How long does tank water last?

Divide the tank size by the flow rate. A 1000-gallon tank at 150 gpm lasts about 6.7 minutes; at 95 gpm it lasts about 10.5 minutes.

How much water do I need for a 20-minute operation?

Multiply the flow rate by the time. 150 gpm for 20 minutes needs 3000 gallons, more than most single engines carry, so a supply is required.

What flow keeps up with my supply?

If your incoming supply delivers fewer gpm than you are flowing, you are drawing down your tank. Match flow to sustainable supply for continuous operations.

Does this account for pump or nozzle losses?

No. This is a straight volume-over-flow calculation. The flow rate you enter should be your actual delivered gpm.

Water supply duration calculator

How long will your water last? Enter the water on hand and the flow rate to get the run time — essential for tank water, tender/tanker shuttles, and rural water supply planning.

Tank / supply preset

Water available (gal)

Flow rate (gpm)

Supply duration — min

Duration is the water on hand divided by the flow rate. For sustained operations, your incoming supply (hydrant, relay, or tender shuttle) must match or exceed the flow rate, or the clock is running on your tank.

How supply duration is calculated

Run time is simply the available water divided by how fast you are flowing it:

Duration (min) = Water (gal) / Flow (gpm)

To work the other way — how much water you need for a set time — multiply:

Water needed (gal) = Flow (gpm) × Time (min)

Worked example

A 1000-gallon tank flowing a 150 gpm attack line with no incoming supply:

Duration = 1000 / 150 = 6.7 min

That is why establishing a continuous water supply early is critical — a single tank empties fast at fireground flows.

Frequently asked questions

How long does tank water last?: Divide the tank size by the flow rate. A 1000-gallon tank at 150 gpm lasts about 6.7 minutes; at 95 gpm it lasts about 10.5 minutes.
How much water do I need for a 20-minute operation?: Multiply the flow rate by the time. 150 gpm for 20 minutes needs 3000 gallons — more than most single engines carry, so a supply is required.
What flow keeps up with my supply?: If your incoming supply (hydrant, relay, or tender shuttle) delivers fewer gpm than you are flowing, you are drawing down your tank. Match flow to sustainable supply for continuous operations.
Does this account for pump or nozzle losses?: No. This is a straight volume-over-flow calculation. The flow rate you enter should be your actual delivered gpm.

Worked Example

The scenario: a 1,000-gallon tank supplying one 1¾" line at 150 GPM.

Duration = usable water ÷ total flow. Figuring ~90% of tank capacity is actually usable: 900 ÷ 150 = 6 minutes. Open a second identical line and it's 3 minutes.

Six minutes is enough to darken a room-and-contents fire — and nowhere near enough for anything bigger. That's the calculation behind the rule every engine company lives by: the attack starts on tank water, but the water supply plan starts before the attack does.

Running out of water is a pump-operator failure mode. Train it out.

PumpForge models tank water, relay, and draft supply so operators learn to manage water before the tank runs dry — realistic panel, automatic scoring, after-action reports.

Explore PumpForge →

For training and estimation; always verify against your apparatus and SOPs. Related tools: required fire flow, pump discharge pressure, and foam concentrate. Water supply management is a core competency in NFPA 1002. Verify hydrant capacity before you rely on it with a fire hydrant flow test.