Whole House Filtration System Size Calculator
Determine the correct whole house water filtration system size based on your household's peak flow rate demand, daily water consumption, and filter capacity requirements.
Formulas Used
1. Total Daily Demand (GPD):
GPD = Occupants × Daily Usage per Person
2. Peak Flow Rate (GPM):
Peak GPM = Bathrooms × 3 fixtures × 2.0 GPM × 0.6 (simultaneity factor)
Capped by pipe capacity: Q = 0.4085 × (P/100)^0.54 × d^2.63 × 7.48
3. Design Flow Rate:
Design GPM = Peak GPM × Sediment Factor (1.0–1.5)
4. Filter Capacity (gallons between changes):
Capacity = GPD × 90 days ÷ Hardness Factor
Hardness Factor = 1 + (GPG ÷ 50)
5. Pressure Drop:
ΔP ≈ 5 + (Design GPM ÷ Tank Rated GPM) × 10 PSI
6. Annual Filter Cost:
Replacements/Year = 365 ÷ (Capacity ÷ GPD)
Annual Cost = Replacements/Year × Cost per Filter
Assumptions & References
- Average US residential water use: 80–100 gallons per person per day (USGS, EPA WaterSense).
- Simultaneity factor of 0.6 applied — not all fixtures operate at peak simultaneously (AWWA M22 guidelines).
- Average fixture flow: 2.0 GPM (conservative; WaterSense fixtures ≤ 1.5 GPM, older fixtures up to 2.5 GPM).
- Pipe flow capacity estimated using Hazen-Williams equation (C = 150 for PVC/copper) with pressure as driving head.
- Filter media life baseline: 90 days at average load; reduced proportionally for harder water (NSF/ANSI 42 & 53).
- Tank sizing follows industry-standard flow ratings: 8" → 7 GPM, 10" → 10 GPM, 12" → 12 GPM, 13" → 15 GPM.
- Pressure drop across filter housing: 5–15 PSI at rated flow (manufacturer typical specifications).
- Water hardness classification per USGS: 0–3 GPG soft, 3–7 moderate, 7–11 hard, 11+ very hard.
- Filter cost estimates are approximate; actual costs vary by brand, media type, and region.
- This calculator does not account for iron, manganese, or chemical contamination — additional treatment may be required.