Water Supply
Estimated Remaining Supply
Project additional hydrant flow at a selected residual pressure.
What It Tells You
An estimate of total and additional hydrant flow available at a chosen residual pressure.
Formula
Projected flow = current flow × ((static − target residual) ÷ (static − current residual))^0.54
What the Variables Mean
- static
- pressure before water is flowing PSI
- current residual
- pressure while the test flow is moving PSI
- target residual
- the pressure where available flow is projected PSI
- current flow
- measured hydrant flow GPM
Plain-Language Explanation
The pressure drop during a hydrant flow test shows how the water system responds to demand. Reverse Flow uses the standard 0.54 projection relationship.
Worked Example
Static is 80 PSI, residual is 50 PSI at 1,000 GPM, and the target residual is 20 PSI.
- Projected = 1000 × ((80 − 20) ÷ (80 − 50))^0.54
Fireground Takeaway
Twenty PSI is the standard reference. Lower targets are aggressive estimates, not rated available fire flow.
Practice This Formula
Open Reverse Flow and start a Practice Quiz filtered to this topic.
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