Culvert Size Calculator

Enter your drainage area and site conditions to get a recommended culvert diameter for your driveway or private road crossing. Always round up when in doubt — undersizing a culvert is far more damaging than oversizing.

Find Your Culvert Size

Upstream Drainage Area (acres) Estimate the total land area that drains to this crossing point. When unsure, round up.

Rainfall Region

Upstream Slope

Land Cover

Consequence of Failure

Low — temporary inconvenience only, road can wash out and be rebuilt Moderate — road damage would be expensive; only access to property High — failure could endanger structures, neighboring property, or people

Recommended Culvert Diameter

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Minimum Size

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Recommended

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Conservative Choice

How to Use This Result

The recommended diameter is your minimum starting point. When you order the pipe, consider going one size larger if:

You're in the upper end of the drainage area range
The crossing is difficult to access for future maintenance or replacement
The crossing is your only access to the property
You experience flash flooding or high-intensity summer thunderstorms

Standard culvert diameters available at most suppliers: 12", 15", 18", 24", 30", 36". Pipe is typically sold in 20-foot lengths. For most residential driveways crossing a drainage ditch, 15 or 18 inches is the correct range.

Read the full culvert sizing guide for the reasoning behind these calculations, or go straight to how to install a culvert once you have your size.

🧮 Gravel Calculator

Calculate tons of gravel needed for your road or repair project.

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📋 PDF Checklist

Free printable seasonal maintenance checklist with culvert inspection items.

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🪚 Install a Culvert

Full step-by-step installation guide with tools list and common mistakes.

Read Guide →