Culvert Size Calculator
Enter the area that drains to the culvert, the design rainfall intensity, the land type, the pipe material and its slope. The calculator estimates peak flow with the Rational Method and sizes the pipe flowing full.
- Culvert size
- Peak flow
- Pipe diameter
Use your local design storm, often the 10 or 25 year storm.
Peak flow is Q = C × i × A, then Manning's equation sizes the pipe. Ten acres of grass and farmland in a 2 in/hr storm gives about 7 cfs. A concrete pipe at a 1% slope needs about 16 in, so use an 18 in culvert.
Culvert size chart
Grass, 2 in/hr, concrete at 1%. Search for an area.
| Area | Peak flow (cfs) | Culvert |
|---|---|---|
| 1 acres | 0.7 | 12 in |
| 5 acres | 3.5 | 15 in |
| 10 acres | 7 | 18 in |
| 20 acres | 14 | 24 in |
| 40 acres | 28 | 30 in |
| 80 acres | 56 | 36 in |
| 150 acres | 105 | 48 in |
Inches.
Drainage area and culvert
Grass at 2 in/hr. Slide the area.
The pipe is drawn under the road.
How to size a culvert
Rain to pipe. Step through it.
Area
Measure what drains to it.
Ask your local highway or drainage authority for the design storm.
Concrete, HDPE or metal
Culvert pipes. Compare them.
Strong.
Light.
Cheap.
Corrugated pipe needs a bigger size for the same flow.
Before you install
Culvert checks. Tick each item.
Tick each item as you go.
Lay the pipe on the stream bed's own slope where you can.
What you get
Culvert sizing. Each tile explains one result.
Standard size.
cfs and m³/s.
Drainage.
Land type.
Percent.
Too small a culvert floods the road and can wash it out.
Frequently Asked Questions
No question matches that search.
How do you size a culvert?
Work out the peak flow from rainfall and area, then pick a pipe that carries it.
What is the smallest culvert size?
Many authorities set fifteen or eighteen inches as the minimum.
What is the Rational Method?
Peak flow equals the runoff coefficient times rainfall intensity times area.
Does slope change culvert size?
Yes. A steeper pipe carries more water, so it can be smaller.
Is plastic or concrete culvert better?
Both last well. Plastic is lighter, concrete is stronger under heavy loads.
Do I need a permit for a culvert?
Often yes, especially in a stream or a public road.
A crossing that stays dry
Size the culvert for the storm and the road stays open when it rains.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.