Enter tank volume and flow rate — mix any units you like
1 — Tank Volume
Tank Volume (total capacity to fill)
2 — Flow Rate (pump or pipe delivery rate)
Flow Rate (incoming water per minute/hour)
Fill Percentage (leave at 100% for a full tank)
%
Water used while filling (optional)
L/min
⚠ Please enter a valid tank volume and flow rate (both must be greater than zero).
WaterTankCalculator.com — Tank Refill Time Results
Your Results
Refill Time
—
minutes
Total Seconds
— s
Total Minutes
— min
Total Hours
— hr
Days
— days
Volume to Fill
— L
Net Fill Rate
— L/min
Refill Progress Visualiser
📖 How To Use
How to Use the Tank Refill Time Calculator
Getting an accurate refill estimate takes under a minute:
Enter the tank volume
Input your tank's total capacity — or use the volume you actually need to fill. Select the matching unit: litres, US/UK gallons, cubic metres or cubic feet. If you don't know the volume, use our Rectangular Tank Volume Calculator first.
Enter the flow rate
This is the rate at which water actually arrives at the tank. A pump's label usually shows its maximum flow at zero or low head; at the height you are pumping to, it delivers less — read the pump's performance curve, or time a bucket at the tank inlet. If people keep using water while the tank fills, enter that rate in the optional box.
Set the fill percentage (optional)
If you're not starting from empty, adjust this to reflect how much of the tank actually needs filling. For example, if the tank is already 40% full, set this to 60%.
Hit Calculate
Refill time appears instantly in seconds, minutes, hours, and days. Copy or print the results with one click.
Important: Always use the net delivery rate — the actual flow reaching the tank, not the pump's maximum. The height you pump to, pipe friction and fittings all reduce real flow, sometimes a lot. The simplest check is a bucket test: time how long it takes to fill a known container at the tank inlet, or use the water flow rate calculator.
📐 The Formula
Tank Refill Time Formula Explained
The calculation is straightforward — divide the volume of water needed by the rate at which water arrives:
Refill Time = Volume to Fill ÷ (Flow In − Water Used While Filling)
Time (minutes) = Volume (litres) ÷ Flow Rate (litres per minute)
Time (hours) = Volume (litres) ÷ Flow Rate (litres per hour)
Time (seconds) = Volume (litres) ÷ Flow Rate (litres per second)
This calculator converts all inputs to litres and litres per minute internally, then derives every other unit from the result, so mixed units — a tank in US gallons and a pump rated in m³/hr, say — work correctly. "Volume to fill" is the tank volume × the fill percentage. The method assumes a steady flow; if your flow varies (as with a pressure-switch pump or a supply that weakens at peak times), use the average.
Flow Rate Conversion Reference
Unit
Equals (L/min)
Typical Use
1 L/min
1 L/min
Small domestic pumps, water features
1 L/hr
0.0167 L/min
Drip irrigation, RO filters
1 m³/hr
16.667 L/min
Commercial pumps, industrial supply
1 US GPM
3.7854 L/min
US pump ratings, fire hoses
1 US GPH
0.0631 L/min
Slow-fill agricultural systems
1 UK GPM
4.5461 L/min
UK/Commonwealth pump ratings
Common Pump Flow Rates vs. Refill Time (1,000 L tank, full fill)
Pump Rating
Flow (L/min)
Refill Time
Typical Application
10 L/min
10
~100 min (1h 40m)
Small domestic booster pump
20 L/min
20
~50 min
Mid-range home pump
40 L/min
40
~25 min
Large residential/small commercial
5 US GPM
18.9
~53 min
Standard US utility pump
1 m³/hr
16.7
~60 min
European commercial pump
100 L/min
100
~10 min
Larger transfer pump
💡 When to Use This
Common Use Cases for the Tank Refill Time Calculator
🏠
Household Rooftop Tanks
Know exactly when your overhead tank will be full so you can schedule your pump timer — avoiding midnight overflow or morning dry-outs.
🌾
Agricultural Irrigation
Calculate how long to run a borehole pump to fill a field storage tank before the next irrigation cycle.
🏗️
Construction Sites
Estimate how long to run a water bowser pump to fill a concrete-mixing tank or dust-suppression reservoir.
🚒
Fire Fighting Reserves
Verify that a fire-reserve tank can be refilled to minimum capacity within a required time window using your supply pump.
