Calculate water loss in gallons per day from a leaking pipe, a pool or tank level drop, or a dripping tap, with percentage lost and optional water cost.

Idealized orifice flow at a steady pressure; real leaks can differ.

Starts at 60 psi; typical home water pressure is 40โ€“80 psi. For a leak to open air, use the gauge pressure.

Opening coefficient, tariff and loss period
0.62 is a typical sharp-edged hole value; real leaks vary.

Optional. kgal = 1,000 US gal; CCF = 100 ftยณ. Include applicable volumetric sewer fees if appropriate; excludes fixed charges and tiers.

Constant rate/duty assumed across the period; leave blank to omit totals.

Water Loss Formula

For a measured volume decrease, use the following formula. Divide by the actual measurement period to obtain a rate; a net change can include evaporation, withdrawals, additions or leaks.

WL = IV - FV

Variables:

  • WL is the water loss (gallons)
  • IV is the initial volume of water (gallons)
  • FV is the final volume of water (gallons)

Subtract final volume from initial volume using consistent units. The calculator also uses Q = Cd ร— A ร— โˆš(2 ฮ”P / ฯ) for an idealized steady opening, with differential pressure across the opening and assumed water density 1,000 kg/mยณ. Cd and geometry require suitable assumptions; pressure at an upstream gauge can differ during flow. USBR explains differential-pressure flow and coefficient calibration.

What is Water Loss?

Water loss refers to the amount of water that is lost from a system due to various reasons such as leaks, evaporation, or overflow. It is an important metric in water management for municipalities, agriculture, and any system where water conservation is important. Understanding and minimizing water loss can lead to significant cost savings and more efficient water use.

How to Calculate Water Loss?

The steps below describe the starting/ending-volume route: select Pool or tank, then Starting and ending volume. For a level-drop measurement, volume decrease = constant surface area ร— drop. An optional bucket comparison estimates positive loss beyond the bucket drop while retaining the signed difference; it cannot establish or rule out a leak. Follow comparable measurement conditions and account for rain, refill and other water changes. See the EPA pool water-efficiency guide.

  1. First, determine the initial volume of water (IV) in gallons.
  2. Next, determine the final volume of water (FV) in gallons.
  3. Next, gather the formula from above = WL = IV – FV.
  4. Enter the actual measurement period and calculate Water Loss in US gallons per day. An unchanged volume is zero loss; a net gain is flagged separately.
  5. For a dripping fixture, choose counted drips or timed collection. The editable default of 15,140 drips per US gallon is the USGS educational approximation, not a standard drip size. Daily run time defaults visibly to 24 hours and can be adjusted from 0 to 24; timed collection avoids relying on drip volume. Optional USD cost uses only the entered variable tariff, excluding fixed charges and tiers.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

Initial volume of water (IV) = 500 gallons

Final volume of water (FV) = 450 gallons. If measured over 24 hours, net decrease = 500 โˆ’ 450 = 50 US gallons, equivalent to 50 US gal/day. This is net observed water loss, not proof of a leak.