Culvert Flow Calculator

Culvert Flow Calculator that finds full-pipe Manning capacity, headwater with HDS-5 inlet and outlet control, and the smallest round or box culvert for the flow at a road crossing.

What the Culvert Flow Calculator Solves

The culvert flow calculator has four modes. Full-Pipe Capacity gives the Manning flow of a barrel running full. Headwater for a Flow finds how high water backs up at the entrance for a design flow. Flow at Headwater Limit finds the most a culvert can pass before water reaches a set level, and Culvert Size for a Flow picks the smallest pipe that works.

Seven entrance types are built in, covering concrete pipe with a square-edge headwall or a projecting groove end, corrugated metal pipe with a headwall, mitered to slope or projecting, and concrete box culverts with flared or parallel wingwalls. Each one carries its own FHWA HDS-5 inlet coefficients and entrance loss value.

Results include velocity, part-full flow, critical and normal depth, the control that governs, and the flow one size up or down. A culvert flow rate chart lists every standard diameter at your slope and roughness, and a step-by-step solution shows each equation with your numbers.

Full-Pipe Flow With Manning's Equation

When a round barrel runs full, its capacity comes from Manning's equation. For a full circular pipe, the flow area is πD²/4 and the hydraulic radius R is D/4. The US form uses 1.486, and the SI form uses 1.0 with Q in m³/s.

$$Q=\frac{1.486}{n}A{R}^{2/3}{S}^{1/2}$$

A 24 in concrete pipe at 1% slope with n = 0.013 has A = 3.142 ft² and R = 0.5 ft. Its full flow is 1.486 / 0.013 × 3.142 × 0.5^(2/3) × 0.1 = 22.62 cfs, or about 10,150 gpm, moving at 7.2 ft/s. In metric, a 600 mm pipe on the same slope carries 0.614 m³/s. Switch the units to mm and m³/s and the culvert flow calculator works fully in metric, with L/s as an option.

Roughness changes capacity in direct proportion. The tool sets n = 0.013 for concrete pipe and 0.024 for corrugated metal, so a 36 in CMP at 1% carries 36.1 cfs flowing full, while the same size in concrete carries 66.7 cfs.

A round pipe carries the most at about 94% depth, roughly 7.6% more than flowing full, because the last sliver of depth adds more wetted perimeter than area. At half depth it carries exactly half the full flow. The Partial Pipe Flow Calculator covers any other depth.

Inlet Control vs Outlet Control

Manning full flow is only the barrel's limit. Most culverts are held back first by the entrance, where water must squeeze into the pipe. That is inlet control, and headwater then depends on the inlet shape, size and flow, but not on barrel length or roughness.

Outlet control applies when the barrel itself cannot pass the flow. Headwater then adds up the entrance loss, friction along the barrel, the exit loss and the water level at the outlet, less the fall of the barrel. The culvert flow calculator runs both checks and reports the higher headwater as the one that governs.

For inlet control, HDS-5 uses regression equations fitted to lab tests. When the inlet is submerged, meaning Q / (A√D) is 4.0 or more, the equation below applies. Below 3.5 the unsubmerged form uses critical depth instead, and the tool blends the two in between.

$$\frac{HW}{D}=c{\left(\frac{Q}{A\sqrt{D}}\right)}^2+Y-0.5S$$

Take the 24 in concrete pipe with a square-edge headwall carrying 20 cfs. Then Q / (A√D) = 20 / (3.142 × 1.414) = 4.50, so the inlet is submerged. With c = 0.0398 and Y = 0.67, HW/D = 0.0398 × 4.50² + 0.67 - 0.005 = 1.47, a headwater of 2.94 ft.

Outlet control for the same pipe, 100 ft long with no tailwater, gives 2.53 ft, or 1.26 D. Inlet control is higher, so it governs. The inlet coefficients match the HDS-5 table reprinted in the Kitsap County stormwater manual, and the entrance loss values match the TxDOT hydraulic design manual.

