Hydrant Flow Test Calculator

Before you can count on a hydrant to supply your fireground, you need to know how much water it can actually deliver. That's what a flow test tells you. It's not guessing — it's real data from the hydrant under real conditions.

During a flow test, you take a static pressure reading with no water flowing, then open an adjacent hydrant and read the residual pressure while measuring the actual flow with a pitot gauge. Those three numbers let you project the available flow at any residual pressure you need.

The formula used across the fire service is:

The exponent of 0.54 accounts for the non-linear relationship between pressure and flow in a water distribution system. A small pressure drop means you have a strong system with plenty of water to give. A large drop means the mains are already working hard, and you'll run out of capacity sooner.

The standard desired residual pressure is 20 PSI. This is the minimum pressure NFPA 291 recommends to keep in the system while flowing water for fire suppression. Drop below 20 PSI and you risk collapsing the water mains, losing supply to other hydrants on the fireground, and contaminating the domestic water system through back-siphonage.

Some departments or water authorities use different target residuals. If your jurisdiction has a specific standard, adjust the desired residual pressure accordingly.