Road Base Calculator — New Hampshire

Road base is crushed angular rock blended with its own stone fines, screened to a set top size. Yards call it crusher run, crush-and-run, dense-graded aggregate base, or the local name your pit uses. It is not a decorative stone and not a clean, washed stone. It is a working material made to pack into a firm platform under a driveway, a slab, or pavers.

Two traits make it work, and both come from the way it is crushed. The stone is angular, so the broken faces catch on each other instead of rolling the way a rounded river stone does. And the blend carries a full range of sizes, from the top stone down to a flour-fine dust. Size this blend in the calculator by the finished compacted depth, not by a loose pile.

The fines are the part people overlook. When you roll the blend, the small particles wedge into the gaps between the larger stones and bind the whole layer. The angular stone gives the strength and the fines give the tight pack. Together they set up into a dense, load-bearing base that a clean single-size stone can never match.

This is the line that separates road base from the stone sold for looks or for drainage. A washed 3/4-inch stone has the fines rinsed out, so it drains fast but stays loose forever. Road base keeps the fines on purpose, because the goal here is a hard base, not a free-draining one. Pick road base when the layer has to carry weight and hold its shape.

Road base comes in a few graded classes, and the names shift by region for the same kind of material. Crusher run and crush-and-run are the common all-purpose dense-graded blends, usually with a top size near one inch down to fines. Aggregate base course, often shortened to ABC, is the same idea under a highway-department name. In the upper Midwest the equivalent spec is called Class 5. Ask your pit which of these they stock, because the product matters more than the label.

A 3/4-inch minus is the blend you reach for on most home jobs. The 3/4-inch top size spreads and screeds smoothly under pavers and in a thinner lift, and the fines still let it lock. A coarser blend with a larger top stone suits a deeper driveway or a soft site, where the bigger rock bridges weak ground. The finer the top size, the smoother the finish, so match the class to the lift you are placing.

Watch out for the clean, single-size stones that sit next to road base in the yard. A number 57 stone or a washed 3/4-inch stone looks similar in the pile, but the fines are gone, so it will not compact into a base. Those stones are for drainage and bedding, not for a load-bearing layer. If a supplier offers you a clean stone for a driveway base, ask instead for a dense-graded base with fines.

For the quantity, the class barely moves the math. The volume comes from your area and depth, and dense-graded blends sit near the working density the calculator uses. Choose the class for the job, then size the order the same way whichever graded base you land on.

A road base layer is often built in two parts, and naming them keeps your order and your depths straight. The lower part is the sub-base. It sits directly on the prepared subgrade, uses a coarser, cheaper blend, and its job is to spread load and bridge soft ground. The upper part is the base course, a tighter dense-graded blend that forms the firm platform right under your slab, pavers, or wearing surface.

On a simple residential driveway you often place a single dense-graded base and skip the split, because the depths are modest. On a deeper build, or over weak clay, the two-layer section earns its place. A coarse sub-base below carries the structural work while a finer base course on top gives a clean, screed-ready surface. Size each layer on its own in the calculator and add the results.

Think of the subgrade as the foundation the whole section rests on, not as part of the order. The calculator sizes the base you buy and spread, not the native ground below it. If that ground is soft, the fix is a deeper sub-base or a fabric layer, which the next sections cover. A strong section is only as good as the ground you build it on.

Match the blend to its role in the section. A coarse, larger-top-size base goes low where it bridges and drains a little. A finer 3/4-inch dense-graded blend goes on top where it screeds flat and locks tight. Getting the layers in the right order is what turns a pile of crushed stone into a base that holds.

Depth drives the order more than any other input, and for road base you always set the finished compacted depth, never the loose pile. Enter that compacted number in the calculator and let it add the allowance for the loose material you actually buy. Set the depth from the load the base will carry and the ground under it.

A path or light foot-traffic surface needs the least, often a shallow compacted layer just deep enough to stay firm and shed water. A paver patio or walkway sub-base usually runs about 4 inches of compacted base below the setting bed. A residential driveway that carries cars and light trucks calls for roughly 4 to 6 inches compacted. The weight the surface sees sets where you land in that range.

Heavier loads need more. A driveway that takes delivery trucks, an RV, or equipment often wants 6 to 8 inches of compacted base, and a parking pad for heavy vehicles follows the same rule. Soft, wet, or clay ground pushes you deeper still, because the base has to bridge a weak subgrade instead of resting on firm soil. When in doubt on poor ground, add depth rather than hope the thin layer holds.

Road base reaches these depths in lifts, not in one dump. Build in 2 to 4 inch lifts and compact each before the next, because a thick single layer never firms up at the bottom. If the depth changes across the site, from a thin edge to a deep center, run the calculator for each zone and add the totals. Always feed it the compacted depth you want to end up with.