Culvert Flow Rate Chart

This chart is for round concrete pipe with a square-edge headwall at 1% slope and n = 0.013. Full flow is Manning capacity. The two headwater columns give the flow the entrance admits before water reaches the crown (HW = D) or 1.5 times the diameter, with inlet control governing every size on a 60 ft barrel.

The gap between columns is the main lesson. A 24 in pipe rated at 22.6 cfs flowing full passes only 12.8 cfs before water reaches its crown, because the entrance limits it. The culvert flow calculator rebuilds this chart for your own slope, roughness and inlet, and Print / PDF saves it as a culvert flow rate chart PDF.

Inlet Type Changes Capacity

The same 24 in culvert passes very different flows depending on its entrance and material. This comparison holds headwater to 1.5 D at 1% slope on a 60 ft barrel with no tailwater.

Concrete with a groove end carries about 38% more than projecting CMP of the same size. The rough corrugations push the steel pipes into outlet control, so a better entrance alone would not help them. For a steel culvert flow calculator check, pick the CMP inlet that matches the end treatment you plan to build.

Box Culvert Flow

Box culverts use span B and rise D in place of a diameter. Pick a box inlet, and the culvert flow calculator becomes a box culvert flow calculator, switching to rectangular geometry and the HDS-5 box coefficients. For a box, the critical specific head in the unsubmerged equation is 1.5 times critical depth.

A 6 ft by 4 ft concrete box with 90° wingwalls at 1% slope, 80 ft long, carrying 200 cfs gives Q / (A√D) = 200 / (24 × 2) = 4.17. The inlet is submerged, and HW/D = 0.0400 × 4.17² + 0.80 - 0.005 = 1.49, a headwater of 5.96 ft under inlet control.

That box sends water out at 13.9 ft/s, and the tool flags it as fast outlet flow that can scour the downstream channel. In the full-capacity mode the fourth card shows how much a 1 ft wider span adds, which is the usual way to grow a box culvert.

Sizing a Culvert for a Design Flow

To calculate culvert capacity the other way round, Culvert Size for a Flow tries each standard diameter in turn and stops at the first one that keeps headwater within your limit. Enter the allowable headwater as a ratio of diameter or as a depth in feet, such as the height to the road shoulder.

For 40 cfs through a square-edge concrete pipe at 1%, 100 ft long, with HW held to 1.5 D, the answer is a 33 in pipe with 3.63 ft of headwater. The 30 in size would back water up 4.31 ft, over its 3.75 ft limit.

Where cover is too shallow for one large pipe, set Number of Barrels to run two or more identical pipes side by side. The tool splits the design flow evenly, so 80 cfs through two barrels sizes each pipe for 40 cfs and also lands on 2 × 33 in, with the same 3.63 ft of headwater. In the capacity modes it multiplies the result, so three 24 in barrels carry 67.9 cfs flowing full.

The design flow usually comes from a runoff estimate. The Storm Water Runoff Calculator gives peak flow by the rational method, and the Channel Slope Calculator and Manning's Flow Calculator check the ditch that feeds the culvert. The Pipe Bedding Calculator estimates the stone under the barrel.

Pipe Flow Rate From Pipe Size

Many flow questions are about pressure pipes rather than culverts. For a full pipe, flow equals area times velocity. In US units, flow in gpm is 2.448 × d² × V, with d as the inside diameter in inches and V in ft/s.

$$Q_{gpm}=2.448d^{2}V$$

The table uses a velocity of 5 ft/s, about 1.5 m/s. Real flow in a supply line depends on the pressure available and the length of pipe, which the Hazen-Williams Equation Calculator works out from head loss.

Use the actual bore, since a pipe sold as 22 mm or 1 in is measured outside or by nominal size and has a smaller inside diameter. The Pipe Flow Calculator runs this for any size, and the Gallons Per Minute Calculator and Liters per Minute Calculator find flow from a timed bucket test.

