How to measure an area for gravel
Measure the area in square feet the same way you would for sod or mulch: length times width for a rectangle, and for an irregular shape, break it into smaller rectangles and circles you can measure separately and add together. A driveway is usually a straightforward rectangle; a gravel path or a circular fire pit area is easier to handle as one or more circles. This calculator's preview panel scales to match what you enter, using the same math you would do by hand: length times width for a rectangle, pi times radius squared for a circle.
Why tons, not just cubic yards
Gravel and other bulk aggregate are almost always sold by weight, in tons, even though you plan the job by volume: an area times a depth. The conversion between the two depends on the material's density, how much a cubic yard of that particular rock actually weighs, which is why this calculator asks which material you are using instead of assuming one fixed number. Get the density wrong and the ton figure is wrong even if your area and depth measurements are perfect, which is the single most common way people over-order or under-order aggregate. Most standard stone and gravel run about 1.3 to 1.4 tons per cubic yard, but lighter materials like lava rock are notably less dense, around 0.9 tons per cubic yard, so the exact same volume of lava rock weighs roughly a third less than the same volume of pea gravel.
Depth by application
How deep you spread gravel depends entirely on what it needs to do. A decorative bed or ground cover only needs about 2 inches since it is not bearing weight. Walkways and paths that see foot traffic hold up better at 2 to 3 inches. Patios generally want 3 to 4 inches for a stable, even surface. Driveways need the most, typically 4 to 6 inches, and are often built in layers rather than one pour: a compacted base course topped with a finer surface layer, sometimes totaling 6 to 8 inches for driveways that regularly see truck traffic. French drains are a special case, typically using around 12 inches of washed, angular stone (often labeled #57) packed around the perforated pipe so water can move through it freely.
Why base layers need extra for compaction
Loose gravel takes up more space than the same gravel after it has been driven over, rained on, and settled. Crushed stone and decomposed granite used as a compacted base can lose roughly 10 to 25 percent of their loose volume once compacted, which means a base layer measured and ordered at the finished depth will come up short. If you are building a driveway or a paver base, order enough extra to cover that settling, on top of the general waste buffer for spillage and uneven ground, rather than treating the two as the same allowance.
The density rule of thumb, and why it changes the ton figure
As a rule of thumb, most standard aggregate runs about 1.3 to 1.4 tons per cubic yard, and that estimate is good enough for a rough budget. But it is only an estimate: the density reference table on this page shows real spread across common materials, from 1.38 tons per cubic yard for dense sand and decomposed granite down to 0.90 for porous lava rock. Because tons is what you are billed for, using the wrong density can throw off your order by a meaningful margin on a large job even when your area and depth are exactly right. When in doubt, ask your supplier for the specific density of the product you are buying rather than relying on a generic average.
Bags vs. bulk
Small decorative areas, a single planter bed or a narrow border, are often cheaper and easier to handle in bagged gravel from a hardware store, sold by the cubic foot. Anything closer to a full cubic yard or more is almost always cheaper delivered in bulk by the ton, and saves you from hauling and emptying a stack of bags. If you are unsure which way to go, price both: multiply the bag count and per-bag price against a bulk quote for the same tonnage before you commit.
If your project is specifically a French drain or a dry creek bed rather than a decorative bed, the river rock calculator is built around those drainage depths and washed-stone guidance instead of the shallow decorative default here. If you are prepping soil rather than topping it with stone, see the topsoil calculator; filling in the lawn itself is a different job again, see the grass seed calculator. Setting pavers rather than spreading a decorative bed calls for a very different depth too, see the sand calculator for bedding, play, and fill sand. Building a compacted DG path or patio rather than a loose decorative bed needs its own compaction allowance on top of waste, see the crushed stone & decomposed granite calculator for that math.