How to Estimate Rock for a Building Pad
If you’re planning to build a shop, shed, mobile home pad, or any other type of foundation in the San Antonio area, one of the most important steps is calculating how much rock you need. Whether you’re using crushed limestone, caliche, road base, or gravel, estimating your material needs ahead of time can save you from over-ordering (or worse, running short halfway through the job). Most contractors recommend compacted limestone under slabs to prevent shifting. Here’s a great overview of concrete slab prep.
In this guide, we’ll walk you through how to estimate the right amount of rock for a building pad, using simple math, helpful tips, and some real-world examples from our work here at Shady Oaks Express, your local source for bulk material delivery and site prep services in South Texas.
Step 1: Measure Your Pad Area
Start by measuring the length and width of the pad area in feet. Then decide how thick you want the rock layer to be. Most building pads use between 4 to 8 inches of base material, depending on soil conditions and what you’re building.
Example: A 20-foot by 30-foot building pad, with 6 inches of crushed limestone base.
Step 2: Convert Inches to Feet
To calculate volume in cubic feet, you’ll need to convert the depth (in inches) into feet.
4 inches = 0.33 feet
6 inches = 0.5 feet
8 inches = 0.67 feet
In our example: 20 x 30 x 0.5 = 300 cubic feet
Step 3: Convert Cubic Feet to Cubic Yards
Rock and fill material are usually sold by the cubic yard, not cubic feet. There are 27 cubic feet in a cubic yard, so divide the number of cubic feet by 27.
300 ÷ 27 = 11.11 cubic yards
It’s always smart to round up and allow for compaction, spreading, or minor grading corrections, so we’d recommend ordering 12 cubic yards in this case.
Step 4: Adjust for Soil Type and Purpose
If you're building on expansive clay soil, sandy terrain, or uneven ground, you may want to lay down more base material to create a strong, level foundation. For mobile home pads, metal buildings, or RV pads, many customers go with 6-8 inches of compacted road base to handle the weight and help with drainage.
We had a customer in Somerset who called in for a 24' x 24' shop pad. They thought 10 yards would do the trick, but once we ran the math, they actually needed closer to 14 yards to account for the extra depth and some sloping in the terrain. A quick adjustment before delivery saved a return trip and kept the job on track.
Quick Estimation Formula
You can use this formula for any size pad:
Length (ft) x Width (ft) x Depth (inches ÷ 12) ÷ 27 = Cubic Yards Needed
This works for all base materials, including gravel, crushed limestone, caliche, and select fill.
Or just give us a call and we’ll do the math for you—we help customers estimate rock daily.
Final Thoughts
Ordering the right amount of rock, gravel, or base material for your building pad doesn’t have to be a guessing game. A little measuring and a quick calculation can make your site preparation go much smoother.
At Shady Oaks Express, we’ll help you estimate the amount of material you need and deliver it right to your job site. Need it spread and leveled? We also offer Bobcat/skid steer services, so you don’t have to lift a shovel.
Give us a call at 210-842-1440 or request a quote online. We provide material delivery, gravel spreading, and site prep work across the San Antonio area and surrounding counties.
Related Posts:
Skid Steer Services for Site Prep: What to Expect
Top 5 Uses for Crushed Limestone in San Antonio
Gravel vs Caliche: What’s Better for Your Driveway?
