The quantities are easy; the sequence is what gets sites shut down
Every number on this page is straightforward arithmetic: perimeter times an overlap allowance gives fence, area divided by roll coverage gives rolls, acres times an application rate gives tons. What is not arithmetic, and what inspectors actually look at, is whether the controls were in the ground before the ground was opened. Perimeter fence, the stabilised construction entrance and inlet protection are installed first. A site that grades for three days and then installs fence has already sent its sediment downstream, and the fence is now decoration.
The second thing inspectors look at is maintenance. Silt fence fills with sediment and stops working, blanket lifts at unanchored edges, the entrance clogs with fines and stops shaking mud off tyres, and inlet protection turns into a dam. All of it needs checking after every significant rain and after any earthmoving that touches it. Budget for replacing a proportion of what you install rather than treating the take-off as a one-time purchase.
Silt fence goes on the low side, along the contour
Silt fence does not filter water in any meaningful sense. It ponds sheet flow behind the fabric, and sediment settles out of the standing water. That mechanism tells you everything about where it works. It needs to be roughly level along its length so water spreads out behind it instead of running along it to the low end. It needs its ends turned uphill, so ponded water cannot simply escape around the termination. And it fails outright in concentrated flow, which is why running silt fence across a swale or a drainage way is one of the most common and most useless installations on any site.
Because of this, fencing the entire perimeter by reflex is usually wrong as well as expensive. What needs fencing is the length across which water can leave the disturbed area, which is the down-gradient side. The calculator flags the case where fence per acre comes out very high, because that pattern normally means either an unusually long thin site or a perimeter that was fenced without looking at where water actually goes.
Blanket, mulch, or nothing yet
| Situation | Usual treatment | Why |
|---|---|---|
| Steep slopes, channels, anywhere flow concentrates | Erosion control blanket, anchored | Holds soil and seed in place under flow that would wash mulch away |
| Flatter graded areas awaiting final work | Seed and straw mulch | Cheap, fast, effective on ground that only sees sheet flow |
| Areas that will be worked again shortly | Temporary measures per the plan | Bare soil has a limited allowable exposure time, and it is set locally |
Blanket take-offs need real overlap. Side laps, end laps and the anchor trench at the top of a slope all consume material, and the trenched-in upslope edge is the detail people skip and then watch peel off in the first storm. Fifteen percent is a reasonable planning allowance on regular slopes and more on anything cut up by obstacles. Staple and pin patterns are specified by the blanket manufacturer and get denser on steeper ground and in channels, so take the density from the product rather than from a default.
What the numbers do not settle
Nothing here decides whether your site needs a permit, an approved control plan, an inspection regime or record keeping, and nothing here says which practices your jurisdiction will accept. Disturbed-area thresholds, allowable time for bare soil, required practices, stabilisation standards and the authority you answer to all vary locally, and some requirements are federal. Land near a wetland, a watercourse or a floodplain brings additional agencies into it. Those questions go to your local building, stormwater or conservation authority before work starts, because retrofitting controls onto a site that has already been cited is far more expensive than installing them in the right order.
Two field realities to plan around. First, call the free national utility locate service before trenching fence or stripping for the entrance — it exists in every state and costs nothing. Second, controls sized for a site that is disturbed all at once cost more than a site phased so that only part of it is open at any time, and phasing is usually the cheapest sediment control there is. Once the grading quantities are settled on the cut and fill calculator, and the flow leaving the site is understood on the culvert and swale flow calculator, the controls follow from where that water actually goes.
Questions people ask
How much silt fence do I need per acre?
There is no reliable per-acre figure, because fence tracks perimeter geometry and topography rather than area. A square 1 acre site with one down-gradient side needs a couple of hundred feet; a long narrow acre falling sideways can need three times that. What matters is the length across which sheet flow can leave the disturbed area, measured along the contour on the low side. This calculator reports fence per acre as a cross-check rather than a design rule — if it comes out very high, look again at whether the whole perimeter really needs fencing.
How many tons of straw mulch per acre?
The application rate is an input here on purpose, because it is set by your specification, your seed supplier or your local requirement rather than by a universal number. Rates are usually expressed in tons per acre and vary with whether the mulch is anchored, crimped or tacked, and with the season and slope. What the calculator does is convert your rate into tons and into bale count at the bale weight you actually have, which is the part people get wrong — bale weights vary widely, so a bale count derived from an assumed weight can be off by half.
Do I need a permit before disturbing soil?
That depends entirely on where you are, how much ground is being disturbed, and what is downstream, and this page will not give you a threshold because stating one as fact would be wrong somewhere. Erosion and sediment control requirements are set locally and some are federal, and work near wetlands, watercourses or floodplains adds agencies. The practical answer is to call your local building department and stormwater or conservation authority before the first machine arrives and ask what applies to your parcel and your scope. It is a short call and it is far cheaper than a stop-work order.
Can silt fence go across a ditch or swale?
No. Silt fence is a sheet-flow control that works by ponding shallow, spread-out flow behind fabric so sediment settles. Concentrated flow in a ditch or swale overwhelms it, undermines it, or blows it out, and a failed fence releases everything it had collected in one slug, which is worse than never installing it. Where flow concentrates, a different practice entirely is called for, and which one is appropriate depends on the flow, the channel and what your jurisdiction accepts. That determination belongs with whoever prepared the control plan.
What does the construction entrance actually accomplish?
It keeps site mud off the public road, which is both the most visible compliance failure and a genuine hazard. A pad of coarse stone at the exit flexes tyres and shakes fines loose before vehicles reach pavement. Two things make it work: coarse enough stone that it does not simply pack solid, and enough length that every wheel makes several full revolutions on it. Its enemy is fines washing in from the site, so it needs topping up or replacing during the job, and the stone volume calculated here should be treated as the initial install rather than the total the job will consume.