How to Work Out a Fill or Drain Time
Why this matters
"How long will it take to fill?" is a scheduling question with a crew, a customer and sometimes a chemical start-up hanging on the answer. Get it wrong one way and a tech drives back to a pool that is still two feet low; get it wrong the other way and a hose runs overnight into an overflowing pool or a pump runs dry at the bottom of an empty one. The arithmetic is one division. What makes the answer wrong is almost always the flow figure that went into it: a number off a label, a guess about "a garden hose," or a pump's starting rate assumed to hold all the way down. This card is the division, the measurement that has to come first, and the corrections for flows that do not stay put.
The arithmetic
Time in minutes = gallons / gallons per minute (GPM). Divide by 60 for hours.
Carry the units and they cancel on paper: gallons divided by gallons per minute leaves minutes. If the answer comes out in "gallons squared per minute," a multiplication crept in where a division belonged. See related: Units Carried Through Every Line of a Calculation.
FILL / DRAIN TIME WORKSHEET
--------------------------------------------------
1 Volume to move ______ gal
(source: ________________)
2 Flow MEASURED at the ______ GPM
discharge end, how: _______
3 Time = L1 / L2 ______ min
4 L3 / 60 ______ h
5 Rough check: L2 x 60 x 24 = ______ gal/day
6 Flow re-measured at ____% ______ GPM
7 Remaining gal / L6 = ______ min left
8 Check-in times set: ______ ______ ______
--------------------------------------------------
The steps
1. Get the volume you are actually moving. A fill moves the volume from where the water is now to where it needs to be, not the whole pool or tank. A drain for a resurface takes it to the floor; a drain to clean a spa filter well takes it a few inches. Pool volumes are figured in How to Estimate Pool and Spa Gallons and tanks in How to Figure the Volume of a Round or Rectangular Tank. Write where the number came from on line 1.
2. Measure the flow at the end of the hose you will use. This is the step that decides whether the answer is right. Flow at an open hose bib is not the flow through 100 ft of 5/8 in hose, because the hose takes pressure to push water through it, and the longer and narrower it is, the less comes out the end. Set up the real hose, open the valve fully, and time a marked container at the discharge end. The method, and how many trials to take, is owned by How to Measure Flow With a Bucket and a Stopwatch. A label or a remembered figure for "a garden hose" goes on line 2 only if you write "estimated" next to it.
3. Divide, convert, and write both units. Minutes on line 3, hours on line 4. Hours are what the schedule needs; minutes are what lets someone check the division.
4. Run the per-day check. Flow x 60 x 24 is gallons per day. It turns an abstract time into a picture: if the hose moves 13,000 gallons a day and the pool needs 17,000, the fill runs past a day, whatever line 4 says. If line 4 and the per-day check disagree, the division is wrong.
5. Decide whether the flow will hold, and re-measure partway if it will not. A fill from a city supply holds roughly steady, though it drops when the house runs other water and can shift with neighborhood demand. A drain with a pump does not hold, for a reason set out below. Either way, plan to re-time the flow once, around the halfway mark, and re-figure the remaining time from that rate on lines 6 and 7.
6. Set check-in times before you leave. Put the first check well before the predicted finish, not at it. A fill predicted to finish at 32 hours gets checked at about 24; a drain predicted at 8 hours gets checked at about 6. The check is what catches a flow that was wrong, and an overflow or a dry pump is a callback that costs more than the visit.
Worked example 1: filling a pool from a hose
A pool needs 17,200 gallons after a drain and refinish. The tech connects the hose that will do the fill, opens the bib fully and times a marked 5 gallon pail at the hose end: three trials average 33.3 seconds.
Line Item Value Unit
1 Volume to move 17,200 gal
2 Flow = 5 x 60 / 33.3 9.0 GPM
3 Time = L1 / L2 1,911 min
4 L3 / 60 31.9 h
5 Check: 9 x 60 x 24 12,960 gal/day
About 1,900 minutes, roughly 32 hours. The per-day check agrees: 12,960 gallons in the first 24 hours leaves 4,240 gallons, and 4,240 / 9 = 471 minutes, a little under 8 more hours, for 32 in all.
The second hose. The customer has a second hose bib on the other side of the house and suggests running both. Do not assume the flow doubles. Both bibs usually hang off the same service line, so the second hose shares the same supply pressure the first one was using. Measure the two together: say the combined timing comes to 14 GPM rather than 18. Then 17,200 / 14 = 1,229 minutes, 20.5 hours. That is a real saving of about 11 hours, and still not half of 32.
