Methodology — How Our Calculators Work
This page documents every assumption, density figure, and conversion factor used by the calculators on Pea Gravel Calculator. If a result looks wrong to you, this page lets you check the underlying numbers and verify them against the primary sources we used.
Density Figures
Pea gravel density varies with stone composition (limestone vs granite vs quartz), moisture content, and how loosely the gravel is piled. Our calculators use these defaults:
| Quantity | Value Used | Source |
|---|---|---|
| Loose density (dry) | 100 lb/ft³ (1,602 kg/m³) | USGS Mineral Commodity Summaries |
| Tons per cubic yard | 1.35 US short tons | Derived: 100 lb/ft³ × 27 ft³/yd³ ÷ 2,000 lb/ton |
| Compacted density | 110 lb/ft³ | ASTM C29 (typical compacted-rodded value) |
| Standard bag size (US) | 0.5 ft³ (~50 lb) | Home Depot, Lowe's, Vigoro retail packaging |
| Standard bag size (UK) | 20 kg (~0.013 m³) | B&Q, Wickes retail packaging |
These figures are conservative — actual loose density of pea gravel ranges 95–105 lb/ft³ depending on stone composition and moisture. We use 100 lb/ft³ as the working default because it matches what most US retail suppliers print on their bags and what the USGS uses in its national aggregate statistics.
Recommended Depths
Depth recommendations come from three source categories: building codes, manufacturer technical specifications, and practical install guides published by major aggregate suppliers.
| Application | Recommended Depth | Source / Reasoning |
|---|---|---|
| Garden path or walkway | 2–3 inches | NRMCA decorative aggregate guide |
| Patio or seating area | 2–3 inches over base | Standard residential install practice |
| Driveway (passenger vehicles) | 4 in gravel + 4 in base | Aggregate supplier consensus; NACE gravel road specs |
| Driveway (trucks) | 6 in gravel + 6 in base | Heavier load distribution requirement |
| French drain | Fill trench fully around pipe | USDA NRCS drainage design guide |
| Playground surfacing | 9–12 inches loose | CPSC Public Playground Safety Handbook |
| Top-dressing flower beds | 1–2 inches | Standard residential practice |
The Formula
The core volume calculation is identical on every calculator page:
Cubic yards = Volume ÷ 27
Weight (lb) = Volume × 100 lb/ft³
Tons = Weight ÷ 2,000
Bags (50 lb / 0.5 ft³) = Volume ÷ 0.5, rounded up to nearest whole bag
For metric inputs, the same calculation runs with 1,602 kg/m³ density and a 1,000 kg/tonne conversion. Mixed-unit inputs — for example, feet for length and centimetres for depth — are normalised to a single unit before the volume calculation runs. The driveway calculator adds a 15% compaction factor on top of the standard 10% waste factor, giving a 25% total overage for driveway projects where gravel compresses significantly under vehicle load.
Cost Estimation
The cost calculator accepts a custom price from the user. The placeholder values shown use these 2026 US averages, refreshed quarterly:
| Unit | Low | Average | High |
|---|---|---|---|
| Per ton (bulk pickup) | $30 | $45 | $70 |
| Per cubic yard (bulk) | $40 | $58 | $95 |
| Per 0.5 ft³ bag (retail) | $4.50 | $5.75 | $8.00 |
| Delivery (local, under 15 mi) | $45 | $85 | $150 |
These prices come from monthly spot-checks at Home Depot, Lowe's, and three regional landscape supply yards (Phoenix, Atlanta, Boston), plus the Texas A&M AgriLife Extension landscape pricing survey. Your local supplier may differ by ±30%. The calculator accepts a custom price input when you have a real quote from your supplier.
Testing and Cross-Verification
Before any calculator goes live, every result is cross-checked against three independent online calculators: gravelshop.com, calculator.net, and a third specialist aggregate calculator. Discrepancies greater than 2% trigger a formula review. A calculator only goes live when our output matches the consensus of the comparison tools within 2%.
Limitations
Our calculators are estimators, not engineering tools. They do not account for:
- Settling and compaction loss — the calculators flag when to add 10–15% overage
- Slope and grade variation — a sloped driveway needs more material than a flat one at the same stated depth
- Local soil conditions — heavy clay subgrade needs a deeper base layer than sandy loam
- Custom blends — mixed sizes or coloured decorative blends may differ in density from standard 3/8-inch pea gravel
For structural projects — retaining walls, load-bearing driveways for commercial vehicles, or drainage systems on slopes above 5% — consult a licensed civil engineer or landscape contractor. Our calculators provide a starting estimate, not a final specification.
Update Cycle
The full site is reviewed quarterly. The next scheduled review is August 2026. Density figures are checked against USGS annual statistics. Price ranges are refreshed against retail spot-checks. Depth recommendations are checked against any updated NRMCA, ASTM, or CPSC guidance. Pages whose underlying data has changed get their "Last reviewed" date updated.
Reporting an Error
If you spot a number that does not match what your local supplier quotes, a formula that disagrees with a reference you trust, or a broken external link, please contact us. Calculation errors are corrected within 24 hours. Broken source links are fixed within one week.