Tank Drain Time Calculator
How long a tank takes to empty through an outlet under gravity — or how fast a pump pulls it down. Works for vertical and horizontal cylinders, rectangular and cone-bottom tanks, with head pressure, flow rates and a drain curve.
How Long Will a Tank Take to Drain?
The honest answer is “it depends on the head” — and the head keeps dropping. A full tank pushes liquid out fast; as the level falls the pressure behind the outlet fades and the flow slows to a trickle. That is why a tank does not empty at a steady rate, and why the last few inches take longer than you would guess.
For a tank draining by gravity through an opening, the instantaneous flow follows Torricelli’s law, and the total time comes from adding up every thin slice of liquid as the level falls:
This calculator does that integration numerically for each tank shape — so a wide-bottomed cone, a horizontal cylinder and a tall vertical tank of the same volume all give different, correct drain times.
Gravity Drain vs Pumped Drain
A gravity drain is free but fades: flow starts strong and tails off as the head disappears, so the average rate is well below the initial rate. A pump holds a steady flow regardless of level, so the time is simply the volume divided by the pump rate — predictable, faster at low levels, but it needs power and has to overcome any suction lift. Use gravity when you have head to spare and time to wait; use a pump when you need a guaranteed rate or must lift the liquid up and out.
Orifice Size and Drain Time
Drain time is governed by the outlet area, and area grows with the square of diameter — so doubling the outlet diameter roughly quarters the drain time. The discharge coefficient matters too: a sharp-edged hole flows at only about 61% of ideal because the jet necks down just past the edge, while a smooth nozzle or a short pipe behaves differently. Add a length of drain pipe and friction reduces the effective coefficient further, which this tool estimates when you enter a pipe size.
Frequently Asked Questions
Why does the last bit drain so slowly?
Flow depends on the square root of the remaining head. Near empty there is almost no head left, so the final inches crawl out – the drain curve flattens toward the end.
Does tank shape really change the time?
Yes. Two tanks of equal volume drain differently because the cross-section at each height differs. A horizontal cylinder is widest in the middle, a cone is narrow at the bottom – both change how fast the level falls.
Why must a closed tank be vented?
As liquid leaves, something must replace it. A sealed tank pulls a vacuum that fights the outflow and can stall it. A vent or vacuum breaker lets air in so the drain runs as calculated.
Will thick oil drain in this time?
Not quite. The orifice formula is for low-viscosity liquids like water. Viscous or cold oil drains slower; treat the result as a best case and lower the discharge coefficient for heavy fluids.
Related calculators
- Orifice Flow Calculator — flow rate through the outlet at a fixed head.
- Flow Rate Converter — convert between GPM, L/s, L/min and m³/h.
- Pipe Volume Calculator — volume held in a pipe or tank length.
- Pump Horsepower Calculator — size a pump for a pumped drain.
- Pipe Flow Calculator — velocity and flow in the drain line.