Water Flow Rate Calculator
Calculate water flow from a measured volume and time, estimate pipe velocity, target fill time, daily volume and water cost.
Default project preview
10 gal/min
Litres per minute
37.85 L/min
Flow per hour
600 gal/h
Pipe velocity
7.26 ft/s
Target fill time
10 min
Calculator
Build the material order
Your order will appear here
Enter the project details and calculate a rounded material estimate.
Input and output guide
What each value means
The labels and units below are part of this calculator’s visible methodology. Replace every default with measurements or product data from the actual project.
Measured water volume
Enter this project assumption in gal. The current imperial default is 5 gal.
Time to collect volume
Enter this project assumption in seconds. The current imperial default is 30 seconds.
Internal pipe diameter
Enter this project assumption in in. The current imperial default is 0.75 in.
Target fill volume
Enter this project assumption in gal. The current imperial default is 100 gal.
Equivalent flow duration
Enter this project assumption in hours/day. The current imperial default is 1 hours/day. Minimum: 0. Maximum: 24.
Water price
Enter this project assumption in per 1,000 gal. The current imperial default is 8 per 1,000 gal. Minimum: 0.
How to read the outputs
Litres per minute
universal conversion. Default preview: 37.85 L/min.
Flow per hour
continuous equivalent. Default preview: 600 gal/h.
Pipe velocity
from entered internal diameter. Default preview: 7.26 ft/s.
Target fill time
100 gal target. Default preview: 10 min.
Daily volume
1 flow hours. Default preview: 600 gal.
Daily water cost
entered water rate. Default preview: $4.80.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Flow = measured volume ÷ collection time. Pipe velocity = volumetric flow ÷ internal pipe area. Target fill time = target volume ÷ measured flow.
Reviewed project guidance
How to use this estimate in a real project
Last reviewed: July 30, 2026
Calculate flow from collected volume and elapsed time or from entered velocity and pipe area.
Calculation steps
- 1Enter the actual project dimensions or operating values.
- 2Apply the calculator formula using consistent units.
- 3Review the unrounded result and the editable allowance.
- 4Round only to the product, stock length or equipment increment actually purchased.
Worked numerical example
Filling a 20 L container in 15 seconds equals 80 L/min, or about 1.33 L/s.
Purchasing and rounding
For a field bucket test, repeat the timed fill several times and use a representative flow rather than purchasing equipment from one reading. For pipe calculations, use actual internal diameter and the relevant operating condition when the result will inform valves, pumps or treatment equipment.
Interpret the result
The result expresses how much water passes per unit time. It does not by itself establish available pressure or system capacity: a fixture can show adequate static pressure yet deliver poor flow because of restrictions, pipe losses or source limitations.
Page-specific assumptions
- Inputs describe one consistent project condition.
- Entered yields and efficiencies match the selected product or equipment.
- Site-specific code and engineering requirements are checked separately.
Common mistakes
- Mixing imperial and metric inputs.
- Using nominal dimensions where actual coverage controls.
- Rounding intermediate values too early.
- Treating a quantity estimate as design approval.
Primary and technical references
BuildMeter provides planning estimates, not engineering, architectural, electrical, mechanical, plumbing, permit or code approval. Review the methodology and verify final decisions with product documentation and qualified professionals.
BuildMeter Guides
Understand the measurements and assumptions
Use these visible reference guides before changing project inputs or planning allowances.
FAQ
Water Flow Rate Calculator questions
Why should the test use the actual outlet?+
Fittings, hoses, valves and pressure losses at that outlet affect the measured flow.
Does flow rate equal water pressure?+
No. Pressure is available force, while flow is volume delivered over time; the two interact through system resistance.
Why is internal diameter important?+
Pipe velocity depends on the true internal flow area, which can differ from nominal pipe size.