1 In 20 Fall Calculator

Quickly calculate the vertical drop needed for a 1:20 fall across a given run length. This premium drainage slope calculator is ideal for builders, landscapers, plumbers, civil contractors, and property owners planning channels, ramps, paving, gutter runs, pipe trenches, and surface water drainage.

Calculate required fall

Enter a horizontal run and choose whether you want the standard 1 in 20 fall or a custom gradient. The calculator returns drop in millimeters, centimeters, meters, and inches, plus percentage slope and angle.

Quick field notes

Use these practical reminders when applying a 1 in 20 fall on-site.

• Keep units consistent. If the run is in meters, the fall result is easiest to understand in meters and millimeters.
• Confirm whether you are measuring horizontal run or actual sloped length. Most drainage specs use horizontal run.
• Check the finished surface depth, bedding thickness, and outlet invert level before setting grades.
• Too little fall can leave standing water. Too much fall can create erosion, scouring, or usability issues depending on the application.

Expert Guide to Using a 1 in 20 Fall Calculator

A 1 in 20 fall calculator helps you convert a slope ratio into a practical, measurable vertical drop. In construction, landscaping, plumbing, drainage design, and site preparation, professionals often specify gradients as a ratio rather than as a percentage or angle. When someone says a surface must have a “1 in 20 fall,” they mean the level should drop 1 unit vertically for every 20 equal units of horizontal distance. The units can be millimeters, centimeters, meters, inches, or feet, as long as the same unit is used on both sides of the ratio.

This matters because water management is one of the most important elements of any outdoor or below-ground build. A paving area with inadequate fall can pond water. A drain trench with a poorly set invert can collect silt or fail to self-drain. A channel that drops too aggressively can move water too fast, increasing wear or causing washout. The purpose of a 1 in 20 fall calculator is to remove guesswork by translating the ratio into an exact drop over your chosen run.

For example, if a path, trench, or slab edge is 10 meters long, a 1 in 20 slope means the required fall is 10 ÷ 20 = 0.5 meters. That equals 500 millimeters or about 19.69 inches. Once you know that value, you can set string lines, laser levels, screed rails, or pipe bed elevations more accurately.

What does 1 in 20 mean in percentage and degrees?

A ratio of 1:20 can be expressed in more than one way:

• Ratio form: 1 in 20
• Decimal slope: 0.05
• Percentage grade: 5%
• Approximate angle: 2.86 degrees

Many people find the percentage useful because it quickly communicates steepness. A 5% slope means the surface drops 5 units vertically for every 100 units of horizontal travel. The angle is usually less useful on-site than the ratio or percentage, but it can help when using digital inclinometers or verifying machine grading settings.

Gradient	Decimal	Percent grade	Approximate angle	Drop over 10 m run
1 in 10	0.10	10%	5.71 degrees	1.00 m
1 in 20	0.05	5%	2.86 degrees	0.50 m
1 in 40	0.025	2.5%	1.43 degrees	0.25 m
1 in 60	0.0167	1.67%	0.95 degrees	0.167 m

The basic formula behind the calculator

The formula is simple:

Fall = Run ÷ Ratio

If the specified fall is 1 in 20, the ratio is 20. If the run is 8 meters, the fall is 8 ÷ 20 = 0.4 meters. If the run is 3,000 millimeters, the fall is 3,000 ÷ 20 = 150 millimeters. This is why using the same units for both the run and the result is so important. The ratio itself is unitless, so the result always comes back in the same unit as the run.

The calculator above also converts the result into multiple units because real projects often mix measurement systems. Plans may be dimensioned in meters while installers work in millimeters. Some equipment settings may be entered in feet or inches. By converting automatically, you reduce transcription errors and can communicate more clearly with everyone involved.

Where a 1 in 20 fall is commonly used

A 1 in 20 fall is considered a relatively pronounced gradient in many drainage contexts, though whether it is appropriate depends on the surface, material, water volume, and user requirements. Typical situations include:

• Short drainage channels where positive runoff is essential
• Paving transitions where a visible slope is acceptable
• Landscaped swales or grading zones that need firm directional runoff
• Ramp-like surfaces where design intent, accessibility, and code requirements are carefully checked
• Temporary site works where ensuring drainage is more important than maintaining a flatter finish

It is not automatically the correct slope for every drain, slab, path, or pipe. In fact, some systems operate effectively at much shallower gradients, while others may require special handling if the slope becomes too steep. That is why this calculator should be used as a calculation tool, not a substitute for local code, engineering design, or product-specific installation guidance.

Worked examples for a 1 in 20 fall

Here are practical examples that show how to apply the math in the field.

1. Concrete apron, 4 meters long: 4 ÷ 20 = 0.2 meters. Required drop: 200 mm.
2. Drainage trench, 12 meters long: 12 ÷ 20 = 0.6 meters. Required drop: 600 mm.
3. Garden path, 2.4 meters long: 2.4 ÷ 20 = 0.12 meters. Required drop: 120 mm.
4. Patio edge, 18 feet long: 18 ÷ 20 = 0.9 feet. Required drop: 10.8 inches.

