Tank Drain Time
Calculator
Results
- Drain time (s)
- 2,393.728266
- Drain time (minutes)
- 39.895471
- Tank volume (L)
- 1,799.999999
- Initial outlet velocity (m/s)
- 4.851387
HVAC and plumbing results
| Drain time (s) | 2,393.728266 |
| Drain time (minutes) | 39.895471 |
| Tank volume (L) | 1,799.999999 |
| Initial outlet velocity (m/s) | 4.851387 |
formula-map diagram
- Drain time (s)
- 2,393.728266
- Drain time (minutes)
- 39.895471
- Tank volume (L)
- 1,799.999999
- Initial outlet velocity (m/s)
- 4.851387
HVAC and plumbing relationship
Formula
t = A × √(2h ÷ g) ÷ (Cd × a)= 2393.7282668868
Note
This is a simplified model: it applies a standard engineering formula to the numbers you entered. Conversions between BTU/h and kW use the exact factor 1 kW = 3412.142 BTU/h, but every sizing figure is an estimate. Air conditioner sizing uses the common 20 BTU/h per square foot rule of thumb, not a room-by-room load calculation, and it ignores insulation, glazing, orientation, ceiling height, infiltration and local climate. Heating loads use a single volumetric heat-loss factor in W/m³·K rather than a fabric-by-fabric U-value calculation. Air properties are fixed at 1.2 kg/m³ and water at 1000 kg/m³ and 4186 J/kg·K, with no correction for temperature, altitude or glycol. Duct and pipe results use the ideal continuity equation and ignore fittings, bends, roughness and system effect unless you enter those losses yourself. The Hazen-Williams equation is valid only for water in full turbulent flow at ordinary temperatures. Water hammer uses the Joukowsky surge, an upper bound for instant closure. Tank drainage assumes a prismatic tank and steady free discharge. Hot water recovery and condensate figures ignore standing losses and coil bypass. Size real systems with a proper heat-loss survey and have the work checked by a qualified HVAC or plumbing professional.
More in HVAC and plumbing
See all →Frequently asked questions
How is the time to drain a tank calculated?+
Drain time depends on the tank's volume and the outlet's flow rate, but because flow rate itself decreases as the water level (and therefore the pressure pushing water out) drops, a full calculation accounts for this changing rate rather than assuming a constant flow throughout.
Why does drain time not decrease in a straight line as the tank empties?+
Flow out of an opening depends on the square root of the remaining head (water depth) above it, so a full tank drains relatively quickly while a nearly empty tank drains much more slowly for the same drop in water level — this is why drain time follows a curved, not linear, pattern.
Does the size of the drain opening matter more than the tank's total volume?+
Both matter, but a small increase in the outlet's diameter has an outsized effect on drain time because area increases with the square of the radius — doubling the outlet diameter roughly quarters the drain time for the same tank.
Why would I need to know how long a tank takes to drain?+
This matters for maintenance scheduling (like draining a water heater to remove sediment), sizing emergency drainage or overflow systems, and estimating downtime for tanks that need periodic emptying for cleaning or inspection.
Does tank shape affect the calculation?+
Yes — a cylindrical tank standing upright drains differently than a wide, shallow tank of the same volume, since the relationship between water depth and remaining volume depends on the tank's cross-sectional shape at each height.