Drainage Slope Calculator
Get the right slope for your drain pipes so water flows properly without backing up or leaving deposits. Works in Imperial and metric units.
Pipe & System Parameters
Standard drainage lines
Override with custom slope percentage
If known, helps verify velocity requirements
Standard Slope Requirements
| Pipe Size | Minimum Slope | Recommended | Typical Use |
|---|---|---|---|
| 1-1/2" - 3" | 1/4" per foot (2.08%) | 1/4" per foot | Sinks, lavatories, small drains |
| 4" | 1/8" per foot (1.04%) | 1/4" per foot | Toilet drains, main branches |
| 6" - 8" | 1/16" per foot (0.52%) | 1/8" per foot | Building drains, sewers |
| 10" - 12" | 1/20" per foot (0.42%) | 1/12" per foot | Main sewers, storm drains |
Common Applications
Residential
- • Kitchen sink drains
- • Bathroom drainage
- • Basement floor drains
- • Washing machine drains
Commercial
- • Restaurant grease interceptors
- • Hospital drainage systems
- • Industrial floor drains
- • Parking garage drainage
Storm Water
- • Roof drainage systems
- • Parking lot drainage
- • Foundation drainage
- • Municipal storm sewers
Frequently Asked Questions
Which drainage type should I select for my project?
Pick the one that matches what you're draining. "General Drainage" works for most basic applications. Choose "Toilet/Water Closet" for any toilet drain (the calculator will enforce the 3" (75mm) minimum pipe size). "Floor Drain" is for areas where debris might be an issue. "Storm Drainage" is for roof drains, parking lots, and outdoor applications. When in doubt, "General Drainage" is usually fine.
Should I enter a custom slope or just use what the calculator recommends?
Start with the recommended slope first - that's what the codes suggest and what works in most situations. Only use custom slope if you have specific site constraints or your engineer specified something different. If you do enter a custom slope, the calculator will tell you if it meets code requirements and has adequate self-cleaning velocity.
Can I use this calculator with metric measurements?
Absolutely. Just toggle to metric mode and enter your pipe length in meters and flow rate in liters per minute. The pipe sizes will show in millimeters, and results will display in metric units. The slope percentages stay the same since they're ratios, but drop distances show in millimeters and velocities in meters per second.
What does the "Total Drop" result tell me?
That's how much lower the end of your pipe needs to be compared to the beginning. So if you've got 50 feet (15 meters) of pipe and the calculator says 12.5 inches (318mm) total drop, your pipe outlet needs to be 12.5 inches (318mm) lower than where it starts. This helps you figure out excavation depth and whether your slope will work with existing connections.
When should I bother entering the expected flow rate?
Only if you know it from fixture unit calculations or have specific flow requirements. Most of the time, leave it blank and let the calculator show you the pipe's capacity instead. But if you're designing for a specific flow (like a commercial kitchen or storm drain), enter it to verify your slope gives adequate velocity for that flow rate.
What does "Self-Cleaning Velocity" mean and why should I care?
This tells you if water flows fast enough to carry away solids and prevent buildup. If the calculator shows "ADEQUATE," you're good. If it shows "LOW," your slope might be too shallow and you could get clogs over time. The minimum is usually 2 feet per second (0.6 m/s) for most drains, higher for floor drains with debris.
What should I do if the calculator says my slope isn't code compliant?
If it's too shallow (below minimum), you need to increase your slope or use a larger pipe. If it's too steep (above 40%), you need to reduce the slope, possibly by using a longer route or step-downs. The calculator shows the acceptable range, so adjust your slope percentage until you're in the green zone.
Why won't the calculator let me use a 2" (50mm) pipe for toilet drainage?
Because codes require minimum 3" (75mm) pipe for toilet drains. The calculator enforces this - if you select "Toilet/Water Closet" drainage type with anything smaller, it'll give you an error. This isn't negotiable; it's a code requirement to handle solid waste properly.
How accurate are these slope calculations for real-world conditions?
Pretty accurate for standard applications. The calculator uses Manning's equation and standard pipe roughness values for PVC, which covers most modern installations. However, it assumes clean pipes and uniform conditions. Real-world factors like debris, pipe joints, and varying flows can affect performance, so the recommendations include safety margins.
Should I size my pipe based on the capacity this calculator shows?
The capacity shown is the maximum theoretical flow for that pipe at the calculated slope. It's useful for verification, but don't size pipes based on this alone. Use fixture unit calculations or engineering design flows first, then use this calculator to verify your chosen pipe size and slope will handle that flow with proper velocity.
My drain line needs to go around obstacles - how do I handle direction changes?
Use sweep fittings instead of sharp 90s whenever possible - they cause way less turbulence and keep things flowing smoothly. If you need to change direction more than 45 degrees, try to maintain at least half your target slope through the bend. Sometimes it's worth running the line a bit longer to avoid sharp turns. Plan your route first, then calculate slope based on the actual path the water will take.
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Complete Your Drainage System Design
Proper drainage slope is essential for system performance. Design complete drainage systems with these related calculators: