Enter the cross-sectional area and an average (mean) flow velocity to estimate the flow rate (discharge) through the culvert using the continuity equation (Q = A·V). You can also use the Rational Method tab to estimate peak runoff (Q = C·i·A) for small drainage areas, which is often used when sizing culverts.

Culvert Capacity Calculator

Enter a pipe size to get its flow capacity, or enter drainage area to size a new culvert.

Pipe Capacity
What Size Do I Need?
Show calculation ▾
Copy result

Culvert Capacity Formula

The following formula is used to calculate flow rate (discharge) through a cross section when the mean velocity is known (continuity equation). Note: a culvert’s hydraulic capacity for a real installation depends on inlet/outlet control, headwater and tailwater, entrance losses, roughness, slope, barrel length, and whether the barrel is flowing full—not only area and velocity.

Q = A * V

Variables:

  • Q is the flow rate (discharge) through the culvert (m³/s)
  • A is the cross-sectional flow area (m²)
  • V is the mean flow velocity at that section (m/s)

To estimate the flow rate, multiply the cross-sectional flow area by the mean velocity at that section. In practice, velocity is often estimated from hydraulic methods (for example, Manning’s equation for open-channel flow in the barrel) or from measured/assumed design velocities.

What is a Culvert Capacity?

Culvert capacity is the maximum flow rate (discharge) a culvert can convey under specified conditions (such as headwater depth, tailwater, inlet geometry, barrel roughness, slope, and length). In many contexts, people also use “capacity” informally to mean the current flow rate through the culvert; the Area–Velocity tab of the calculator estimates that flow rate when area and mean velocity are provided.

How to Calculate Culvert Capacity?

The following steps outline how to calculate the Culvert Capacity.

  1. First, determine the cross-sectional flow area of the culvert (A) in square meters.
  2. Next, determine or estimate the mean velocity of the water flow (V) in meters per second (for example from a hydraulic method or a design assumption).
  3. Next, gather the formula from above = Q = A * V.
  4. Finally, calculate the flow rate (Q).
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

cross-sectional area of the culvert (A) = 5 square meters

velocity of the water flow (V) = 10 meters per second

Solution: Q = A × V = 5 × 10 = 50 m³/s.