Help & Formulas
How to Use This Calculator
Standards Reference
Supported Plumbing Codes:
- RNPCP (Philippines) - Revised National Plumbing Code of the Philippines
- IPC - International Plumbing Code
- UPC - Uniform Plumbing Code
Complete Formulas Used
Sanitary System Calculations
Total Fixture Units (FU) = Σ(Fixture Count × Fixture Unit Value)
Drainage Load (L/s) = Total Fixture Units × 0.8 (conversion factor)
Drainage Load (gpm) = Drainage Load (L/s) × 15.85032314
Pipe Sizing Logic:
- ≤ 21 FU → 50 mm pipe
- 22-40 FU → 75 mm pipe
- 41-180 FU → 100 mm pipe
- 181-800 FU → 150 mm pipe
- > 800 FU → 200 mm pipe
Storm Drain System Calculations
Q (L/s) = C × I (mm/hr) × A (m²) ÷ 360
Q (gpm) = C × I (in/hr) × A (ft²) ÷ 96.23
Where:
Q = Runoff rate
C = Runoff coefficient (0.1-1.0)
I = Rainfall intensity
A = Drainage area
Pipe Sizing Logic:
- ≤ 1.2 L/s → 75 mm pipe
- 1.3-2.5 L/s → 100 mm pipe
- 2.6-5.3 L/s → 150 mm pipe
- 5.4-15.0 L/s → 200 mm pipe
- > 15.0 L/s → 250 mm pipe
Cost & Labor Calculations
Total Material Cost = Σ(Quantity × Unit Cost)
Material Cost with Waste = Total Material Cost × (1 + Waste Factor%)
Labor Hours = Σ(Task Quantity ÷ Productivity Rate)
Labor Cost = (Plumber Hours × Plumber Rate) + (Helper Hours × Helper Rate)
Subtotal = Material Cost with Waste + Labor Cost
Total with Overhead = Subtotal × (1 + Overhead%)
Final Project Cost = Total with Overhead × (1 + Profit Margin%)
Productivity & Labor Allocation
Pipe Laying Days = Total Pipe Length ÷ Pipe Laying Productivity
Manhole Days = Total Manholes ÷ Manhole Installation Productivity
Fitting Days = Total Fittings ÷ Fitting Installation Productivity
Total Labor Days = Pipe Days + Manhole Days + Fitting Days
Labor Allocation: 60% Pipe Work, 25% Manholes, 15% Fittings
Fixture Unit Reference Table
| Fixture Type |
Fixture Units (FU) |
Notes |
| Water Closet | 6 | Per fixture |
| Lavatory | 1 | Per fixture |
| Kitchen Sink | 2 | Per fixture |
| Shower | 2 | Per fixture |
| Bathtub | 2 | Per fixture |
| Urinal | 4 | Per fixture |
| Floor Drain | 2 | Per fixture |
Runoff Coefficient Reference
| Surface Type |
Coefficient (C) |
| Roofs (smooth) | 0.90-0.95 |
| Pavement (asphalt/concrete) | 0.85-0.95 |
| Gravel surfaces | 0.25-0.60 |
| Lawns (flat, sandy soil) | 0.05-0.20 |
| Lawns (steep, clay soil) | 0.15-0.35 |
Rainfall Intensity Guidelines
| Rainfall Type |
Intensity (mm/hr) |
Intensity (in/hr) |
| Light Rain | 2.5-10 | 0.1-0.4 |
| Moderate Rain | 10-50 | 0.4-2.0 |
| Heavy Rain | 50-100 | 2.0-4.0 |
| Cloudburst | >100 | >4.0 |
Default Spacing Standards
| Component |
Default Spacing |
Purpose |
| Cleanouts | Every 30 meters | Access for maintenance |
| Manholes | Every 50 meters | Access and direction changes |
| Fittings/Elbows | Every 10 meters | Pipe direction changes |
| Downspouts | Per 100 m² roof area | Roof drainage |
| Catch Basins | Every 60 meters | Surface water collection |
Tips for Accurate Calculations
- Always verify local code requirements before finalizing designs
- Adjust productivity rates based on your crew's experience and site conditions
- Consider soil conditions when estimating excavation and installation times
- Update material costs regularly to reflect current market prices
- Use appropriate safety factors for critical applications
- Consult licensed engineers for complex projects or formal submissions
Important Disclaimer: This calculator provides estimates based on standard engineering principles.
Always consult local building codes, regulations, and licensed professionals for formal design and approval processes.
Results should be verified by qualified engineers for critical applications.