Hazard during a fill: the hose end goes over the edge and stays above the water surface, or the bib carries a hose-connection vacuum breaker. A hose lying submerged in the pool is a cross-connection: a drop in supply pressure can siphon pool water back into the drinking water line. An unattended fill also needs its check-in, because an overflow can run water against a foundation or into a neighbor's yard.
Worked example 2: draining with a submersible pump
A second job: a pool with 17,200 gallons has to be drained for repair.
Hazards on a drain, settled before the pump goes in:
- Shell uplift. An in-ground pool emptied while the ground around it is saturated can be pushed up by groundwater and crack or lift. Whether, when and how far to drain is a decision for the pool professional following the shell builder's guidance and the hydrostatic relief arrangement for that pool, made before the pump goes in, not partway through.
- Discharge. Where pool water may go (sanitary cleanout, storm drain, landscape) and what it must be treated to first is set by local ordinance. Ask the jurisdiction; the answer varies.
- Electrical. The submersible pump plugs into a GFCI-protected receptacle, the cord connection stays dry and out of the water, and nobody handles or repositions the pump while it is plugged in. Unplug it first.
- The circulation pump. Turn off the pool's own circulation system and its heater before the level falls below the skimmer, and lock out its breaker so a timer cannot start it dry. Restore it only after the refill reaches the skimmer and the pump is primed.
With those settled, the tech sets a submersible pump on the floor and runs its discharge hose to the approved discharge point. At the start, the pump fills a marked 30 gallon container at the hose end in 45 seconds.
Line Item Value Unit
1 Volume to move 17,200 gal
2 Flow at start = 30 x 60 / 45 40 GPM
3 Straight-line time = L1 / L2 430 min
4 L3 / 60 7.2 h
Why 7.2 hours is optimistic: a pump's flow depends on how high it has to lift the water and how much friction the hose adds. The pump's maker publishes a curve of flow against that total head. At the start, the water surface is near the deck and the lift is short. As the pool drains, the surface falls and the pump sits at the bottom pushing water up a taller column, so it moves down its own curve to less flow. The time stretches toward the end of the drain, not evenly.
The tech re-times at about the halfway point, when 8,600 gallons are out: 30 gallons now take 53 seconds, which is 34 GPM (30 x 60 / 53 = 34.0). The first half took 232 minutes by the clock, consistent with an average near 37 GPM (8,600 / 232 = 37.1).
For the remaining 8,600 gallons, the flow will keep falling. Assume it drops by about the same 6 GPM it lost over the first half, so it runs from 34 down toward 28 and averages about 31 GPM:
- Remaining time = 8,600 / 31 = 277 minutes
- Total = 232 + 277 = 509 minutes, 8.5 hours
That is about 18 percent longer than the straight-line 430 minutes (509 / 430 = 1.18). The extra hour and a quarter is exactly the window in which nobody was planning to be on site.
When the flow falls on its own: gravity drains
A tank draining by gravity through a short outlet has the same problem as the pump, from the other direction. The flow through the outlet depends on the depth of water above it, roughly with the square root of that depth, so as the tank empties the flow slows. For a vertical tank of constant cross-section draining that way, the empty time works out to about twice what the starting flow predicts. That factor of two holds only for a straight-walled tank draining through a short outlet. Through a long hose the friction changes the relationship, and the honest answer is to re-time the flow at half depth as in example 2.
What changes the answer
- The supply is a well. A long fill can draw a private well down faster than it recovers, and the pump may short-cycle or the well may run dry partway. Ask the owner about the well's yield before starting a long fill, and expect the measured flow to fall during the job.
- The water is delivered. A water hauler brings a fixed quantity, so the question becomes truckloads, not hours: volume divided by the truck's stated capacity, rounded up.
- The pool must not stand partly empty. If the resurfacing schedule or the weather means the shell cannot sit low for a day and a half, a 32-hour hose fill is the wrong plan, and the calculation is what tells you to arrange more supply or a delivery before you drain.
How to verify you got this right
- The flow was measured at the discharge end, on the real setup, and the sheet says how.
- The per-day check agrees with the time on line 4.
- The halfway re-time was done on any pump drain, and the finish time was re-figured from it.
- The check-in came before the predicted finish, and the level at the check-in matched the plan within the error you would expect from the measured flow.
- Everything disabled for the job is back. The circulation pump and heater are restored only after refill and priming, and the vacuum breaker or air gap stayed in place throughout.
References
- See related: How to Measure Flow With a Bucket and a Stopwatch
- See related: How to Estimate Pool and Spa Gallons
- See related: Units Carried Through Every Line of a Calculation
- Pump manufacturer performance curves (flow against total head)
- Local ordinances on pool water discharge, and the shell builder's draining guidance