How much gravel do I need, and the divisor everybody forgets
Area in square feet, times the finished depth in inches, over 324. That is cubic yards. A 600 square foot driveway at 4 inches is 7.4 cubic yards.
The 324 does two conversions at once: 12 inches to the foot, and 27 cubic feet to the cubic yard. People remember the first and drop the second, and the answer comes out 27 times too big, which is the kind of error that is obvious on a driveway and expensive on an invoice.
Order loose, specify finished. That is the whole trick.
A base is specified by the depth it ends up at, and it is delivered as a loose pile. For anything with fines in it those are different numbers, because the fines let the stone lock together and the pile shrinks under a compactor.
Crusher run and road base lose close to a third of their loose volume compacting. So a driveway worked out at 7.4 finished yards needs closer to 9.6 ordered, and somebody who orders 7.4 finds out on the last strip, which then gets laid thin because nobody is going back to the yard for one yard of stone.
But clean single-sized stone does not do this. No. 57 arrives as stone with gaps between it and stays as stone with gaps between it. Applying a compaction factor to it over-orders by a quarter or more. Most calculators either ignore compaction entirely or apply one blanket factor to everything, and both are wrong: one leaves you short on base, the other sells you stone you cannot use.
The size numbers run backwards
Crushed stone is ordered in America by an AASHTO M 43 number, jointly held with ASTM D448, and a bigger number means a smaller stone. Nominal maximum sizes, from the FHWA tabulation of the M 43 gradations:
- No. 5 and No. 56, nominal 1.5 inches.
- No. 57, No. 6 and No. 67, nominal 1 inch.
- No. 7, No. 78 and No. 68, nominal 3/4 inch.
- No. 8 and No. 89, nominal 1/2 inch.
- No. 9 and No. 10, nominal 3/8 inch.
Two numbers together mean a blend spanning both: a No. 57 is graded across the range a No. 5 and a No. 7 cover between them. The practical difference on site is whether the stone is single-sized, which drains and stays loose, or graded to fines, which beds and compacts.
Rounded stone rolls, angular stone locks
This decides more than the size does. Pea gravel and river rock are rounded, so under load the stones roll past each other: they rut, they migrate to the edges, and no depth fixes it. Crushed stone has broken faces that jam against each other and hold.
So pea gravel is right for drainage, pipe bedding and a path nobody drives on, and wrong for a driveway at any depth. It is one of the few places in this trade where the cheap answer and the correct answer are genuinely different materials rather than different quantities.
Tons or cubic yards, and why both are on the page
Bulk aggregate is sold by weight in some yards and by volume in others. The conversion runs on the loose density, which for ordinary stone is about 2,700 pounds a cubic yard, near 3,200 for road base, and lower for sands and soils. Those are typical figures and they move with grading and moisture, which is why the density is a field you can overwrite rather than a constant baked in.
If you have been quoted a price per ton and a price per yard for the same material and they do not reconcile at roughly 1.35 tons a yard, one of them is worth a phone call.
Depth is design, so the depths here are reported and not recommended
The tool lists what people conventionally use, because that is useful, and it does not calculate a depth for you, because it cannot: how deep a base needs to be depends on what the ground under it is doing and what is going to drive on it. A calculator that answers that question from an area and a material has invented the answer.
What it will tell you is that anything past about 6 inches goes down in layers. A plate compactor only works through so much at a time, so two 4 inch lifts end up compacted and one 8 inch lift ends up with a loose bottom half, which is the half carrying the load.
Type 1 and hardcore are the British words for this
The British page is called Aggregate and hardcore, works in square meters, millimetres, tonnes and 850 kilogram bulk bags, and is built around MOT Type 1 and pea shingle. Nothing on it is wrong and an American cannot use a single field of it, nor would they ever find it, because nobody in the United States searches for hardcore or Type 1.
The densities and the compaction factors are shared between the two pages on purpose, in one file, so they cannot drift apart and quietly disagree. 1.6 tonnes per cubic meter and 2,700 pounds per cubic yard are the same physical fact stated twice.
Typical densities, not your weighbridge ticket
It does not price anything. It does not choose a depth or a material for a structural base, and it does not know your ground. Densities are typical rather than measured, so treat the weight figures as close rather than exact, and use your supplier figure where you have one.