This is the one estimating trap that catches road base more than any other material. Road base arrives loose and full of air, then packs down hard as you roll it. The finished compacted layer is thinner than the loose pile you spread, so the loose tonnage you must buy is more than the compacted volume suggests. Order only for the finished depth and you will run short partway through.

The calculator handles this with a compaction allowance, set to 10 percent by default. It sizes the volume from your area and compacted depth, then adds that allowance so the loose material you buy packs down to the depth you planned. Keep the allowance on. It also soaks up the stone lost to an uneven subgrade, to spillage while spreading, and to the edges of the work.

Dense-graded base can shrink more than a light cover stone, because the fines drive out the air under a roller. On a deep base, on soft ground, or when you place thick lifts, the real shrink can run past the default. Raise the allowance in those cases rather than trust the floor. A surplus of a fraction of a ton is cheaper than a second delivery and a stalled roller.

The fines also need the right moisture to pack to full density, so a blend that is bone dry or soaking will not hit the compacted depth you entered. The tonnage assumes the base actually reaches that density when you roll it. Treat the compaction allowance as the bridge between the loose stone on the truck and the hard layer you want on the ground.

The tonnage assumes the base sits on firm, prepared ground, so start below the base. Strip the sod, roots, and topsoil, because that organic ground rots and settles and leaves voids under your layer. Then compact the exposed subgrade until it holds. A loose subgrade sinks under the base and the traffic above it no matter how well you measured the stone.

Over soft or clay ground, lay a geotextile separation fabric on the subgrade before the first stone. The fabric keeps the fines from pumping down into the mud and stops the soft soil from working up into the base. This one step saves a base that would otherwise sink into wet clay. On firm, well-drained soil you can often skip it.

Place the base in 2 to 4 inch lifts and compact each lift before adding the next. A plate compactor suits a path, a patio, or a tight spot, while a vibratory roller earns its keep on a driveway or a larger pad. The fines bind best at a slight dampness, so a light water down helps the layer reach density. A single thick dump packs only at the top and leaves a loose zone below that settles later.

Build a crown or a cross slope into the finished grade so water runs off instead of ponding. Water that sits in or under the base softens the subgrade and undoes the compaction you worked for. Check the finished level at many points, not just the edges, because a low spot in the middle means an uneven base. A base that drains and holds its shape is the base that lasts.

Once you know your compacted depth, the calculator turns it into both cubic yards and tons so you can order either way. It works out the volume from your area and depth, converts cubic feet to cubic yards by dividing by 27, adds the compaction allowance, and applies a working density near 1.4 tons per cubic yard. Road base sits at that heavier density because the fines fill the gaps and the blend packs tight.

Suppliers differ on the unit they quote. Some price bulk base by the cubic yard, which matches a volume plan, and others weigh the loaded truck at the pit, so the ticket lists tons. Reporting both lets you match the yard and check your estimate against the weight slip when the truck arrives. If you hold one number and need the other, multiply cubic yards by about 1.4 for tons, or divide tons by 1.4 for yards.

Bulk road base comes by dump truck, and one truck holds roughly 10 to 14 tons. Divide your total by that range to see how many loads to expect. A short driveway or a small pad may fit in part of one load, while a long driveway or a wide parking area can run past a single truck. Ask your pit what their trucks carry, because it varies by yard.

Name the product when you order, not just road base. Ask for the local dense-graded blend by its regional name, such as crusher run, ABC, or Class 5, and confirm the top size before the truck loads. Pick a drop spot close to the work but clear of the area you must grade, and lay a tarp under the pile to keep the fines off a lawn or a drive.

Most base shortfalls and failures trace to a short list of road base errors. The first is buying a clean, single-size stone instead of a dense-graded base with fines. The clean stone drains but never locks, so it stays loose under load. If you want a hard base, order a blend that keeps its fines.

The second is sizing for the compacted depth and forgetting that loose stone packs down. Road base shrinks as you roll it, so the loose tonnage you buy must exceed the finished volume. Keep the compaction allowance on, and raise it on a deep base or soft ground. People who order for the finished depth alone come up short after the first pass of the roller.

The third is dumping the full depth in one thick layer and rolling the top. The bottom of a thick lift never reaches density, so it settles later and sinks the surface. Build in 2 to 4 inch lifts and compact each one. The fourth is going too thin for the load, such as a few inches of base under a driveway that carries trucks, which ruts and fails.

The fifth is skipping the fabric over soft clay, so the base pumps down into the mud and disappears. The sixth is leaving the grade flat with no crown, so water ponds, soaks the base, and softens the subgrade. The last is measuring a curved drive or an odd pad as one clean rectangle. Break an odd shape into simple rectangles, size each in the calculator, and add them, then round the order up so a little margin covers the uneven ground.