How to Calculate Bale Density
Why Does Bale Density Matter?
When constructing a straw bale home, it’s important to make sure the straw bales are dense enough to meet structural and energy performance needs. According to Appendix BJ of the 2024 International Residential Code (IRC), straw bales used in wall construction must have a minimum dry density of 6.5 pounds per cubic foot. That means we have to take moisture content into account when doing the math.
In this guide, we’ll break down how to calculate bale minimum dry density so you can determine whether your bales meet the necessary specifications for a safe and durable structure.
Understanding Bale Density Requirements
Straw bale density is determined by the size, weight and moisture content of the bale. The density you need depends on:
Building Codes: The 2024 IRC requires a minimum dry density of 6.5 pounds per cubic foot for straw bales used in wall construction.
Structural Strength: Higher density means better compression resistance and stability.
Thermal Insulation: Proper bale density helps maintain energy efficiency and prevents settling over time.
How to Calculate Bale Density
To determine bale density, use the following formula:
Dry Density = (Measured Weight ÷ Volume) ÷ (1 + Moisture Content)
This formula accounts for the moisture in the bale and gives you the dry density, which is what the code requires.
Below, we’ll walk through the same process in smaller steps for clarity.
Step 1: Measure the Bale
Start by measuring the bale’s width, height, and length. In this example, we’ll use the dimensions of a typical two-string bale.
Length: 36 inches
Width: 18 inches
Height: 14 inches
Multiply those together:
36 × 18 × 14 = 9,072 cubic inches
Then convert to cubic feet:
9,072 ÷ 1,728 = 5.25 cubic feet
Step 2: Weigh the Bale
Next, weigh the bale. In this example, the bale weighs 45 pounds.
Step 3: Measure Moisture Content
Use a moisture meter to determine the bale’s moisture content by weight. Let’s say the bale is at 10% moisture.
Step 4: Calculate Dry Density
Now plug everything into the formula:
Dry Density = (45 ÷ 5.25) ÷ 1.10 = 7.79 pounds per cubic foot
This result represents the dry weight density of the bale after adjusting for moisture content.
What This Tells Us
This bale has a dry density of 7.79 pcf, which is well above the 6.5 pcf minimum required by code. That means it’s good to go. ✅
Not sure if your bales meet code? Check out our Straw Bale Building Codes blog and follow links from there to the IRC Appendix on Straw Bale Construction.
Final Thoughts
If you’re building with straw bales, be sure to:
Measure the bale’s volume
Weigh the bale
Test for moisture
Use the dry weight in your density calculation
Even if a bale feels heavy, it might fall short if it holds too much moisture. On the other hand, a smaller, lighter bale can pass if it’s dry and compact. That’s why doing the math matters. It gives you confidence that your bales meet code and will perform well in the wall.
If your density calculation comes in under 6.5 lbs/ft³, the bales may compress too much under load or fall short of code requirements. In that case, you may need to find denser bales, retie what you have, or adjust your building approach.
Need to learn how to retie bales efficiently and effectively? A key skill in straw bale construction for sure! Join us at a workshop for hands-on learning and until then, practice for yourself using this guide: Straw Bale Construction - How to Retie to Custom Sizes.
Stuck in Density?
Understanding bale density is one thing—ensuring your entire structure meets engineering and code requirements is another.
If you want expert guidance on choosing the right bales, verifying density, or planning a structurally sound straw bale home, consulting a professional is the best way to ensure a safe, durable, and code-compliant build. I’m happy to support your project in any way I can by answering your (or your builders!) questions to short cut your learning curve and save you time and money, or help you with initial design concepts.
👉 Learn more about scheduling a Consultation with me
Bale on,
Timbo
UPDATED 03/2025
