PondTools

Pumps and plumbing

How to calculate pond pump head and pick the pipe size

Total dynamic head is the vertical lift from the pond surface to the top of the waterfall plus the friction the pipe, elbows and check valve add at the flow you want. You read the pump's published chart at that head instead of trusting the number on the box. For a backyard pond moving 3,000 to 6,000 gallons an hour, friction in 1.5 inch pipe is usually larger than the lift itself, so the pipe decides more than the pump does. To keep the water under about 5 feet per second, push no more than roughly 1,800 GPH through 1.5 inch pipe, 3,000 GPH through 2 inch and 6,000 GPH through 3 inch.

What the engineering references say

Friction in plastic pipe is calculated with the Hazen-Williams formula, which The Engineering Toolbox writes as "h100ft = 0.2083 (100 / c)^1.852 q^1.852 / d^4.8655", with h in feet of head lost per 100 feet of pipe, q in gallons per minute and d the inside diameter in inches. Its coefficient table lists "Polyvinyl chloride, PVC, CPVC" at 150, Spears Manufacturing says "150 is the commonly accepted value for PVC and CPVC pipe," and the Uni-Bell PVC Pipe Association notes that new PVC tests at "155-165 that may decrease to 150 over the life cycle of the pipe."

Pumps are rated in GPH and friction tables in GPM, so the rows below are the Engineering Toolbox's Schedule 40 PVC figures with the flow converted by dividing GPH by 60. Its 1.5 inch table stops at 60 GPM.

Friction head lost per 100 feet of Schedule 40 PVC, in feet, from the Engineering Toolbox table (velocity in feet per second in parentheses)
Flow1.5 inch pipe2 inch pipe3 inch pipe
1,200 GPH (20 GPM)2.6 (3.2)0.8 (2.0)0.1 (0.9)
1,800 GPH (30 GPM)5.5 (4.9)1.6 (2.9)0.2 (1.3)
2,400 GPH (40 GPM)9.4 (6.5)2.8 (3.9)0.4 (1.8)
3,000 GPH (50 GPM)14.3 (8.1)4.2 (4.9)0.6 (2.2)
3,600 GPH (60 GPM)20.0 (9.7)5.8 (5.9)0.9 (2.7)
4,500 GPH (75 GPM)not tabulated8.8 (7.3)1.3 (3.3)
6,000 GPH (100 GPM)not tabulated15.0 (9.8)2.2 (4.4)

Each fitting adds friction as if it were an extra length of pipe. The Engineering Toolbox's equivalent length table covers elbows and tees but has no check valve row; PlumbingSupply.com's table of the same kind does, and one fully open swing check valve costs about as much as three elbows.

Equivalent feet of straight pipe for common fittings
Fitting1.5 inch2 inch3 inchSource
90 degree elbow4.05.27.7Engineering Toolbox
45 degree elbow2.22.84.1Engineering Toolbox
Tee, flow through the branch8.110.316.3Engineering Toolbox
Swing check valve, fully open13.417.225.5PlumbingSupply.com

On velocity, the Toolbox table carries the note "Velocities should not exceed 5 feet per second," and Spears says "Flow velocities in excess of 5.0 feet per second are not recommended for closed-end systems." Hayward Flow Control allows "no more than 8 feet per second for piping systems that do not have fast opening or closing valves" and 5 where there are solenoid valves, both to limit water hammer.

What pond suppliers and contractors do

The pond trade uses a shortcut. The Pond Guy says "A foot of vertical height or 10' of tubing is equivalent to 1' of head pressure," so "a feature that is 3' tall with 16' of tubing would have 5' of head pressure," then "you just have to match up the numbers" on its pump charts. Hydrosphere Water Gardens adds that "Every 90° elbow or reducer fitting adds 1′ of head to the dynamic pressure," and Pond Informer adds "Most pressurized filters add 3 to 5 feet of head" and "UV clarifiers add 1 to 2 feet."

The suppliers who publish friction numbers show how much the pipe matters. Hydrosphere's tubing page says "1 ½" pipe has 19.98′ of friction loss per 100′ of pipe when water is pumped through it at approximately 3600 GPH" while "2" pipe has only about 3' of friction loss in 50' of tubing" at the same flow, and that a 3,300 GPH Tsurumi on 50 feet of 1.5 inch delivers about 1,700 GPH against about 3,000 GPH through 2 inch. In POND Trade Magazine, Demi Fortuna works a 6,000 GPH waterfall with a 50 foot run, an elbow, a male adapter and a check valve: in 3 inch pipe friction is "0.03 x 50 feet = only 1 ½ feet" and the job needs "6,000 gph at 6 feet TDH" (a 520 watt Atlantic TT7500); in 2 inch it is "0.22 x 50 feet = 11 feet of friction head" and needs "a pump that could provide 6,000 gph at a whopping 21 feet of head" (an 1,160 watt A-31). His 3 inch fitting allowances, 11 feet for the elbow and 27 for the check valve, run higher than the tables above.

