Yard & Garden calculator

French Drain Slope CalculatorDrain Grade and Total Drop

Calculate French drain slope from run length and total drop. Get percent grade, inches per foot and drop per 100 feet for trench layout checks.

Updated August 20, 2026

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✓Clear formulaSee how the result is built

✓Editable inputsMatch your product

✓Purchase planningCalculator stays easy to use

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Project details

Enter your measurements and material details.

Planning estimate only. Verify product specifications and project requirements before ordering.

💡Tip:

Measure elevation with an appropriate level, laser or survey method; visual ground slope is not accurate enough for drainage layout.

How the French Drain Slope Calculator works

Drain slope describes the change in elevation over a horizontal run. This calculator converts a measured total drop into percent grade and inches of fall per foot so you can compare the layout with a project specification.

Formula

Slope % = vertical drop ÷ horizontal run × 100. Inches per foot = total drop in inches ÷ run length in feet.

Measure before you buy

Measure horizontal drain length and the elevation difference between the start and end points. Do not use the pipe length along a steep slope as the horizontal run if precise grade is required. For long drains, verify intermediate elevations rather than assuming the trench is uniformly sloped.

Quantity and slope are separate issues; use the gravel calculator for stone.
Avoid relying on visual grade over long distances.
Drainage design and discharge location can be subject to local requirements.

Common questions

How do I calculate French drain slope?

Divide vertical drop by horizontal run and multiply by 100 for percent slope.

How many inches of fall per foot is 1%?

About 0.12 inch per foot.

Can the drain be too steep?

Hydraulic and installation considerations depend on the system; this calculator only reports geometry.

Should I measure pipe length or trench length?

Use horizontal run for grade calculations.

Does this tell me the required slope?

No. It calculates the slope from your measurements; use project guidance for the target value.

Worked example: French Drain Slope Calculator

Using the example inputs shown in the calculator — Drain run length (ft) = 60, Total elevation drop (in) = 7.2 — the result is 1% slope. This demonstrates the formula with a complete set of numbers rather than a hidden assumption.

The result breakdown also shows Fall per foot: 0.12 in/ft; Drop per 100 ft: 12 in. Replace the example values with field measurements and the exact product coverage, yield or package size you plan to buy.

Recalculate the french drain slope calculator whenever the measured dimensions or product specification changes. Keep the calculator result separate from any structural or code decision.