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EstimatorKit

Pond gallons from soundings, not a guess

A pond’s volume comes from its length and width at the waterline, the way its outline runs, and the average of stick readings taken across it, which is the only defensible way to measure a floor nobody poured. Out comes the volume in gallons, liters and cubic feet, and with it the three figures the volume is actually wanted for: the liner sheet to cut, the flow a pump needs to turn the pond over, and the count in thousands of gallons that every treatment on the shelf is dosed against.

  • Averages your soundings
  • Liner cut list
  • Pump gph
Outline at the waterline

The usual dug pond: a curved edge that bulges and pinches. Measure the longest and widest points of the water.

Soundings (inches)

Walk a stick round on a rough grid rather than down the middle. A pond dug with a marginal shelf spends a third of its surface at nine inches, and readings taken only from the deep pocket will price a liner for a pond that does not exist.

Water held

1,269 US gal

Liters
4,802
Cubic feet
170
Surface
107 sq ft
Units of 1,000 gal
1.27

6 readings averaging 19 in, deepest 30 in, on 14 × 9 ft at 0.85 of the box — a working figure for the kidney and lobed outlines garden ponds are dug to, which sit between an oval and its bounding box. It shifts with how deeply the waist is cut in — a barely waisted pond is nearer 0.9, a tight one 0.75.

Liner to cut

Bare: edge to edge, down and up
19 × 14 ft
Plus 12 in each edge to anchor
21 × 16 ft
Off the roll
20 ft wide × 21 ft
Paid for, against needed
420 vs 336 sq ft

Both liner dimensions use the deepest sounding, not the average: the sheet has to reach the bottom of the deep pocket and climb out again, and it cannot do that on an average.

Pump to move it

1,269 gph puts the whole pond through the filter once every 1 h.

Pumps are labeled at zero head. Every foot the water is lifted and every bend in the hose takes some of that back, so read the pump’s own head curve at the height of your outfall rather than the number on the box.

Measuring the water

The second step is the one that decides whether the answer is worth anything.

  1. Measure the water at its longest and widest, then say what shape it is

    Take both measurements at the waterline and square to each other, not around the curve and not at the top of the excavation — a hole measured at ground level includes the batter of its own sides. Then pick the outline: squared off keeps the whole rectangle, a clean oval keeps 0.7854 of it, and the usual lobed garden pond keeps about 0.85.

  2. Sound it in several places, including the shelf

    Push a marked stick to the bottom at points spread over the whole surface, and add a reading for each one. This is the step that separates the answer from a guess: a pond with a marginal shelf at 9 in around a third of its edge and a 30 in pocket in the middle averages nowhere near 30, and sounding only the pocket prices 58% more water than is there.

  3. Take the gallons, the liner and the pump

    The volume comes out in gallons, liters and cubic feet, along with the count in thousands of gallons that every treatment label doses against. Beside it is the liner sheet to cut — worked from the deepest sounding, not the average — matched to the nearest roll width, and the flow a pump needs to turn the whole pond over on the schedule you set.

Liners, pumps and soundings

How many depth readings does a pond need?

Enough to hit every level the pond has, which is usually five or six. The point of averaging is not precision at each spot, it is catching the shelf, the slope and the deep pocket in proportion to how much surface each covers. The six readings this page opens with — 9, 16, 28, 30, 20 and 11 in — average 19 in, where a single sounding dropped in the deep end would have said 30 and put the volume 58% high.

What size liner do I need for a pond?

Length plus twice the deepest point plus twice your overlap, and the same again for the width. For a 14 by 9 ft pond 30 in deep with a foot of overlap all round, that is a sheet 21 by 16 ft — a 20 ft roll cut to 21 ft, because the roll width has to cover the 16 and the length is simply cut off. Liner is made in fixed widths and sold by the running foot, so the width is the only real constraint and getting the length right costs nothing.

How big a pump does a pond need?

One that moves the whole volume through the filter on a schedule — once an hour where fish are stocked heavily, once every two hours for a planted water garden. The 1,269 gallon pond the page starts with therefore wants about 1,269 gph at the hourly setting. Set the schedule yourself in the box, and then read the pump’s head curve rather than the number on the carton, because every foot the water is lifted and every bend in the hose takes some of it back.

Sounding a hole that was dug rather than built

Nothing about a pond is a dimension you can read off a drawing. The edge wanders, the floor was shaped by a digger operator working to the eye, there is a planting shelf a foot down along part of the margin and a pocket at one end deep enough to keep fish below the ice. A single depth cannot describe that, and the usual advice to “use the average depth” quietly assumes you already know it. So the input here is a set of soundings taken on a rough grid, and the honest instruction is to include the boring places: the shallow beach, the shelf, the edge where it tails out. Readings taken only where the water is interesting are readings taken only where it is deep, and the tool says so on screen when every one of them clusters near the maximum.

The volume itself is rarely the point. It matters because three other numbers descend from it and each fails differently. A liner is cut from the deepest sounding rather than the average, because a sheet has to reach the floor of the pocket and climb out again, and it is bought by roll width because that is how the material is made — length is cut by the foot and costs nothing to get right. Turnover is a rate: the whole pond through the filter every hour where the stocking is heavy, every two hours for a planted pond with a few fish, and always read against a pump’s head curve rather than its box, because a pump rated at zero head is a pump measured lying on the floor. Dosing is the one that repeats forever — every bottle on the shelf is written per 1,000 gallons, so a volume that is 25% out mis-doses the pond every time anything goes in it for as long as the pond exists.

The digging is worth a line of its own, because it happens before any of the above. The default pond on this page is a little over six cubic yards of spoil — close to thirty contractor barrow loads — which is well past the point where it goes anywhere except a truck, a dumper, or a new bank somewhere else in the yard. If the spoil becomes beds, the construction calculator set to mulch will tell you what it takes to top them off, and the margin around the water usually wants the same treatment. If the outline is complicated enough that a single factor annoys you — a stream running in, a bog garden lobed off one side — trace the waterline and break it into rectangles on the square footage calculator, then treat that as your surface. A manufactured basin with a sloping floor — a swimming pool included — is measured the same way: squared off at the waterline, with the soundings spread evenly from the shallow end to the deep one.