Why does the end of a gravity drain take so long?

The drain rate falls with the square root of driving head, so flow slows as the level drops. Draining to the outlet with no drop below it takes exactly twice the constant-rate time.

How do I measure the drain rate at the start?

With the valve fully open, time the first few inches of level drop and multiply inches per minute by gallons per inch. On a 96 in tank, 2.2 in per minute is about 69 gal/min.

Does a drain hose that runs downhill empty the tank faster?

Yes: every inch the discharge sits below the outlet adds driving head, though line friction takes some back. Re-measure the starting rate after changing the line, because the model scales from it.

Can I use this for a tank emptied by a pump?

No. A pump-out follows the pump curve rather than the square-root law, so estimate it as volume ÷ measured pump rate with the Pump Runtime calculator.

Process Manufacturing calculator

Tank Empty Time Calculator

Find how long a gravity drain takes when the flow slows as the level falls. You need the diameter, start and stop levels, the drain line's drop and one measured drain rate.

What this calculator does

Minutes to gravity-drain a vertical tank between two levels as flow slows with falling head, against your changeover window.

Formula used

A = π × inside diameter² ÷ 4; volume drained = A × (start level − stop level) ÷ 231
Driving head H = level above outlet + discharge drop; drain rate = rate at start × √(H ÷ H start)
Drain time = 2 × A × √H start × (√H start − √H stop) ÷ (rate at start × 231)
Average rate = volume drained ÷ drain time; constant-rate time = volume drained ÷ rate at start
Time to spare = time until tank is needed − drain time

Inputs explained

Inside Diameter: Shell inside diameter from the vessel drawing or nameplate.
Starting Level Above Outlet: Liquid height above the drain outlet when draining begins.
Stop Level Above Outlet: Height where you close the valve; 0 to drain to the outlet.
Discharge Drop Below Outlet: Outlet down to the drain line's open end, above any liquid.
Drain Rate at Start: Flow meter reading or timed level drop at the starting level.
Time Until Tank Is Needed: Minutes until the tank must be empty; 0 if none.

How to use the result

Best suited to planning a drain before clean-in-place or changeover, top-inlet gravity transfer to a lower tank.
Viscous products in laminar flow slow more steeply near the end than this model shows. Covers gravity drains only; a pump-out follows its pump curve, not the square-root law.

Current U.S. benchmarks

The producer price index for industrial chemicals stands at 336.006 (BLS, Aug 2026), up 13.3% from a year earlier. Quotes priced off last quarter's material cost miss this move.
The U.S. has 14,543 chemical manufacturing establishments employing about 911,245 workers (Census County Business Patterns, 2023).

Common questions

Why does the end of a gravity drain take so long? The drain rate falls with the square root of driving head, so flow slows as the level drops. Draining to the outlet with no drop below it takes exactly twice the constant-rate time.
How do I measure the drain rate at the start? With the valve fully open, time the first few inches of level drop and multiply inches per minute by gallons per inch. On a 96 in tank, 2.2 in per minute is about 69 gal/min.
Does a drain hose that runs downhill empty the tank faster? Yes: every inch the discharge sits below the outlet adds driving head, though line friction takes some back. Re-measure the starting rate after changing the line, because the model scales from it.
Can I use this for a tank emptied by a pump? No. A pump-out follows the pump curve rather than the square-root law, so estimate it as volume ÷ measured pump rate with the Pump Runtime calculator.

Last reviewed 2026-10-01.