How Is Septic Tank Size Determined? A Guide for Spokane and Coeur d'Alene Homeowners

At MAGNUS Solutions, LLC, serving Spokane, WA, and Coeur d'Alene, ID, we know that choosing the right septic tank size is critical for a reliable, regulation-compliant system. An undersized tank can lead to backups, while an oversized one increases costs unnecessarily. Here’s how septic tank size is determined, based on Washington (WAC 246-272A) and Idaho (IDAPA 58.01.03) regulations and local factors.

Key Factors in Determining Septic Tank Size

Septic tank size is calculated to handle your household’s wastewater while ensuring effective treatment and compliance with state codes. The main factors include:

1. Number of Bedrooms:

   • Both Washington and Idaho use the number of bedrooms as a proxy for household water usage.

   • Washington: Minimum 1,000 gallons for homes with up to 4 bedrooms, plus 250 gallons per additional bedroom (e.g., 1,250 gallons for 5 bedrooms).

   • Idaho: Minimum 1,000 gallons for 3-4 bedrooms, with 250 gallons added per extra bedroom.

   • For example, a 5-bedroom home in Spokane or Coeur d'Alene typically requires a 1,250-gallon tank.

2. Daily Wastewater Flow:

   • Residential systems assume 120-150 gallons per person per day. A 4-person household generates about 480-600 gallons daily.

   • For non-residential properties (e.g., businesses), WA requires tanks sized at 3 times the daily flow, while ID uses twice the daily flow or 1,000 gallons minimum.

3. Soil and Site Conditions:

   • Hilly or rocky lots, common in the Inland Northwest, may require larger tanks or alternative systems (like mound or aerobic units) to compensate for poor soil percolation. Local health districts (Spokane Regional Health District or Panhandle Health District) assess soil type during permitting.

4. System Type:

   • Gravity systems often need larger tanks for adequate settling time, while pumped or advanced treatment systems may allow smaller tanks with enhanced treatment. Both states require two-compartment tanks for better solids separation.

Regulatory Requirements

• Washington: Tanks must be watertight, made of concrete or approved materials, and sized to handle at least 3 days of wastewater flow. The 2025 updates emphasize secure lids and compliance during inspections.

• Idaho: Tanks require a minimum liquid depth of 2.5 feet, 15% air space for scum, and must be engineer-designed for durability. Connection to public sewers is required if within 200 feet.

Why Proper Sizing Matters

A correctly sized tank ensures efficient wastewater treatment, prevents overflows, and extends system life. Undersized tanks risk frequent pumping or drain field failure, while oversized tanks increase installation costs. Our team conducts site evaluations to match tank size to your property’s needs and local codes.

Trust MAGNUS Solutions, LLC

Whether you’re building a new home in Spokane or upgrading a system in Coeur d'Alene, MAGNUS Solutions, LLC designs and installs septic systems tailored to your needs. We coordinate with local health districts for permitting and compliance. Need help determining your tank size? Contact us for a free consultation today!