🏊
Swimming Pools
Work out how many hours a garden hose or fill pump needs to run to top up your pool after evaporation or backwash.
🆘
Emergency Preparedness
Plan how quickly you can refill emergency water stores during a mains outage, using a tanker delivery or secondary source.
Pump Sizing Tip: If the calculated refill time is too long for your needs, use this formula in reverse — divide your required volume by your maximum acceptable fill time to find the minimum flow rate your pump must deliver. Then cross-reference with our Pump Horsepower & Flow Rate Calculator.
🧮 Worked Example
Worked example: topping up a rooftop tank while the house uses water
A 1,000 L rooftop tank is 40% full, so 60% needs filling. A bucket test at the tank inlet shows the pump delivers 18 L/min, and the household draws about 3 L/min while it runs.
Volume to fill: 1,000 × 60% = 600 L.
Net fill rate: 18 − 3 = 15 L/min.
Refill time: 600 ÷ 15 = 40 minutes.
With no water being used, the same top-up takes 600 ÷ 18 = 33.3 minutes. A full fill from empty at 18 L/min takes about 55.6 minutes — set a pump timer a little longer than that, or fit a float switch so the pump stops when the tank is full.
❓ FAQ
Frequently Asked Questions
How long does it take to fill a 1,000-litre water tank?
It depends entirely on your pump's flow rate. At 20 L/min it takes 50 minutes; at 10 L/min it takes 100 minutes; at 40 L/min it takes just 25 minutes. Use this calculator with your pump's rated flow to get the precise answer for your setup.
What flow rate should I use — the pump's rated maximum or something lower?
Use the actual delivery rate at your operating conditions, not the pump's maximum nameplate rating. Pump output drops as head (the height and pressure the water must be pushed against) and pipe friction rise, and the figure on the label is usually the maximum at little or no head. Read the flow at your head from the pump's performance curve, or — most reliable — do a bucket test at the tank inlet.
How do I calculate tank refill time manually?
Divide the volume to be filled by the flow rate, both in matching units. For example: 500 litres ÷ 25 L/min = 20 minutes. If your units don't match, convert first — 1 US GPM = 3.785 L/min, 1 m³/hr = 16.667 L/min. This calculator handles all conversions automatically.
What is a typical household pump flow rate?
Small domestic pumps commonly deliver somewhere around 10–50 L/min (about 2.6–13 US GPM) under working conditions, but it varies a lot with the pump and the head. Check the pump's performance curve at the height you are pumping to, not the headline maximum — or time a bucket at the tank.
Does pipe diameter affect refill time?
Yes. At the same water velocity, doubling the bore carries four times the flow, because area grows with the square of the diameter. Over a long pipe with the same pressure drop, the gain is even larger — about six times by the Hazen-Williams formula — because a bigger pipe also has less friction. However, the pump or supply pressure is often the real limit. A large pipe from a small pump won't significantly improve flow; both the supply line and the pump must be appropriately sized for your required fill rate.
How long does it take to fill a 500-gallon tank with a garden hose?
It depends on your water pressure and the hose's diameter and length, so time a bucket to find your rate. At 10 US GPM, a 500-gallon (US) tank takes about 50 minutes; at 5 GPM, about 100 minutes. Use the fill percentage field if the tank is already partially full.
Can I use this calculator for pools, septic tanks, or industrial vessels?
Yes — the formula applies to any container filled at a known flow rate, regardless of the liquid or the container type. Enter the total volume (use the appropriate volume calculator for irregular shapes) and your supply flow rate, then read the refill time. The main assumption is a steady flow throughout the fill; if the flow changes (as with variable-speed pumps or mains pressure that drops at busy times), use the average rate.
How do I find my pump's flow rate in L/min if it's rated in m³/hr?
Multiply the m³/hr figure by 16.667. So a 3 m³/hr pump delivers 3 × 16.667 = 50 L/min. Alternatively, select "m³/hr" from the flow rate unit dropdown in this calculator and enter the value directly — no manual conversion needed.
✅ Sources & Review
Sources and review
The method is basic arithmetic — volume divided by flow. The conversions and pipe-flow statements rest on these references:
NIST SP 811 — unit definitions and conversions (US and UK gallons, cubic metres, cubic feet).
The Hazen-Williams pipe-friction formula (standard water-supply hydraulics): for the same pressure loss per length, flow rises with diameter to the power 2.63, so doubling the bore gives about 22.63 ≈ 6.2 times the flow.
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.
