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The length of fence measured along the toe of the slope, before any upslope returns.
Measure along the contour where the fence will actually stand — not around the property boundary. Silt fence works by ponding sheet flow behind it, so a run that cuts across a contour concentrates flow at its low point instead of spreading it.
Tools needed: Measuring wheel, Site plan with contours
The disturbed area whose sheet flow drains to this length of fence.
Trace the catchment on the grading plan: everything upslope of the fence that sheds to it, including any area routed to it by a diversion. Concentrated flow from a channel, pipe or ditch does not belong here at all — silt fence is a sheet-flow control and fails when a channel is aimed at it.
How far upslope the disturbed ground runs before the grade breaks or another control intercepts it.
State manuals cap this as well as the area, and the two caps tighten together as the ground steepens. This page reports the area your slope length implies so you can see whether it agrees with the catchment you entered above; the slope-length limit itself is a table in your own manual.
The average grade of the ground draining to the fence, as a percentage.
A 4:1 batter is 25%; a 2:1 batter is 50%. Steeper ground both shortens the slope length a manual will allow above a fence and raises the odds that flow concentrates rather than sheets, which is the failure mode silt fence cannot survive.
Centre-to-centre spacing of the support posts along the run.
Unsupported fabric is normally posted at 6 ft (1.83 m) centres; fabric backed by wire mesh can usually be stretched to 8 or 10 ft (2.44 or 3.05 m). Follow the spacing in the detail your permit was issued against — this field only counts posts, it does not judge whether the spacing suits the fabric.
How many times the run is turned upslope, at its ends and at any low point along it.
Both ends of every run need one, and long runs on rolling ground need intermediate ones wherever the fence line dips. A run that simply stops on a contour discharges everything it has collected around its own end.
How far each return runs back uphill from the main line.
The return has to reach far enough upslope that its end sits above the ponded water surface behind the fence. On flat ground that is a short hook; on a steep face it is shorter still, because the contour climbs quickly. Take the figure from the detail on your plan.
The limit your erosion and sediment control manual sets on catchment per unit of fence.
The default is 0.25 acre per 100 ft (about 1,010 m² per 30 m), the figure most state manuals carry. Some jurisdictions are tighter, some allow more behind a wire-backed or reinforced fence. Replace it with the number in the manual your permit cites — this page checks against whatever you enter, not against a national constant.
Silt fence to install
427 ft
The catchment behind this run is inside the allowance entered. That is a quantity check, not an approval: slope length, slope steepness and the fabric's own flow rate are separate limits in the same manual, and none of them is tested here.
- Main run before returns
- 395 ft
- Added by the J-hook returns
- 32 ft
- Posts required
- 75 posts
- Bays along the main run
- 66
- Post centres along the main run
- 5.98 ft
- Contributing area per 100 ft (30 m) of run
- 3,270.89 ft²
- Allowance entered, for the same length
- 10,890 ft²
- Share of the allowance used
- 30.04 %
- Catchment the slope length above implies
- 13,035 ft²
- Fall over the slope above the fence
- 3.3 ft
They open the calculator with your figures already in it
Silt Fence Run and Contributing Area Calculator: 427 ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- ASTM D6462 Standard Practice for Silt Fence Installation — trenched and sliced installation, post embedment, and attachment of the geotextile to the posts
- A ceiling of 0.25 acre (about 1,010 m²) of contributing drainage area per 100 ft (30 m) of silt fence is the figure carried by most state erosion and sediment control manuals. It is an editable input here, not a constant, because the number that governs your site is the one printed in your own state's manual
- Turning the ends of a run upslope so runoff cannot escape around them — a J-hook — is standard field practice in state erosion and sediment control manuals. The return LENGTH is a rule of thumb rather than a published figure, which is why it is asked for rather than assumed
Inputs used
- Silt Fence Run Length
- 395 ft
- Contributing Drainage Area Behind the Run
- 12920 sq ft
- Slope Length Above the Fence
- 33 ft
- Average Slope Steepness Above the Fence (%)
- 10
- Post Spacing
- 6 ft
- Number of J-Hook Returns
- 4
- Length of Each J-Hook Return
- 8 ft
- Allowable Contributing Area per 100 ft (30 m) of Run
- 10890 sq ft
Intermediate steps
- Main run before returns
- 395 ft
- Added by the J-hook returns
- 32 ft
- Posts required
- 75 posts
- Bays along the main run
- 66
- Post centres along the main run
- 5.98 ft
- Contributing area per 100 ft (30 m) of run
- 3,270.89 ft²
- Allowance entered, for the same length
- 10,890 ft²
- Share of the allowance used
- 30.04 %
- Catchment the slope length above implies
- 13,035 ft²
- Fall over the slope above the fence
- 3.3 ft
Confidence note: The catchment behind this run is inside the allowance entered. That is a quantity check, not an approval: slope length, slope steepness and the fabric's own flow rate are separate limits in the same manual, and none of them is tested here.
What this calculation does not cover
- Sheet flow only. Silt fence is not a check dam and must never be placed across a channel, swale or ditch line.
- The slope-length and slope-steepness limits in your manual are not applied here — only the contributing-area limit you entered.
- Post count assumes one continuous line. Breaking the perimeter into separate runs adds an end post to each.
- Trench excavation, backfill, wire backing and fabric overlap at joints are separate quantities.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations3
- ASTM D6462 Standard Practice for Silt Fence Installation — trenched and sliced installation, post embedment, and attachment of the geotextile to the posts
- A ceiling of 0.25 acre (about 1,010 m²) of contributing drainage area per 100 ft (30 m) of silt fence is the figure carried by most state erosion and sediment control manuals. It is an editable input here, not a constant, because the number that governs your site is the one printed in your own state's manual
- Turning the ends of a run upslope so runoff cannot escape around them — a J-hook — is standard field practice in state erosion and sediment control manuals. The return LENGTH is a rule of thumb rather than a published figure, which is why it is asked for rather than assumed
Which documents these citations point at
Standards referenced: ASTM D6462 (ASTM International, United States).
Cite this page
Your workspace
Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.
On contributing area: 304 m² (3271 sq ft) drains to each 100 ft (30 m) of run, at or under the 1012 m² (10890 sq ft) allowance this convention gives. The catchment entered is consistent with the slope length above the run, which is the cross-check worth making before trusting either number.