How To Calculate Fill Dirt On A Slope

How to Calculate Fill Dirt for a Slope (Accurately and Easily)

Filling in a slope isn’t just about dumping dirt—it’s about getting the right amount, accounting for settling, and making sure your base is stable. Whether you’re leveling a backyard, grading a construction site, or prepping for a foundation, knowing how much fill dirt to order will save time, money, and stress.

Here’s a clear, step-by-step method to calculate how much fill dirt you’ll need—plus how to adjust for slope shape and compaction.

What Is Fill Dirt, Exactly?

Fill dirt is the dense, mineral-rich layer of subsoil found beneath topsoil. Unlike topsoil, it contains little to no organic material, making it more stable and compactable. That’s why it’s ideal for:

• Raising low areas

• Leveling sloped land

• Supporting patios, roads, and building pads

• Filling behind retaining walls

Step-by-Step: Calculating Fill Dirt on a Slope

Step 1: Measure the Slope Area

Start by collecting three basic measurements:

• Length (L) – horizontal distance across the slope

• Width (W) – left to right, perpendicular to the slope

• Height (H) – vertical rise from bottom to top

If you’re leveling from the bottom to a flat top edge, you’re working with a triangular cross-section—basically a wedge.

Step 2: Use the Wedge Volume Formula

Use this formula to estimate the amount of fill dirt needed:

Cubic Feet = 0.5 × Length × Width × Height
Cubic Yards = (0.5 × L × W × H) ÷ 27

Example:

A slope that’s:

• 40 feet long

• 15 feet wide

• 4 feet high at the deepest point

Calculation:

(0.5 × 40 × 15 × 4) ÷ 27 = 1,200 ÷ 27 = 44.4 cubic yards

You’ll need about 44.4 yards of fill dirt before accounting for compaction.

Step 3: Account for Compaction

Fill dirt settles after it’s placed and compacted. To avoid falling short, increase your total based on a compaction factor.

Typical compaction allowances:

• 10–15% for screened fill

• Up to 25% for loose or heavy clay soils

Formula:
Adjusted Volume = Raw Volume × (1 + Compaction Rate)

Example (15% compaction):
44.4 × 1.15 = 51.1 cubic yards

Order Fill Dirt the Easy Way

At Hello Gravel, we simplify the process from calculation to delivery. No chasing quotes. No back-and-forth. Just the right materials delivered directly to your site.

Get a Quote Today

We offer:

• Clean, compactable fill

• Honest pricing and fast delivery

• No-stress online ordering

Measure the area → Let us handle the rest.