Your machine's GPM rating times the time on the trigger tells you the gallons a job drinks — the number that sizes your buffer tank and answers "can I use your spigot?"
A 4 GPM machine running 2.5 hours uses about 600 gallons — more than a 275-gallon tote, so plan the water source.
The example
Three jobs, each with the machine's GPM and hours of trigger time.
| A | B | C | D | |
|---|---|---|---|---|
| 1 | Job | GPM | Hours | Gallons |
| 2 | House wash | 4 | 2.5 | 600 |
| 3 | Driveway | 4 | 1 | 240 |
| 4 | Fleet lot | 5.5 | 4 | 1,320 |
The formula
Three factors, one multiplication:
How it works
GPM is per minute, so hours need the ×60:
B2is the machine's rated gallons per minute.C2is hours of actual spraying time, not total time on site.*60converts hours to minutes so the units line up: gallons/min × minutes = gallons.
Trigger time is usually 60–80% of on-site time — you are not spraying while moving hoses.
Try it: interactive demo
Enter your machine's GPM and the spraying hours.
Variations
Will the tank last?
Divide your buffer tank size by GPM for minutes of runtime.
Cost of the water
Multiply gallons by your water cost per gallon.
Pitfalls & errors
Use trigger hours, not hours on site — billing 8 site hours as spray time triples the real water number.
A 275-gallon tote at 4 GPM buys you about 68 minutes of continuous spraying — know your runway before the driveway is half done.
Practice workbook
Frequently asked questions
Is rated GPM what the machine really flows?
How do I size a buffer tank?
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