How this calculation works
The difference between constant simultaneous rates determines whether the tank fills or drains. A target is reachable only in the direction of that net change; an already reached target takes zero time.
Net inflow = Qin−Qout; time = (target−start)/(Qin−Qout)A worked example
Moving from 200 L to 800 L with 30 L/min inflow and 10 L/min outflow requires 600/20 = 30 minutes.
Good to know
Constant entered rates only. Gravity-driven outflow can change with the liquid level and is not calculated here. The starting and target volumes must remain within capacity; the projection stops at empty or full.
Reference: OpenStax: flow rate and continuity (opens in a new tab) ↗
Subtract simultaneous flows first
With 30 L/min entering and 10 L/min leaving, the volume rises at 20 L/min. Moving from 200 L to 800 L therefore takes 600/20 = 30 minutes. Dividing by the 30 L/min inlet alone would ignore water leaving during the interval.
The balance also works for draining. If outflow exceeds inflow, the net change is negative. Both the desired volume change and net rate must have the same sign for a positive time to the target.
The pipe flow calculator can express an observed inlet rate, while the orifice discharge calculator evaluates a head-dependent outlet at one fixed head.
An already reached target needs no waiting
If starting and target volumes are equal, the answer is zero even when inflow and outflow cancel or both are zero. If the target differs and rates cancel, the volume never reaches it under this constant-rate model.
The timeline stops at a reachable target inside capacity. Time to empty or full is shown separately to expose the next physical boundary. The pipe flow calculator can convert a known pipe-average speed into an inlet-flow assumption before using this balance.
A gravity outlet may not have constant flow
A hole draining under liquid head usually slows as the level falls. Treating its initial flow as constant can understate the time to drain. The orifice discharge calculator evaluates one stated constant head; neither page integrates a changing-level drainage process.
Pump switching, leakage changes and variable demand also break the constant-rate assumption. Use measured or justified average rates only when that approximation matches the question, and keep all volume and rate units consistent.
A few questions, answered
What does Target not reached mean?
The target differs from the start and the net rate is zero or points in the opposite direction.
Can I enter a target above capacity?
No. The target and starting volumes must lie inside the stated tank capacity.
Can this calculate the fill time with a drain running?
Yes, if both entered rates can reasonably be treated as constant during the interval.