Culvert Flow FAQs

These answers cover the culvert and flow questions people search most, with numbers from the culvert flow calculator.

How much water can flow through a 24 inch culvert?

A 24 in concrete culvert at 1% slope carries 22.6 cfs flowing full by Manning's equation, about 10,150 gpm. With a square-edge headwall, the entrance lets in only 12.8 cfs before water reaches the top of the pipe and 20.4 cfs at a headwater of 3 ft.

A 24 in projecting corrugated metal pipe carries less, about 16.4 cfs at a 3 ft headwater on a 60 ft run. Steeper slopes raise full-flow capacity but barely change the inlet limit, which is why headwater, not Manning flow, usually sets the real answer.

How much water can a 36 inch culvert handle?

A 36 in concrete culvert at 1% slope carries 66.7 cfs flowing full, about 29,900 gpm. Under inlet control with a square-edge headwall, it passes 35.3 cfs at a headwater equal to its diameter and 56.1 cfs at 4.5 ft of headwater.

The same size in corrugated metal carries 36.1 cfs flowing full at 1%, about 54% of the concrete value, because n rises from 0.013 to 0.024. Choose the CMP inlet in the tool to see where it lands under headwater limits.

What is the standard size of a culvert pipe?

Round culvert sizes in the tool run from 12 in to 120 in. They step by 3 in up to 36 in, then by 6 in, so common driveway and road sizes are 12, 15, 18, 24, 30 and 36 in. Metric pipe follows 300, 375, 450, 600, 750 and 900 mm.

Local road departments usually set a minimum diameter for driveway and road crossings, so check that rule before sizing down to the smallest pipe that passes the flow.

What are the three types of culverts?

By shape, the three main types are pipe culverts, which are round, box culverts, which are rectangular, and arch or pipe-arch culverts, which are lower and wider for shallow cover. Pipe culverts come in concrete, corrugated steel, aluminum and plastic.

The culvert flow calculator covers round concrete and corrugated metal pipe plus concrete box culverts. Arch and pipe-arch shapes use their own HDS-5 coefficients, which the tool does not include.

How do you measure a culvert pipe?

Measure the inside diameter across the opening, not the outside of the wall, since flow depends on the open area. For corrugated pipe, measure from the inside crest of the corrugations. Then measure the barrel length along the pipe and the invert elevation at each end.

The fall between the two inverts divided by the length gives the barrel slope. A 60 ft pipe that falls 0.6 ft has a 1% slope, which you enter directly as a percent.

How do you build a pipe culvert?

Set the pipe on a firm, shaped bedding of granular stone, laid on the design slope from inlet to outlet. Backfill in thin lifts compacted evenly on both sides, then add enough cover over the crown for the traffic it will carry before any vehicle crosses.

Protect both ends with a headwall, end section or riprap, since the entrance type sets capacity and the outlet is where scour starts. Match the inlet you build to the inlet you selected in the calculator.

Can I do culvert flow calculations in Excel?

Yes. For full flow in a 24 in pipe at 1%, =1.486/0.013*PI()*2^2/4*(2/4)^(2/3)*SQRT(0.01) returns 22.62 cfs. For submerged inlet control, =(0.0398*(Q/(A*SQRT(D)))^2+0.67-0.5*S)*D returns the headwater in feet.

The step-by-step solution in the tool lists each value in order, so you can rebuild a culvert flow calculator in Excel and check it against the result.

How much pressure is 20 ft of water?

Each foot of water exerts 0.4335 psi, so 20 ft of water is 20 × 0.4335 = 8.67 psi. In metric, 1 m of water is 9.81 kPa. This is why headwater depth matters at a culvert: deeper water pushes more flow through the inlet.

For a pump or supply line, the Pump Pressure Calculator converts between head in feet and pressure in psi in both directions.