Three things change and they do not change together
Laying a bank back is usually described as one decision, and it has three separate consequences that scale differently. Take the default: a 6 ft bank, 40 ft long, going from 2:1 to 3:1.
| What changes | By how much | What it scales with |
|---|---|---|
| Horizontal run | 12 ft becomes 18 ft — 6 ft more ground | Height only. Length does not enter it. |
| Material moved | 26.7 cubic yards | Height squared, times length. |
| Face area | 537 sq ft becomes 759 — 41 percent more | Height and length, and it goes up as the slope gets flatter. |
The land take is the one that stops projects. Six extra feet is nothing on a field and it is the whole problem on a suburban lot where the toe of the bank is already four feet from the fence. The page checks that for you against distances you measure, because the room available is a property fact rather than an arithmetic one, and any required separation from a line is on your plan set rather than here.
The face gets bigger when the slope gets gentler
This one is genuinely counterintuitive and it costs real money. The face of a bank is the hypotenuse of a triangle whose vertical is the height and whose horizontal is height times the ratio. Its length per foot of height is the square root of 1 plus the ratio squared: 2.236 at 2:1, 3.162 at 3:1, 4.123 at 4:1. Flatter always means longer.
So the 40 ft bank in the example goes from 537 square feet of face to 759. Erosion blanket, seed, plants and the hours somebody spends mowing all price by the square foot of face rather than by the square foot of plan. Flattening a slope to make it mowable makes more of it to mow. That is still usually the right decision, because mowing a 3:1 face is possible and mowing a 2:1 face mostly is not, but the quantity goes the opposite way from the intuition.
Cut from the top, fill at the toe, or both
Same finished slope, three different jobs. Cutting the top back moves the crest 6 ft into whatever is up there and produces 26.7 bank yards that leave as 33.3 loose — three truck loads. Filling the toe out advances the toe 6 ft into whatever is down there and needs 26.7 yards of compacted fill, which takes 30.3 bank yards of material to build because of shrink, delivered as about 38 loose yards.
Splitting it moves each end 3 ft and looks like the free option, because the cut feeds the fill. It nearly is. Half the volume is 13.3 bank yards off the top, which compacts down to 11.7 yards of fill against the 13.3 the toe wants — 1.6 yards short, needing about 1.8 bank yards from somewhere. Shrink always runs one way and it always leaves you short, which is why nobody who does this for a living orders exactly the balanced quantity.
What the arithmetic will not do
It will not tell you a slope is stable. Not at 2:1, not at 3:1, not at any ratio, because that depends on the material, the water in it, what is loaded above it and what a wet spring does — an engineering judgement about your particular ground. The page does the geometry for whatever two ratios you type, including a steeper second one, and it says nothing about whether the result stands up.
Nor does it know what happens to the water. Changing the shape of a bank changes where runoff goes and how fast it gets there, which is a matter with local rules attached and a neighbour downhill of it. Take the geometry and the volumes to somebody who can look at the soil, and treat every number here as the input to that conversation rather than the end of it.
Questions people ask
How much room do I need to flatten a slope?
The extra horizontal run is the bank height multiplied by the change in ratio, and the length of the bank does not come into it. A 6 ft bank going from 2:1 to 3:1 needs 6 more feet whether it runs 40 ft or 400. A 12 ft bank making the same ratio change needs 12. That is why height is the measurement worth taking carefully — it drives the land take directly and the volume by its square.
Why does a flatter slope have more surface area?
Because the face is the hypotenuse. Per foot of bank height the face measures the square root of 1 plus the ratio squared: 2.236 ft at 2:1, 3.162 at 3:1, 4.123 at 4:1. Flattening always lengthens it, so the 40 ft bank in the defaults goes from 537 square feet of face to 759, up 41 percent. Anything you buy or do by the square foot of slope — blanket, seed, plants, mowing — goes up with it, even though the slope itself got gentler.
Is it cheaper to cut the top or fill the toe?
It depends on what is at each end and on where material is coming from, but they are not symmetric. Cutting produces material you have to move away and pay to dispose of; filling consumes material you have to buy and place in compacted lifts, and shrink means you need more bank yards than the finished volume. In the defaults, cutting is 26.7 bank yards out and three truck loads; filling is 30.3 bank yards in. Splitting it between the two moves half as far at each end and still leaves you about 1.8 bank yards short because of shrink.
What slope ratio is safe for a bank?
There is no answer to that on a web page and this one will not pretend otherwise. Whether a slope stands depends on the soil, its water content, what loads sit above it, how it drains and what happens in a wet season, and those are engineering judgements made by somebody looking at the ground. The calculator does the geometry for whatever two ratios you enter, including making a slope steeper, and it makes no claim about either the before or after condition.
How do I measure my existing slope ratio?
A level, a tape and a straight board. Hold the board level out from the face, measure the horizontal distance to a point, then measure straight down from that point to the slope surface. The horizontal divided by the vertical is the ratio — 2 ft out for 1 ft down is 2:1. Take it in three or four places, because banks are rarely one clean plane, and use the flattest reading if you want the conservative land take and the steepest if you want the conservative volume.