📖 How To Use
How to Use the Tank Drainage Time Calculator
Finding out how long your tank takes to drain is a two-step process:
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Enter your tank volume
Type in how much water the tank currently holds (or its full capacity). Pick the right unit — litres, gallons, cubic metres, or cubic feet. If you only want to drain part of the tank, adjust the "Drain From %" field (default is 100%, meaning fully drained).
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Enter the outlet flow rate
This is how fast water leaves through the outlet pipe — typically in litres per minute (L/min) or US gallons per minute (GPM). Check your pump spec sheet, pipe flow tables, or measure it with a bucket and stopwatch. Choose the matching unit from the dropdown.
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Optional: enter pipe diameter
If you don't know your flow rate but know the pipe size, enter the pipe diameter. The calculator will estimate a theoretical gravity-flow rate using Torricelli's theorem as a starting reference.
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Hit Calculate
The total drain time appears instantly in minutes, hours & minutes, and seconds. Copy or print the results with one click.
Tip: Real-world drain times are almost always longer than the calculated value. Friction losses, partially open valves, pipe bends, and declining head pressure as the tank empties all reduce effective flow rate. Add 15–25% to the calculated time as a practical buffer.
📐 The Formula
Tank Drainage Time Formula
The core calculation is straightforward division:
Drain Time (minutes) = Volume to Drain (litres) ÷ Flow Rate (litres/min)
Drain Time (hours) = Volume (L) ÷ (Flow Rate L/min × 60)
Drain Time (seconds) = Volume (L) ÷ Flow Rate (L/sec)
All unit combinations are converted to litres and litres-per-minute internally before dividing, so you can mix any units freely.
Torricelli's Theorem (gravity drain estimate)
When draining by gravity (no pump), the theoretical maximum flow rate through an orifice is given by:
Q = Cd × A × √(2gh)
Q = flow rate (m³/s)
Cd = discharge coefficient (~0.6 for a sharp-edged orifice)
A = cross-sectional area of outlet (m²)
g = 9.81 m/s²
h = water height above outlet (m)
Note that gravity-drain flow rate slows as the water level drops — the calculator uses the average initial flow for simplicity. For a precise time accounting for declining head, the actual drain time is approximately 2 × the value from the constant-flow formula.
Flow Rate Unit Conversion Reference
| Unit | Equals L/min | Common Use |
|---|
| 1 L/min | 1 L/min | Small pumps, taps |
| 1 L/sec | 60 L/min | Large pumps, fire hose |
| 1 L/hr | 0.0167 L/min | Drip irrigation |
| 1 m³/hr | 16.667 L/min | Industrial pumps |
| 1 US GPM | 3.785 L/min | US plumbing |
| 1 US GPH | 0.0631 L/min | Low-flow applications |
❓ FAQ
Frequently Asked Questions
How do I calculate how long it takes to drain a water tank?
Divide the volume of water in the tank by the outlet flow rate, using matching units. For example, a 1,000-litre tank draining at 20 litres per minute takes 1,000 ÷ 20 = 50 minutes. This calculator handles all unit conversions automatically.
Why does it take longer to drain in practice than the calculator says?
Several factors reduce real-world flow below the theoretical rate: pipe friction losses, bends and fittings, partially open valves, and — for gravity drains — declining water head pressure as the level drops. For gravity-only systems, multiply the calculated time by approximately 2. For pumped systems, add 15–25% as a buffer.
How do I find the flow rate of my outlet pipe?
The simplest method is the bucket test: open the outlet fully and time how long it takes to fill a known container (e.g., a 10-litre bucket in 30 seconds = 20 L/min). Alternatively, check your pump's data sheet for rated flow, or use a clamp-on ultrasonic flow meter on existing pipework.
What is a typical flow rate for a household tank outlet?
A standard 25 mm (1-inch) gravity drain at 2 m head flows at roughly 15–25 L/min. A 40 mm (1.5-inch) pipe at the same head delivers about 50–70 L/min. Submersible pumps common for household tanks typically range from 30 to 150 L/min depending on head and model. Always verify against the pump curve at your specific head height.
How long does it take to drain a 1,000-litre tank?
It depends entirely on the flow rate. At 10 L/min: 100 minutes. At 20 L/min: 50 minutes. At 50 L/min: 20 minutes. At 100 L/min (large pump): 10 minutes. Use this calculator with your specific flow rate for an accurate answer.
Can I calculate drainage time for a partially filled tank?
Yes. Enter the tank's full capacity in the volume field, then set the "Drain From %" to the current fill level. For example, a 2,000-litre tank at 75% full holds 1,500 litres — entering 2,000 L and 75% produces the same result as entering 1,500 L at 100%.
What pipe diameter do I need to drain a tank in a specific time?
Work backwards: target flow rate = Volume ÷ Target Time. Then use the pipe flow rate chart or our Pipe Size Flow Rate Calculator to find the pipe diameter that delivers that flow at your available head pressure. Common gravity drain pipe sizes range from 25 mm to 100 mm.