Water Sources
Available Fire Flow
Available fire flow is an estimate of how much more water a hydrant can still deliver, based on how far its pressure dropped between static and residual. A small drop means a lot left; a large drop means little.
Rather than guess, the operator measures: note the static pressure, flow a known line, read the residual, and the size of that drop estimates how many more lines of similar flow the hydrant can support.
It turns a vague sense of a 'good' or 'bad' hydrant into a number the operator can plan around. Knowing the available flow tells you whether the hydrant can feed another attack line or a master stream before you commit to it.
Formula
percent drop = (static − residual) ÷ static
Flow one known line and compare: a small percent drop means the hydrant has a lot left, a large drop means little. The size of the drop maps to how many more lines of similar flow the source can support — a training estimate, not a guarantee.
Common questions
- What is available fire flow?
- Available fire flow is an estimate of how much more water a hydrant can still deliver, based on how far its pressure dropped between static (at rest) and residual (under flow). A small drop means a lot left; a large drop means little.
- How do you estimate available fire flow at a hydrant?
- Read the static pressure, flow a known line, and read the residual. The percent the pressure dropped — (static minus residual) over static — maps to how many more lines of similar flow the hydrant can support. It is a planning estimate; verify against your department's method before committing to it.
Where this shows up
Calculator
Hydrant Available Water