What is the culvert entrance loss coefficient Ke?

Ke is a dimensionless coefficient that gives the head lost at a culvert inlet under outlet-control flow: h_e = Ke·V²/(2g), where V is the barrel velocity. It depends on the inlet geometry — projecting, headwall, mitered, beveled, or tapered — and on the barrel material.

What is a typical Ke value?

Common values: square-edge inlet with headwall Ke = 0.5; groove/socket end of concrete pipe Ke = 0.2; projecting corrugated metal pipe Ke = 0.9; mitered to slope Ke = 0.7; beveled edges Ke = 0.2. Lower Ke means a more efficient, hydraulically smooth entrance.

When does Ke matter — inlet control or outlet control?

Ke is used in outlet-control headwater calculations, where the entrance loss is one term in the total head: HW = TW + (1 + Ke + 29n²L/R^1.33)·V²/2g − L·So. In inlet control the inlet geometry is captured instead by the inlet-control nomograph or polynomial coefficients, not Ke.

How do I reduce culvert headwater with the inlet?

Improve the inlet: replacing a square edge (Ke 0.5) with a beveled or groove end (Ke 0.2) cuts the entrance loss by ~60%. Side- or slope-tapered inlets reduce required headwater further and can avoid upsizing the barrel in inlet-controlled crossings.

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Culvert Entrance Loss Coefficients (K_e) — HDS-5

By Michael Flynn, PE · water resources & dam safety engineer · Last updated Sep 8, 2026

Entrance loss under outlet control: h_e = K_e·V²/(2g), with V the barrel velocity. K_e depends on inlet geometry and barrel material. Values are from FHWA HDS-5, Table 12 (Hydraulic Design of Highway Culverts).

Concrete Pipe

Inlet configuration	K_e
Projecting from fill, socket (groove) end	0.2
Projecting from fill, square-cut end	0.5
Headwall / headwall & wingwalls, socket end	0.2
Headwall / headwall & wingwalls, square edge	0.5
Headwall, rounded edge (radius ≈ 1/12 D)	0.2
Beveled edges (33.7° or 45° bevels)	0.2
Side- or slope-tapered inlet	0.2

Sizing a culvert run, not one entrance? HydroComplete carries inlet and outlet control across every crossing in the conveyance network.

Corrugated Metal Pipe (CMP)

Inlet configuration	K_e
Projecting from fill (no headwall)	0.9
Mitered to conform to fill slope	0.7
Headwall or headwall & wingwalls, square edge	0.5
End section conforming to fill slope	0.5
Beveled edges (33.7° or 45° bevels)	0.25
Side- or slope-tapered inlet	0.2

Box Culvert (Reinforced Concrete)

Inlet configuration	K_e
Wingwalls 30°–75° to barrel, square edge at crown	0.4
Wingwalls 30°–75°, crown edge rounded (r ≈ 1/12 D)	0.2
Wingwalls 90° & 15° to barrel, square edge	0.5
Wingwalls parallel (extension of sides), square edge	0.7
Beveled edges on 3 sides	0.2

Outlet-control headwater.

HW = TW + H − L·S_o, H = (1 + K_e + 29n²L/R^1.33)·V²/(2g)

where H is total head loss (entrance + friction + exit), TW is tailwater depth, L barrel length, S_o barrel slope, R hydraulic radius. The "1" is the exit (velocity-head) loss. Design HW is the larger of the inlet-control and outlet-control results.

Source: FHWA, Hydraulic Design of Highway Culverts (HDS-5, Publication FHWA-HIF-12-026), Table 12. Inlet-control headwater uses the nomographs or the K, M, c, Y coefficients of HDS-5 Table 9 / Appendix A, not K_e.

Sizing a crossing? Open the culvert calculator → · Need the design flow first? NRCS runoff or Rational Method.

Related cheat sheets and tools

Pair K_e with the inlet-control constants K, M, c, Y, the culvert hydraulics tool and a barrel Manning's n. Get the design discharge from NRCS runoff or the Rational Method, and see the full inlet-vs-outlet worked example. For routing a crossing within a whole watershed model, see HydroComplete.