Enter tank volume and outlet flow rate — choose your units freely
Step 1 — Tank Volume
Tank Volume
Drain From %
%
Step 2 — Outlet Flow Rate
Flow Rate
Pipe Ø (optional)
Water Depth Above Outlet (full)
Outlet Inner Diameter
Outlet Type
For a straight-sided tank draining by gravity. "Drain From %" sets how full the tank is when you start; it drains down to the outlet. Valves, bends and hoses add friction, so real times are longer.
⚠ Please enter valid values for Tank Volume and Flow Rate (both must be greater than zero).
WaterTankCalculator.com — Tank Drainage Time Results
Drainage Results
Total Drain Time
—
minutes
Hours & Minutes
—
Total Seconds
— sec
Total Hours
— hr
Volume to Drain
— L
Flow Rate
— L/min
Drain %
— %
Drainage Progress Visual
📖 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:
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 the tank isn't full, set "Drain From %" to its current level (default 100%, a full tank drained to empty).
Enter the outlet flow rate
If a pump empties the tank, or you have measured the outlet flow, use the Pump or measured flow tab and enter the rate — read the pump's curve at your head, or time a bucket. If the tank drains by gravity through an outlet, use the Gravity drain tab instead, because the flow slows as the level drops.
Optional: enter pipe diameter
In the Gravity drain tab, enter the depth of water above the outlet when the tank is full and the outlet's inner diameter, and pick the outlet type. The calculator uses Torricelli's law to work out the drain time as the level falls.
Hit Calculate
The total drain time appears instantly in minutes, hours & minutes, and seconds. Copy or print the results with one click.
Tip: Real drain times are usually longer than calculated. Partly open valves, bends, hoses and pipe friction all slow the flow. For gravity drains, the Gravity drain tab already allows for the falling level, so the remaining difference comes from those losses — measure the actual flow where you can.
📐 The Formula
Tank Drainage Time Formula
For a pump or any steady, measured flow, the 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 (about 0.61 for a sharp-edged hole; about 0.82 for a short pipe stub)
A = cross-sectional area of outlet (m²)
g = 9.81 m/s²
h = water height above outlet (m)
Because h falls as the tank empties, the flow slows. Integrating Torricelli's law for a straight-sided tank with footprint Atank gives the drain time used in the Gravity drain tab:
t = (Atank ÷ (Cd × A)) × √(2h₁ ÷ g)
h₁ = starting depth of water above the outlet (m); the tank drains down to the outlet
Atank = tank volume ÷ full depth above the outlet (straight sides)
Draining to empty this way takes exactly twice as long as it would if the starting flow held steady, so dividing the volume by the starting flow underestimates the time by half. Tapered, spherical or horizontal-cylinder tanks behave differently, and long pipes or hoses add friction, so treat the result as the fastest likely time.
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
🏗 When to Use This
Common Use Cases for Tank Drainage Calculations
🔧
Tank Maintenance
Schedule cleanings and inspections by knowing exactly when the tank will be empty, avoiding unplanned water outages.
🚒
Emergency Planning
Firefighting, disaster response teams, and municipal engineers need to know how long reserves will last at peak draw-down rates.
🌾
Irrigation Scheduling
Calculate how long a holding tank can supply a drip or sprinkler system before needing a refill from the mains or a bore.
🏗
Construction & Dewatering
Civil engineers calculate pit and basement dewatering times to schedule concrete pours and safe working windows.
🏭
Industrial Processing
Batch processing plants, breweries, and chemical facilities need accurate drainage times to sequence production steps.
🏠
Rooftop & Household Tanks
Plan how long it takes to empty an overhead tank for cleaning, through its drain valve or with a pump.
🧮 Worked Example
Worked example: draining a 1,000 L tank by gravity
A straight-sided 1,000 L tank holds 1.0 m of water above a 25 mm sharp-edged outlet (Cd 0.61), so its footprint is 1.0 m².
Dividing 1,000 L by the starting 79.6 L/min would suggest only 12.6 minutes — half the real figure, because the flow slows as the level falls. If a pump empties the same tank at a steady 40 L/min instead, it takes 1,000 ÷ 40 = 25 minutes.
❓ 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?
Real outlets lose flow to pipe friction, bends, fittings and partly open valves. For gravity drains, the flow also falls as the level drops — dividing the volume by the starting flow underestimates the time by half, which the Gravity drain tab allows for. For pumps, use the flow from the pump curve at your head, or measure it.
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?
It varies a lot with the outlet and what is attached to it. In theory a 25 mm sharp-edged hole with 2 m of water above it starts at about 112 L/min (Torricelli, Cd 0.61), but a real outlet with a valve, bends and a hose delivers much less, and the flow slows as the level drops. Pumps vary with head and model. The reliable answer is a bucket test at the outlet, or the pump curve at your head.
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?
For a gravity drain, try outlet sizes in the Gravity drain tab until the time fits — doubling the outlet diameter cuts the time to about a quarter. For a pumped drain, work out the flow needed (volume ÷ target time) and check the pipe with our Pipe Size Flow Rate Calculator.
✅ Sources & Review
Sources and review
The steady-flow method is simple division; the gravity method is Torricelli's law for flow through an orifice, integrated over the falling water level. References:
Discharge coefficients used here (about 0.61 for a sharp-edged hole, about 0.82 for a short pipe stub) are standard textbook values for water at ordinary temperatures.
Reviewed by John Mac, PE, Water Systems Specialist
Texas-licensed Professional Engineer with 13+ years in residential water storage and 180+ tank projects. John writes and reviews every calculator on this site.
