A berm is mostly width
The height is the number people choose and the width is the number that decides whether it fits. At 4:1 in front and 3:1 behind, a 3 ft berm with a 3 ft crest has a base 24 ft wide — the crest is an eighth of it and the two faces are the rest. Add end tapers at 5:1 and each end runs 15 ft, so a 24 ft crest becomes 54 ft overall.
That shape sits on 936 square feet. On a quarter acre lot it is a serious commitment of ground, and it is the reason so many home-built berms end up steep: the ground ran out, the slopes got pushed to 2:1, and now the soil washes off the face every storm and nothing will mow it. Deciding the base width first and letting the height follow is the way round that works.
Where the yards go
| Part | Default figures |
|---|---|
| Cross section | 40.5 sq ft — 9 of it crest, 31.5 the two faces |
| Body, 24 ft of crest | 972 cu ft, 36 cu yd |
| Two tapered ends | 405 cu ft, 15 cu yd |
| Finished shape | 1,377 cu ft, 51 cu yd |
| Topsoil cap, 4 in | 12 cu yd of that |
| Fill core | 39 cu yd |
| Ordered loose at 20 percent settlement | 48.8 cu yd of fill and 15 of topsoil — five loads and two |
The tapered ends are worth noticing: they are 29 percent of the volume at these proportions and they are the part sketched last and estimated never. Each one is a pyramid running from the full cross section out to nothing, so it holds a third of what the same length of body would.
Settlement is not swell
Two different factors get called the same thing and they measure opposite events. Swell is the expansion from undisturbed ground to a loose pile — it is what makes an excavation produce more truck loads than the hole suggests. Settlement here is the reverse trip for delivered material: soil arrives loose on a truck, gets pushed into shape, gets walked on and rained on, and ends up occupying less room than it did in the box.
At 20 percent, five loose yards place as four, so the 39 yards of finished core takes 48.8 loose yards to build. Berms are usually placed by dumping and shaping rather than compacting in lifts, which is why the allowance is large and why a berm keeps sinking for a year. If you are building out of spoil from your own excavation rather than delivered soil, do the bank-to-loose conversion first and bring the loose number here.
What a berm does to water
It is a dam. Whatever used to sheet across that ground now goes around the ends or ponds behind the uphill face, and on a flat lot that is usually fine and on a sloping one it is the whole design problem. This is the practical reason the end tapers matter beyond how they look — they are the route the water takes.
Where that water ends up is a local question with a neighbour attached to it, and in a lot of places changing the way a site drains has rules around it. That is a call to your local building or stormwater authority rather than something a volume calculation can settle. The arithmetic here tells you what the berm costs in soil and ground. It has no opinion at all about whether the shape is stable, whether the slopes will hold their planting, or whether the water will behave.
Questions people ask
How much soil does a berm take?
For the default shape — 3 ft high, 3 ft crest, 24 ft of crest length, 4:1 in front and 3:1 behind, 5:1 end tapers — the finished berm is 51 cubic yards, of which 12 is the 4 in topsoil cap and 39 is fill core. Allowing 20 percent settlement on delivered material, that is 48.8 loose yards of fill and 15 of topsoil to order: five truck loads and two at 12 yards each, and 70.1 tons on the ground.
How wide does a berm need to be for its height?
The base is the crest width plus the height times the sum of the two side slope ratios. A 3 ft berm with a 3 ft crest at 4:1 and 3:1 is 24 ft wide at the base. Four feet tall at the same slopes is 31 ft. The relationship is why height is the wrong dimension to fix first on a tight lot — pick the base width you can give it, then see what height the slopes you want will allow.
How much does a berm settle?
More than compacted fill and for longer, because berms are normally dumped and shaped rather than placed in lifts and rolled. The allowance on this form is expressed as a share of the loose delivered volume, so 20 percent means five loose yards end up as four in place. The real figure depends on the material, how wet it was, how much it got tracked over and how long it has to settle before you plant. Overbuilding the height slightly and letting it come down is the usual answer.
Should the topsoil cap be on top of the height I entered?
No. The dimensions describe the finished shape, so the cap is the outer skin of that shape and the fill core is what sits underneath. That is why the page subtracts the cap volume from the total rather than adding it. If you would rather build the fill core to the full height and then cap it, the berm ends up 4 in taller and correspondingly wider at the base — enter the taller height instead and read the numbers again.
Can I build a berm out of the spoil from a dig on the same property?
Often, and it saves both the haulage out and the delivery in, which is the biggest saving available on a berm. Two cautions. Whether the material is suitable is a soil question rather than a volume one — heavy clay makes a berm that stays wet and grows nothing without a decent cap. And the units have to be handled carefully: excavated spoil is measured in bank yards in the ground and loose yards on the pile, so convert to loose first and bring that figure here, since this page works from loose delivered volume.