On the Garden Pond Forum, Waterbug answered davepratt's question about running a 7,600 GPH pump through 1.25 inch hose with a loss table (22 feet for 1.25 inch, 4.3 for 1.5 inch, 1.3 for 2 inch) and the verdict that it "would only move about one half to one third as much water with a 1.25" pipe as 2" or 3" pipe." In texmaster's thread on a 5,800 GPH pump, crsublette put 2 inch at "1.24 friction head per 10 horizontal feet" at 5,400 GPH against 0.18 for 3 inch, while mgmine held that "There is no reason to go larger than 2"." On check valves, budgenator found the ones sold locally "had why too much resistance to work with reasonable pumps" and built his own, and Waterbug warns that check valves in pond water clog and grow biofilm, cutting flow over time.

Where the shortcut and the tables disagree

The "1 foot of head per 10 feet of tubing" rule has no fixed basis, because friction depends on flow and pipe diameter, and the rule accounts for neither. At 3,600 GPH in 2 inch pipe, 10 feet costs 0.58 feet, close to the rule. In 1.5 inch at the same flow it costs 2.0 feet, twice the rule; at 1,200 GPH in 2 inch it costs 0.08 feet, under a tenth of it. The "1 foot per elbow" rule is off the same way: a 2 inch elbow is 5.2 feet of pipe, which at 3,600 GPH is 0.3 feet of head, while the check valve the rule ignores costs three times that.

The supplier pipe tables are generous too. The Pond Guy allows "Up to 3,600 GPH: 1-1/2 inch tubing" and "Up to 5,400 GPH: 2 inch tubing," and Russell Watergardens publishes the same ceilings. Both sit at about 9 to 10 feet per second in the Toolbox table, twice what Spears recommends, and 1.5 inch at 3,600 GPH is exactly the 20 foot loss Hydrosphere uses as its warning. Those ceilings mark where the pump will still move water. No pond source I found states a velocity limit at all; the 5 feet per second figure comes from the pipe industry, and Hayward shows the industry itself is not unanimous.

What to do

  1. Decide the flow first. For a waterfall the spillway width sets the GPH (see what size pump a pond waterfall needs); for circulation, divide the pond volume by the turnover time.
  2. Measure the static lift from the pond surface to the top of the falls (the pump's depth does not count), and the pipe one way including slack. Add the fitting equivalents: a 50 foot run with two 2 inch elbows and a check valve is 50 plus 10.4 plus 17.2, or 77.6 equivalent feet.
  3. Pick the pipe by velocity. In the Toolbox table, 1,800 GPH in 1.5 inch, 3,000 GPH in 2 inch and 6,000 GPH in 3 inch each run just under 5 feet per second. A 1,000 gallon pond turning over hourly is fine on 1.5 inch; a 3,000 gallon pond turning over hourly wants 2 inch; anything moving more than about 3,000 GPH, which includes a 6,000 gallon pond turning over hourly or a 10,000 gallon koi pond, wants 3 inch, and a 20,000 gallon pond may want two 3 inch runs. Never go smaller than the pump's discharge.
  4. Add it up. At 3,600 GPH through 77.6 equivalent feet of 2 inch, friction is 5.8 feet per 100 feet times 0.776, or 4.5 feet; with 4 feet of lift the total dynamic head is 8.5 feet. The same layout in 1.5 inch is 71.4 equivalent feet at 20.0 feet per 100, or 14.3 feet of friction, for 18.3 feet of head.
  5. Read the pump chart at that head. Atlantic's TT7500 is listed at 7,650 GPH maximum, "6,300 GPH" at 5 feet, "4,640 GPH" at 10 feet, "2,860 GPH" at 15 feet and "1,250 GPH" at 20 feet. At 8.5 feet it gives a little over 5,000 GPH, more than the 3,600 target, so you could step down a model. At 18.3 feet it reads under 2,000 GPH, so with 1.5 inch pipe it never reaches 3,600, and no bigger pump fixes that as cheaply as the next pipe size.
  6. Let the calculator do the arithmetic and the curve reading. The PondTools pump calculator (also standalone) takes the pond volume, the static lift in feet, the pipe inside diameter in inches, the one way pipe length in feet, a flow to check in GPH and two points off the pump's chart; it applies the Hazen-Williams formula with C set to 150 and finds where the pump curve meets your plumbing to give the GPH you will actually get. It assumes smooth pipe, so add the fitting equivalents to the length yourself; a pond traced in the PondTools app fills in the volume.
  7. Do not size the pipe from the pump's outlet thread, do not run corrugated tubing on a long line, and fit a check valve only where the waterfall filter would otherwise drain back into the skimmer.

Sources