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Water loss calculator
Compare what your well pumped with what reached users' meters. The gap is where leaks, slow meters and unmetered use show up, and it's often the biggest cost a small system never sees.
Showing an example: a 40-lot park over 30 days. All meters should be read on the same days, and in gallons. Convert cubic feet first (× 7.48).
14.4% not accounted forSee results
- Metered to users148,900 gal · 84.2%
- Known unmetered use2,500 gal · 1.4%
- Not accounted for25,400 gal · 14.4%
What makes up the gap
EPA's water audit guidance, which follows the terms used by the American Water Works Association, splits water that goes missing into two kinds:
- Real losses: water that physically leaves the system. That means leaks on mains and service lines, leaks and overflows at storage tanks.
- Apparent losses: water that was used but not counted. That means meters that under-read (older meters tend to slow down), unauthorized use, and reading or data errors.
Water you use on purpose without metering it (flushing mains, cleaning a tank, fighting a fire) isn't a loss. Estimate it and enter it as known unmetered use, so it doesn't inflate the result.
Getting a number you can trust
- Read the source meter and every user meter on the same day. Timing is the biggest source of error.
- Use a month or longer. Short periods magnify small timing and rounding differences.
- Make sure every user meter is included, including the office, clubhouse and maintenance shop.
- Keep a note of unusual events in your daily log. A one-off spike with a known cause shouldn't send you hunting for a leak.
Tracking down a leak
- Watch the night flow. Read the source or master meter late at night and again before sunrise. Steady use in the small hours points to a leak or a running fixture.
- Isolate sections. Close valves one section at a time while watching the meter. The section that stops the flow holds the leak.
- Walk the lines in dry weather. Look for green or soft ground, standing water, and water in valve and meter boxes.
- Compare pumping with demand. Rising pump run time with steady use is another sign: see the well pump GPM calculator.
Daily production figures come from the meter usage calculator. A sustained step up that doesn't match occupancy or weather is worth chasing early.
Questions
How much water loss is normal?
There's no single federal limit for small systems. EPA's water audit fact sheet cites an average loss of 16 percent across water systems. Some states and funding programs set their own targets. For your system, the trend matters most: a percentage that climbs month over month deserves a look even if it's below someone else's benchmark.
My park has no lot meters. Can I still estimate losses?
Not as a percentage, because there's nothing to compare production with. What you can do is watch the source meter at night, when little water should be used. Record it before and after a quiet stretch; steady flow through the small hours usually means a leak or a running fixture.
Why is metered use higher than production?
Almost always timing: the source meter and the user meters weren't read over the same days. Otherwise one of the meters is reading high, or a reading was mistyped. Re-read everything on the same day and try again.
Sources
Last reviewed September 28, 2026. Rules change and states add their own; confirm with your state drinking water program.