Gravel Under Concrete Calculator
A 10×10 pad with a 4" compacted base needs about 1.36 yd³ (~1.90 tons) of crushed stone, including 10% waste for uneven subgrade. Enter your footprint below; then size the concrete with the slab bag calculator.
Volume = length × width × thickness. Weight assumes 1.4 tons per cubic yard of crushed stone / road base (planning default; confirm with your quarry). Bag count uses 0.5 ft³ landscape bags; bulk delivery is cheaper past a few yards. Compact in lifts until firm under foot.
Gravel for common pads (4" base, +10% waste)
| Footprint | Volume | Tons @ 1.4/yd³ | 0.5 ft³ bags |
|---|---|---|---|
| 4×4 AC pad | 0.22 yd³ | 0.30 | 12 |
| 8×8 shed base | 0.87 yd³ | 1.22 | 47 |
| 10×10 patio | 1.36 yd³ | 1.90 | 74 |
| 12×12 patio | 1.96 yd³ | 2.74 | 106 |
| 10×20 parking pad | 2.72 yd³ | 3.80 | 147 |
| 24×24 garage | 7.82 yd³ | 10.95 | 423 |
After the base is in, size the pour with the concrete slab calculator, count reinforcement with the rebar and wire-mesh calculator, and check bag yields on the yields chart. Past about a yard of concrete, see bags vs ready-mix.
Building a base that stays flat
- Strip organics: dig out topsoil and roots; pour on undisturbed or well-compacted subgrade, not loose fill.
- Compact in lifts: 2–3 inch lifts with a plate compactor beat dumping 6" and hoping. The base should feel solid under a boot heel.
- Crown and drain: a slight slope (about 1/8–1/4" per foot) away from buildings keeps water from sitting under the slab.
- Bulk vs bags: landscape bags work for a 4×4 AC pad. A 10×10 patio is already a dump-trailer or bulk-bag job; bag math is mostly a sanity check.
FAQ
How much gravel do I need under a concrete slab?
Plan on 4 inches of compacted crushed stone or road base for most patios, shed pads and walkways; driveways often want 4–6 inches. A 10×10 pad at 4" is about 1.36 yd³ (~1.90 tons) including 10% waste. Deeper base or soft clay soils may need more; your local code or engineer sets structural jobs.
What kind of gravel goes under concrete?
¾" crushed stone, crusher run / road base, or dense-graded aggregate that locks when compacted. Pea gravel rolls and does not compact into a stable base for slabs. Clean drainage stone is fine under the base layer in wet sites, but the layer you screed and pour on should compact firm and flat.
How many tons of gravel per cubic yard?
This calculator uses 1.4 tons per cubic yard as a planning default for crushed stone / road base. Quarries and suppliers publish ticket weights that often land around 1.3–1.5 tons/yd³ depending on rock type and moisture. Order from yards (or tons) and confirm density with your supplier; do not treat the default as a weigh-ticket.
Do I need gravel under every slab?
Almost always for exterior slabs on grade: uniform support and drainage cut cracking risk. Interior slabs on well-compacted engineered fill sometimes use a thinner granular layer or vapor barrier details per code. Never pour on loose topsoil, mulch, or uncompacted backfill.