As you can see, the slope becomes significant over long runs. A 1 in 20 fall across 20 meters produces a full 1-meter drop. On some sites that may be easy to achieve. On others, it may clash with thresholds, retaining wall heights, or connection points to existing drainage infrastructure. The calculator helps identify these issues early.

Key takeaway: A 1 in 20 fall is equal to a 5% grade. Over every 1 meter of horizontal run, the drop is 50 millimeters. This mental shortcut is useful for quick checks on-site.

Common conversion table for 1 in 20 fall

The following values are exact calculations and are helpful for layout checks, quotations, and installation planning.

Horizontal run	Required fall	Required fall in mm	Required fall in inches
1 m	0.05 m	50 mm	1.97 in
2 m	0.10 m	100 mm	3.94 in
5 m	0.25 m	250 mm	9.84 in
10 m	0.50 m	500 mm	19.69 in
15 m	0.75 m	750 mm	29.53 in
20 m	1.00 m	1000 mm	39.37 in

Why drainage slope selection matters

Water that does not move as intended can create a chain of avoidable problems: freeze-thaw damage, slippery surfaces, staining, erosion at discharge points, undermining of bases, mosquito breeding in standing water, or infiltration near foundations. The right fall supports function and longevity. The wrong fall can lead to expensive rework.

In practice, “right” depends on the specific system. Surface paving usually has one set of constraints. Underground storm or wastewater piping may have another. Accessibility standards can further affect how much slope is acceptable on walkable routes or ramps. That is why good installers combine the mathematics of slope with local regulations, material tolerances, and end-use requirements.

How to measure and set out a 1 in 20 fall

1. Measure the horizontal run accurately between the start point and endpoint.
2. Use the calculator to find the required vertical drop.
3. Establish a benchmark or known level reference.
4. Set the high point and low point using a laser level, dumpy level, transit, or water level.
5. Run a string line or screed guides between points and check intermediate levels.
6. Confirm the finished surface or pipe invert, not just the excavation depth.
7. Recheck after bedding, compaction, or formwork movement.

On larger projects, checking every few meters is wise. Small deviations can accumulate. A few millimeters lost at each support point may flatten the final gradient enough to create performance issues, especially on systems intended to discharge freely.

How 1 in 20 compares with accessibility-related slopes

One reason people search for a 1 in 20 fall calculator is to understand whether a slope is mild or steep. In many design contexts, a 1:20 slope is an important threshold because it is equivalent to a 5% grade. Accessibility guidance frequently treats 1:20 as a significant benchmark for walking surfaces and ramps, although the exact legal and technical treatment varies by jurisdiction and application. If your project involves public access, building compliance, or mobility needs, always verify the latest local requirements and project-specific code interpretations before construction.

For example, U.S. accessibility guidance from the Access Board is a useful reference point for understanding slope thresholds on accessible routes and ramps. Similarly, stormwater and site grading resources from government agencies and land-grant universities help explain how slope influences water movement and erosion control.

Authoritative references for slope, drainage, and site design

• U.S. Access Board: Ramps and curb ramps guidance
• U.S. Environmental Protection Agency: Stormwater and NPDES resources
• Penn State Extension: Stormwater basics and drainage problem identification

Frequent mistakes when calculating fall

• Mixing units: Entering feet and expecting millimeters without conversion is a common source of error.
• Using sloped length instead of horizontal run: Most gradient calculations are based on horizontal distance.
• Ignoring build-up layers: Paving thickness, mortar beds, screed layers, and coverings change final levels.
• Not checking outlet levels: The calculated fall may be correct, but the receiving drain or channel may sit too high.
• Assuming one ratio fits all conditions: Pipe diameter, material, discharge volume, and code requirements still matter.

When should you use a custom ratio instead of 1 in 20?

The custom ratio option is useful whenever your specification calls for a different slope. Some surfaces only need a gentle fall to shed water. Some pipe runs are designed around hydraulic behavior and may use gradients quite different from 1:20. Some ramps must satisfy accessibility requirements that are stricter than a simple drainage target. The calculator lets you switch from the preset 1 in 20 ratio to any custom value, so you can compare options instantly.

For instance, if you are balancing visual flatness with runoff on paving, you may compare 1:40 against 1:20. On a 10-meter run, 1:40 gives a 250 mm drop, while 1:20 gives a 500 mm drop. That difference can be decisive near building thresholds, garage entries, door clearances, or tie-ins to existing hardscape.

Final advice for using a 1 in 20 fall calculator effectively

A 1 in 20 fall calculator is most valuable when used early and often. Use it during concept design to test whether the site geometry supports the intended drainage path. Use it again during setting out to establish exact high and low levels. Use it during installation to verify that the built work still meets the design intent after excavation, compaction, and finishing layers are added.

The math is straightforward, but the real-world application is where quality matters. A well-calculated and well-set 1 in 20 fall can improve runoff performance, reduce future maintenance, and support a more durable installation. Whether you are laying paving, preparing a channel drain, grading a landscaped area, or checking a sloped route, the calculator above gives you a fast, reliable way to turn the ratio into actionable dimensions.

Note: This tool provides geometric calculations only. Always confirm design suitability, safety, and code compliance for your specific project and jurisdiction.