Compaction Rates & the 10% Waste Buffer Explained
What compaction does to gravel volume, why a 10% waste buffer is the standard residential default, and when to adjust it up or down for your specific site.
When you dump a ton of crushed stone on your driveway, then run a plate compactor over it, the pile gets shorter. That's not weight disappearing — it's air being squeezed out from between the stones. Understanding how much shrinkage to expect is the difference between ordering once and ordering again.
What compaction actually is
Loose gravel is roughly 30–40% air by volume. The voids between stones are filled with… nothing. Compaction physically rearranges the stones so smaller fragments fit into the gaps between larger ones, and the whole mass gets denser.
Two things drive how much a material compacts:
- Gradation. A graded mix with stones of many sizes (CA6, ABC) packs tightly because the small stuff fills the voids. Single-size stone (#57) can't do this — there are no fines.
- Effort. A walk-behind plate compactor produces meaningful compaction in 2-inch lifts. A roller or jumping jack does more. Foot traffic alone does very little.
Typical compaction rates by material
| Material | Compaction (loose → final) | Notes |
|---|---|---|
| CA6 / 21AA / Class 5 | 20–25% | Highest compaction. Ideal sub-base. |
| ABC / Crusher run | 20–25% | Same family as CA6. |
| MOT Type 1 (UK) | 20–25% | Equivalent UK spec. |
| Crushed stone #57 | 5–10% | Open-grade — limited compaction. |
| Crushed stone #8 | 8–12% | Finer than #57; packs slightly more. |
| Crushed stone #4 | 5–8% | Coarse; minimal compaction. |
| Pea gravel | 0–3% | Rounded — won't compact. |
| River rock | 0% | Won't compact at all. |
| Recycled concrete | 20–25% | Behaves like crushed limestone with fines. |
Why a waste factor exists
Even if you ordered the exact right volume and lost zero to compaction, you'd still come up short. Here's where the rest goes:
- Tailgate spillage. Some falls outside the work area when the truck dumps.
- Irregular edges. The actual perimeter is rarely as clean as the measured one.
- Over-fills. When spreading by hand, depth varies an inch in either direction. The deep spots use extra material.
- Measurement error. A 50-foot driveway measured with a 25-foot tape is usually off by 1–2%.
- Wheelbarrow loss. Each load loses a handful or two over uneven ground.
The 10% buffer rule
A 10% buffer covers the typical combination of compaction and waste for an average residential project. The calculator applies it by default. Adjust based on your site:
- 5% buffer: Flat, square slab. Tight measurements. Open-grade stone (no compaction). E.g., decorative bed on level ground with edging already in place.
- 10% buffer (default): Most residential projects. Some compaction, some shape irregularity, normal measurement precision.
- 15% buffer: Sloped sites, irregular perimeters, or any project requiring heavy compaction (driveways, paver bases, French drains).
- 20–25% buffer: Heavy compaction + irregular site + soft sub-grade. Or any project where a second delivery would be very expensive.
How much extra to order (compaction multipliers)
The compaction rates above tell you how much a material shrinks. To turn that into an order quantity, flip it around: you need enough loose gravel to reach your finished, compacted depth. That's the compaction multiplier — order this much loose for every unit of finished depth or volume.
| Material | Compaction | Order (loose per finished unit) |
|---|---|---|
| CA6 / 21AA / ABC / crusher run | 20–25% | 1.25–1.35× |
| Recycled concrete | 20–25% | 1.25–1.35× |
| Crushed stone #8 | 8–12% | 1.10–1.15× |
| Crushed stone #57 | 5–10% | 1.05–1.10× |
| Crushed stone #4 | 5–8% | 1.05–1.10× |
| Pea gravel / river rock | 0–3% | 1.00–1.05× |
Worked example: a 4-inch finished CA6 base needs about 4.8–5.4 inches of loose material (4 × 1.25 to 4 × 1.35). On a 1,000 sq ft base that's roughly 15 loose cubic yards to finish at about 12.3 compacted cubic yards. If your supplier sells by the ton, the compacted density already reflects the packing — order to the tonnage the calculator gives plus the 10% buffer, and don't double-count. The multiplier matters most when you're buying loose volume by the yard.
How to compact gravel properly
Compaction isn't just "walk on it a few times." For a base that lasts decades, follow this sequence:
- Spread in 2-inch lifts. Don't dump 6 inches of base material and try to compact it all at once — only the top 2 inches will compact. Spread, compact, spread the next layer, compact, repeat.
- Wet it lightly. A garden hose mist (not soaking) helps fines bind and prevents the compactor from kicking up dust. Skip this for clean stone.
- Use a plate compactor for dense-grade base. Rent one for $50–$80 per day. A 200 lb plate compactor handles 4-inch lifts. Walk overlapping passes, slow and steady — about 1 ft per second.
- Tamp by hand at edges. A plate compactor can't reach within 6 inches of walls or edging. Use a hand tamper there.
- Check for ruts after one pass. If the compactor leaves visible tracks, the material isn't binding. Add a fine spray of water and try again.
For open-graded stone like #57 or pea gravel, full plate-compaction isn't needed (and doesn't accomplish much). Just spread evenly, then walk it in or run a small hand tamper for finishing. The stones lock together by gravity and traffic over the first few weeks.