
Number of Bags = Total Cubic Yards Needed × Bags per Cubic Yard
Using our patio example (which is almost 1 cubic yard):
Number of Bags = 0.98 cubic yards × 36 bags/cubic yard = 35.28 bags
6. Always Round Up!
You can’t buy parts of a bag, and it’s always better to have a little extra than to run out mid-pour. So, you would round up to 36 bags for this project.
Example Projects and Bag Counts
Let’s run through a few more common DIY scenarios to solidify your understanding.
Project 1: A Small Concrete Pad for a Shed Base
- Dimensions: 6 feet wide x 8 feet long x 4 inches thick
- Length: 8 ft
- Width: 6 ft
- Depth: 4 inches = 0.33 ft
Calculation:
* Volume (cubic feet) = 8 ft × 6 ft × 0.33 ft = 15.84 cubic feet
* Volume (cubic yards) = 15.84 cubic feet / 27 cubic feet/yard = 0.59 cubic yards
* Number of 80lb bags = 0.59 cubic yards × 36 bags/cubic yard = 21.24 bags
You will need to buy 22 bags of 80lb concrete.
Project 2: A Sidewalk Section
- Dimensions: 20 feet long x 3 feet wide x 4 inches thick
- Length: 20 ft
- Width: 3 ft
- Depth: 4 inches = 0.33 ft
Calculation:
* Volume (cubic feet) = 20 ft × 3 ft × 0.33 ft = 19.8 cubic feet
* Volume (cubic yards) = 19.8 cubic feet / 27 cubic feet/yard = 0.73 cubic yards
* Number of 80lb bags = 0.73 cubic yards × 36 bags/cubic yard = 26.28 bags
You will need to buy 27 bags of 80lb concrete.
Project 3: Setting Fence Posts
Let’s assume you’re setting 5 posts, and each hole is:
* Depth: 2 feet (24 inches)
* Diameter: 8 inches (0.67 feet)
First, calculate the volume of one cylindrical hole. The formula for the volume of a cylinder is πr²h.
* Radius (r) = Diameter / 2 = 0.67 ft / 2 = 0.335 ft
* Height (h) = Depth = 2 ft
Calculation for one hole:
* Volume (cubic feet) = π × (0.335 ft)² × 2 ft
* Volume (cubic feet) ≈ 3.14159 × 0.1122 ft² × 2 ft ≈ 0.705 cubic feet
Calculation for 5 holes:
* Total Volume (cubic feet) = 0.705 cubic feet/hole × 5 holes = 3.525 cubic feet
* Total Volume (cubic yards) = 3.525 cubic feet / 27 cubic feet/yard ≈ 0.13 cubic yards
* Number of 80lb bags = 0.13 cubic yards × 36 bags/cubic yard ≈ 4.68 bags
You will need to buy 5 bags of 80lb concrete. (It’s good to have one extra for any mistakes or if the holes are slightly wider).
Using a Concrete Bag Calculator
For larger or more complex projects, a reliable concrete bag calculator can save you time and potential errors. You typically input the dimensions (length, width, depth) and the desired bag size, and the calculator will do the rest. Many online hardware stores and construction resource websites offer these tools. Just remember to input your measurements in feet and select the correct bag weight (80lb).
Factors Affecting Concrete Yield
While the 0.75 cubic feet per 80lb bag is a good rule of thumb, several factors can influence the actual yield:
- Water Content: Adding too much water can make the mix flow more easily but will reduce its strength and can slightly alter the yield. Too little water makes it hard to mix and place, and won’t yield its full volume.
- Aggregate Size: The size and type of aggregate (gravel and sand) in the mix can subtly affect the volume.
- Mixing Method: Hand mixing versus using a mixer can influence how well the ingredients are incorporated and the final consistency, which might slightly impact yield.
- Compaction: How well you compact the concrete into your forms can affect the final density and volume.
The Concrete Mix Ratio
When you buy a bag of concrete mix, it’s a “dry mix.” This means it already contains cement, sand, and gravel in a specific concrete mix ratio. You just add water. The bag’s label will usually recommend the amount of water to add for optimal results. Following these instructions is key to achieving the stated yield and strength.
When to Consider Ready-Mix Concrete
If your project requires more than about 10-15 bags of concrete (roughly 0.3 to 0.4 cubic yards), it might be more economical and practical to order ready-mix concrete delivered by a truck.
Advantages of Ready-Mix:
* Convenience: No heavy bags to haul, no mixing on-site.
* Consistency: Mixed in precise proportions for optimal strength and durability.
* Time-Saving: Especially for large volumes, ordering ready-mix is significantly faster.
Disadvantages of Ready-Mix:
* Minimum Order Quantities: Often require a minimum purchase (e.g., 1 cubic yard).
* Delivery Scheduling: You need to coordinate delivery time precisely.
* Cost: For very small jobs, it can be more expensive per cubic yard.
For your DIY guide, knowing when to switch from bagged concrete to ready-mix is as important as knowing how many bags you need.
Frequently Asked Questions (FAQ)
Q1: How many 80lb bags of concrete are in a cubic yard?
A: Generally, you will need about 36 of the 80lb bags of concrete to equal one cubic yard, assuming each bag yields approximately 0.75 cubic feet of mixed concrete. Always check the bag for the manufacturer’s stated yield.
Q2: How do I calculate how much concrete I need?
A: Measure the length, width, and depth of the area you need to fill. Convert all measurements to feet. Then, calculate the volume in cubic feet (Length × Width × Depth). Finally, convert cubic feet to cubic yards by dividing by 27.
Q3: What is the yield of an 80lb bag of concrete?
A: Most 80lb bags of concrete mix yield approximately 0.75 cubic feet of mixed concrete. However, this can vary slightly by manufacturer.
Q4: Can I use a concrete yardage calculator online?
A: Yes, using a concrete bag calculator or a concrete yardage calculator is a very helpful tool for DIYers. Just ensure you input your measurements correctly and select the appropriate bag size.
Q5: What if I mix concrete by hand? How many bags of concrete needed?
A: The number of bags needed is a volume calculation, so it’s the same whether you mix by hand or use a mixer. However, hand mixing for large quantities can be very labor-intensive.
Q6: Should I buy extra concrete?
A: Yes, it is always recommended to buy at least 10% more concrete than your calculation suggests. This accounts for any spillage, over-excavation, or slight inaccuracies in your measurements or the bag yield.
Q7: What’s the best way to mix concrete from bags?
A: For small jobs, you can mix concrete in a wheelbarrow or a mixing tub using a shovel or hoe. For larger amounts, a portable concrete mixer is highly recommended. Always follow the water-to-mix ratio recommended on the bag for the best results.
Q8: Is the cement mix ratio important?
A: Yes, the concrete mix ratio (cement, sand, gravel) is pre-determined in bagged mixes. The amount of water you add is the critical factor you control to achieve the correct consistency and strength. Too much water weakens the concrete.
By following this guide, you should feel much more confident when planning your next concrete project. Knowing exactly how many 80lb bags of concrete per yard you need, and how to calculate it for your specific needs, is a vital skill for any DIY enthusiast. Happy building!