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.

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).