Fill and drain time come from the same relation: how much water the pool holds versus how fast water moves. Flow is given in gallons per minute (GPM), so the first step converts it to gallons per hour — GPH = GPM × 60 — and then dividing the pool's gallons by that GPH gives the duration.

gph    = gpm × 60
hours  = volumeGal / gph          (= volumeGal / (gpm × 60))
finish = startTime + hours
cost   = (volumeGal / 1000) × rate

bucket test:     gpm = 300 / seconds_to_fill_a_5-gal_bucket
dimensions mode: volumeGal = pool-volume engine (shape × avg depth × 7.48 gal/ft³)
                 avgDepth = (shallow + deep) / 2 (deep blank ⇒ shallow)

Watch the units. Dividing gallons by GPM alone gives minutes, not hours — off by 60×. We always convert to GPH first and echo it (e.g. 9 GPM × 60 = 540 GPH) so you can sanity-check the math. In From dimensions mode the gallons come from the same engine the Pool Volume Calculator uses (7.48 gal/ft³; primary-source-validated shape constants), not a rule-of-thumb multiplier.

Draining is an honest extrapolation. It uses the identical volume ÷ (GPM × 60) math with a pump's GPM, but a real drain runs longer as the pump loses prime near the bottom — so the drain figure is a planning minimum, not a precision model. The water cost is advisory: the rate is a number you type, not a live utility feed.

Most reasons to run this clock are someone else's job upstream. A partial drain to bring cyanuric acid, calcium or salt down is planned on the drain and dilution calculator, which hands you the gallons this page turns into hours, and the gallons themselves come from the pool volume calculator when you do not already know them. If the fill or drop is part of a paid visit, the hours belong in the task box of the pool opening pricing calculator or the pool closing pricing calculator rather than being absorbed.

Sources: Larry's Pool — Pool Fill Time Calculator (GPM preset tiers, water-cost formula); Pool Chemical Calculator — Pool Fill Time Calculator (bucket test, Xh Ym output); Omni Calculator — GPM Calculator (time = volume ÷ GPM).