Gravel Base Calculator
Tons of crushed stone to order for a compacted base, and why the number you place is not the number you buy.
Loose volume — suppliers sell by the ton
Ordered loose, placed compacted — the two numbers differ by 20%. Order the top one.
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How to work it out yourself
- 1
Work out the compacted volume
Length × width × depth in feet, divided by 27, gives the cubic yards you want sitting under the slab once it is rolled. Four inches is standard under a residential slab, 6 to 8 inches under a driveway that takes trucks.
- 2
Multiply by the compaction factor
Stone arrives loose and ends up compacted. Roughly 20% of the loose volume disappears as the plate compactor closes the voids, so multiply by about 1.2 to get what you actually need delivered.
- 3
Convert to tons
Suppliers sell crushed stone by the ton, not the yard. Crushed limestone runs about 1.4 tons per cubic yard; washed gravel is nearer 1.35. Ask which product you are getting — the density difference is worth several tons on a large driveway.
Worked example: A 24 × 36 driveway base at 6 inches
| Area | 24 × 36 = 864 sq ft |
| Compacted volume | 864 × 0.5 ÷ 27 = 16 yd³ |
| Loose volume (×1.2) | 16 × 1.2 = 19.2 yd³ |
| Tons (×1.4) | 19.2 × 1.4 = 26.9 tons |
Order 27 tons. Ordering the 16-yard figure leaves you roughly a fifth short with the plate already on site.
Loose against compacted
This is the mistake that sends someone back to the quarry mid-morning. The volume you calculate is the finished, compacted base — but stone is delivered loose and loses about 20% of its bulk as it is rolled. Order the compacted figure and you are short before lunch, with a crew standing around. Order the loose figure and it comes out right. Compaction also varies with material and moisture: well-graded crushed limestone tightens more than round washed gravel, which barely compacts at all and should not be used under a slab.
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Common questions
How many tons of gravel do I need for a driveway?
For a 24 × 36 driveway with a 6 inch compacted base, about 27 tons of crushed limestone. As a rule of thumb, each 100 sq ft at 6 inches needs roughly 1.55 tons once compaction is allowed for.
How many tons are in a cubic yard of gravel?
About 1.4 tons per cubic yard for crushed limestone, and around 1.35 for washed gravel. Damp material is heavier than the same volume dry, which is one reason a load can weigh in differently than expected.
How thick should the gravel base under a concrete slab be?
Four inches compacted is standard under a residential slab, and 6 to 8 inches under anything carrying trucks or on poor subgrade. On clay, more base does less good than proper drainage and a geotextile between the soil and the stone.
What gravel goes under concrete?
Well-graded crushed stone with fines — sold as crusher run, road base or #57 with a fine layer over it, depending on the region. Round washed gravel will not compact and lets the slab settle unevenly. The word to ask your supplier for is angular.
Do I need gravel under a concrete slab?
Yes, on almost any soil. It gives you a level, drainable, compactable surface so water does not sit under the slab and heave it, and so the slab bears evenly instead of bridging soft spots. Skipping it is the most common cause of cracking on otherwise good concrete.
Put this calculator on your own site
Free to use anywhere — a supplier’s resources page, a builder’s blog, a trade forum answer. It works on a phone, needs no account from your visitors, and we will not change it out from under you. All we ask is that you keep the credit line underneath, which is the second half of the snippet.
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