Drainage Slope Calculator
Enter the drainage length and slope percentage to compute the fall (mm) and slope ratio (1:N), and check against Hong Kong's common minimum 1:80 (≈1.25%) requirement.
Input Data
Results
At a glance:Drainage slope is given as a percentage: units dropped per 100 horizontal units. Fall = length(m) × slope% × 10 (to mm). Slope ratio = 1 : (100 ÷ slope%). In Hong Kong, waterproofed indoor floors (bathrooms, kitchens) commonly need a fall no less than 1:80, i.e. ≈1.25%; too flat ponds water, too steep is awkward to build and use.
Formula
Fall (mm) = length (m) × slope% × 10
Slope ratio = 1 : (100 ÷ slope%)
Compliant if slope% ≥ 1.25
$$f = L\times p\times 10$$$$R = 1 : \frac{100}{p}$$$$p \ge 1.25\%$$How to Use
- Enter the horizontal drainage length (m).
- Enter the slope percentage (Hong Kong floors commonly 1–2%).
- The tool outputs fall, slope ratio and a drainage compliance check.
Case Studies
Bathroom floor
Length 3 m, slope 1.5%.
Fall 45 mm, ratio 1:67, compliant with the ≥1.25% requirement.
Balcony drainage
Length 5 m, slope 1%.
Fall 50 mm, ratio 1:100, below 1.25% — increase the slope.
FAQ
What is a 1:80 slope?
It means 1 unit drops per 80 horizontal units, i.e. 1/80 = 1.25%. Hong Kong waterproofed floors commonly use this as the minimum.
Is steeper always better?
No. Too steep makes the floor visibly tilted and awkward to walk and place items, and harder to tile; 1–2% is usually best.
Related Tools
References
Content review: Calculatorism Construction Team. Results are for reference only; please refer to the relevant authorities for the official figures.