Common questions
How much gravel do I need?
Area in square feet, times the finished depth in inches, divided by 324. That gives cubic yards. A 600 square foot driveway at 4 inches is 7.4 cubic yards. The 324 is 12 inches to the foot times 27 cubic feet to the cubic yard, and forgetting the second half is the commonest mistake in the whole subject: it puts the answer out by a factor of 27.
How many square feet does a cubic yard of gravel cover?
It depends entirely on the depth, which is why a single number gets quoted wrongly so often. At 2 inches, 162 square feet. At 3 inches, 108. At 4 inches, 81. At 6 inches, 54. The arithmetic is 324 divided by the depth in inches, and it is worth knowing because it is the figure a yard will quote you over the phone and the one to sanity check your own answer against.
How much does a cubic yard of gravel weigh?
Around 2,700 pounds, so about 1.35 short tons, for ordinary crushed stone or gravel. Crusher run and road base are heavier at roughly 3,200 because the fines fill the gaps between the stone. Sand is a little lighter, topsoil lighter again. These are typical figures rather than measurements: density moves with grading and with how wet the pile is, so if your supplier quoted a weight per yard, put theirs in the box and use it.
What is No. 57 stone, and why is No. 8 smaller?
They are AASHTO M 43 size designations, jointly held with ASTM D448, and the numbering is not intuitive: a bigger number means a smaller stone. No. 57 is a nominal 1 inch maximum and is the general purpose driveway and drainage stone. No. 8 is a nominal half inch, used for paver bedding and topping. No. 67 is also nominal 1 inch but graded down to fines rather than single-sized, so it beds tighter. If somebody says "57s" they mean the first one.
Do I need to order extra for compaction?
For some materials, and not for others, and that is the part every other calculator gets wrong in one direction or the other. Crusher run and road base contain fines that let them lock up, so they lose close to a third of their loose volume under a plate compactor: you specify the base finished and you buy it loose, and the difference has to be ordered. Clean single-sized stone like No. 57 does not compact meaningfully, so adding an allowance to it just sells you a yard you do not need. This tool applies the factor per material and shows it as its own line so you can see it.
How deep should a gravel driveway be?
Conventionally 4 to 6 inches of compacted base for cars, and 8 to 12 for trucks or over soft ground, usually in two or more layers. Those are reported figures rather than a recommendation, because depth is a function of the ground underneath and the loads on top, and choosing it is a design decision rather than an arithmetic one. Anything deeper than about 6 inches goes down in lifts: a compactor only works through so much at a time, and one 8 inch lift leaves the bottom half loose.
Is pea gravel any good for a driveway?
Not really, and the reason is the shape rather than the size. Pea gravel is rounded, so the stones roll over each other instead of locking together. It ruts under a wheel, it migrates towards the edges, and it will not hold a load however deep you make it. It is excellent for drainage, pipe bedding and paths where nothing drives. For anything carrying weight you want angular crushed stone, because the faces jam against each other.
Does it work for landscape rock and decorative stone?
Yes, and those are the easy case: river rock, lava rock and decorative stone are laid loose and never compacted, so the loose volume and the finished volume are the same number and there is no allowance to argue about. Depth is the thing to get right. Two to three inches is enough to cover the ground and hide the fabric, and going deeper mostly means stone that migrates when somebody walks on it. Pick river rock or No. 57 in the material list and the compaction line will tell you it is adding nothing.
Should I order gravel by the ton or by the cubic yard?
Whichever your supplier quotes, and the tool gives both so you can check one against the other. Bulk material is sold by volume in some places and by weight in others, and the conversion between them runs on a density that varies. If you are given a price per ton and a price per yard for the same material and they do not reconcile at roughly 1.35 tons per yard, one of them is worth querying before the truck is loaded.
Does this work in Canada?
Yes. Canadian yards work in cubic yards and tons for aggregate much as American ones do, and the tool also prints metric tonnes because some suppliers quote those. What differs is the material naming: the AASHTO numbers are used in Canada but provincial specifications carry their own granular designations, Granular A and Granular B being the common ones in Ontario, which are the same idea as road base. The arithmetic and the compaction behavior are identical.