Pond Calculator

Pond Calculator turns one set of pond measurements into every number you need before you dig: water gallons, liner sheet size, pump flow and head, filter and UV size, safe fish load, treatment dose and weekly top-up. It works fully offline — nothing you enter ever leaves your phone.

HOW IT WORKS
A pond is a living system, so the app runs six linked calculations off the same measurements.

Water surface. Rectangle = length x width; round = pi x diameter² / 4. A freeform pond uses the builders' shape factor: length x width x 0.85.
Volume as layers. Real ponds are stepped, not bathtubs. The whole surface counts down to the shelf, then only the deep zone's share counts below it. A 12 x 8 ft freeform pond with a 1 ft shelf and 4 ft deep zone over 60% of the area holds 1,709 US gallons (6,470 L).
Liner size — the number people get wrong and can't fix once cut. Liner side = pond side + 2 x (depth + overlap). That pond needs a 22 x 18 ft sheet. Underlay is cut to match.
Pump turnover and head. Koi ponds turn over hourly, ornamental every two hours — so 1,709 gallons means 1,709 GPH. Head combines static lift plus pipe friction (Hazen-Williams), here 6.11 ft total. A pump's headline figure is measured at zero head, so read its curve at your head.
Filter, UV and fish. Sized by volume and fish load — a heavy koi load calls for a filter rated to 3,418 gallons and a 36 W UV. Stocking uses two ceilings and the lower wins: about six adult koi here.
Dosing and top-up. Treatments are dosed per 1,000 gallons (1.71 units). A 0.10% salt target needs 14.3 lb. At 1 in/week evaporation the pond loses ~51 gallons.

WHAT YOU CAN DO

Pick freeform, rectangle, circle or oval and read gallons, litres and cubic feet instantly
Set a shelf and deep zone and see both layers separately
Get the exact liner and underlay sheet, plus the next stock size up
Switch between koi and ornamental turnover for the pump flow it demands
Enter lift, pipe run and bore for real total head, with a velocity warning
Size the filter and UV by volume and fish load
Compare volume and surface-area stocking limits and the adult koi count
Work out pond salt, dose units and weekly top-up
Open Details for a full breakdown and a liner-size-by-depth table

BUILT FOR CLARITY
A lotus-and-koi design puts the big number first, supporting figures beside it. Light and dark themes, a tiny download, no clutter, no account.

AN ESTIMATE, NOT A POND DESIGN
The 0.85 factor, UV wattage and stocking rules are industry rules of thumb. Rocks, gravel and baskets displace water, so a filled pond holds a little less. Stocking depends on species, temperature, aeration and filter maturity — and the classic one-gallon-per-inch aquarium rule is wildly wrong for koi (24–36 in). Pump flow shown is what the plumbing demands, not what a pump delivers — check the manufacturer's curve. Measure twice and buy the next liner size up. For general information and education only; not professional landscaping, aquaculture or engineering advice.

PRIVACY
No personal data collected; fully offline. Ads via Google AdMob, which may use an advertising identifier per Google's policies.

Download Pond Calculator and get the volume, liner and pump right before the first spade goes in.