Drainage Slope Calculator

How to use this calculator

1. Enter the horizontal run in feet.
2. Enter the vertical drop in inches over that run.
3. See slope in percent and inches per foot to compare against drainage recommendations.

Inputs explained

Run

Horizontal distance the slope covers (feet).

Drop

Vertical fall over that distance (inches).

When to use it

• Verifying driveway or patio slope for water runoff.
• Checking yard grading away from a foundation (e.g., 2% slope).
• Planning drain tile or French drain fall per foot.

Tips & cautions

• Typical drainage slopes: ~1–2% (≈1/8–1/4 inch per foot) for patios/sidewalks; foundations often want ~6 inches fall over 10 feet (~5%).
• Avoid over-steep slopes that cause erosion; use small falls over longer runs when possible.
• Use a level and string line on site to verify measurements.
• Simple slope math; does not account for soil type, permeability, or rainfall intensity.
• Assumes straight run; curved/irregular grades need more detailed layout.
• No hydraulic sizing—this is grade only, not pipe sizing or flow capacity.

Worked examples

Deep dive

FAQs

What slope is recommended away from a foundation?

A common guideline is about 6 inches of fall over the first 10 feet (~5%). Check local codes and soil conditions.

Is 1% enough for patios?

Many patios/sidewalks use 1–2% (about 1/8–1/4 inch per foot). Increase slope or improve drainage if water ponds.

Does soil type matter?

Yes. Clay drains slowly and may need more slope/drainage features. This calculator doesn’t model soil permeability.

How do I measure slope on-site?

Use a level and string line or laser level: measure drop over a known run and plug into this tool.

Can I use metric?

Convert to feet/inches for now; this tool outputs percent and inches per foot.

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