The common 25% rule compares the combined grease and solids layers with the total liquid depth. If grease plus solids equals 25% or more, service is often considered due.
FOG / solids percentage calculator
Formula: (grease depth + solids depth) / total liquid depth × 100. Use inches or another consistent unit for all three fields.
How the calculation works
Add the grease layer depth and solids layer depth, divide by the total liquid depth, then multiply by 100. For example, 3 inches of grease plus 2 inches of solids in 24 inches of liquid is 20.8%.
How FOGProof uses the reading
FOGProof stores the reading on the service log and can flag records that are near or at the common threshold so follow-up is visible later.
Important limitation
This calculator is an operational aid. Local rules and measurement methods vary, and this does not replace jurisdiction-specific instructions.
Grease trap FAQ
Is the 25% rule used everywhere?
No. Many programs reference a 25% threshold, but local rules and measurement methods vary. Follow the authority that regulates the location.