When a project spec says “stabilize at 4 percent” or “add 5 percent cement,” your spreader controller usually needs a surface spread rate in pounds per square yard. Here is a simple, reliable way to convert that percentage into the number your controller expects.
The Core Formula
Spread rate (lb/yd²) = 0.75 × T × D × P
Where
- T = treatment thickness in inches
- D = average dry density of the in-place soil, in lb/ft³
- P = stabilizer percentage, as a decimal (5% becomes 0.05)
This relationship is the standard field shortcut for turning a design percentage into a surface target. The 0.75 constant handles the unit conversions between inches, feet, and square yards.
Why the 0.75 works
Depth in inches is T/12 feet. Converting from square feet to square yards multiplies by 9. So 9 × (T/12) simplifies to 0.75 × T. That factor carries the unit math for you, which is why the field formula stays compact.
Typical inputs
- Thickness (T): use the planned stabilization depth in inches taken from the design or preconstruction plan.
- Dry density (D): if a project-specific value is not provided, a common planning value is 110 lb/ft³. Use lab data when available.
- Percentage (P): convert percent to a decimal. Example 5 percent becomes 0.05.
A quick field shortcut when D = 110 lb/ft³
With D set to 110 lb/ft³, the formula collapses to:
Spread rate = 82.5 × T × P
since 0.75 × 110 = 82.5. This is handy for mental math.
Worked example
Task: 5 percent Portland cement at a 12 inch treatment depth.
Solution: Spread rate = 0.75 × 12 × 110 × 0.05 = 49.5 lb/yd².
Enter 49.5 lb/yd² into the controller as your target rate.
Mini Lookup Table (Planning Values)
Using D = 110 lb/ft³
| Depth (in) | 3% | 4% | 5% | 6% | 7% | 8% |
| 6 | 14.9 | 19.8 | 24.8 | 29.7 | 34.7 | 39.6 |
| 12 | 29.7 | 39.6 | 49.5 | 59.4 | 69.3 | 79.2 |
All numbers are lb/yd². Use project-specific density if supplied, which will scale the results.
Practical Tips
- Use the right density: If your geotechnical report provides a representative dry density, plug that in for D rather than the 110 lb/ft³ planning value.
- Confirm thickness: Measure or verify the actual stabilization depth. If the milling or mixing depth varies, adjust T accordingly.
- Convert percentage correctly: 4 percent is 0.04. This is the most common input mistake.
- Document your target: Keep a note of T, D, and P so everyone on the crew knows where the number came from.
- Spot check in the field: After a load, verify that your actual application aligns with the target by checking material used against the area covered. This keeps calibration tight as conditions change during the day.
Summary
When you are given a percentage, convert it to a surface spread rate with Spread rate = 0.75 × T × D × P. Start with the planned depth, a representative dry density, and the decimal form of the percentage. Enter the resulting lb/yd² into the controller and you are set for a clean, verifiable application.