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Calculate how long a water tank takes to fill from tank capacity, current water level and pump flow. You can also find the flow rate needed for a target filling time.
Inputs
1
Calculation mode
Choose whether you want filling time, required pump flow, or water delivered during a known period.
2
Tank details
The calculator first determines how much water is currently in the tank and how much is required.
Enter the total capacity of the tank. Select the correct unit below.
Enter 0% for an empty tank. Enter the actual level if the tank is partly filled.
Use 100% for a full tank, or choose a lower target to avoid filling completely.
3
Flow and time
Use actual delivered flow whenever possible because pump catalogue flow can reduce when head and pipe losses increase.
Use the actual delivered flow at the tank if it is known.
Optional. Use this only when you want to reduce rated flow for head, pipe friction, fittings or other real-world losses.
4
Electricity estimate
Optional. Add pump power only if you also want estimated kWh and electricity cost.
Optional. Enter 0 if you do not want an electricity-use estimate.
How to use this calculator: Tank Filling Time Calculator
Calculate how long a water tank takes to fill from tank capacity, current water level and pump flow. You can also find the flow rate needed for a target filling time. The example below is calculated by this page's real engine from the displayed inputs.
1Select Fill Time, Required Flow Rate or Water Added. This changes the inputs and calculation without creating another calculator page.
2Enter the total capacity and choose litres, cubic metres, US gallons or UK gallons.
3Use either the current percentage or the quantity of water already stored. Use zero for an empty tank.
4For filling calculations, enter the final level you want. Use 100% for a full tank.
5For Fill Time enter pump flow. For Required Flow enter the desired filling duration.
6If the entered flow is a catalogue or rated value rather than measured delivery, you may apply a reduction for real-world conditions.
7This is optional. Enter HP, electrical-input kW or electrical-input watts.
8Check remaining water, effective flow, filling time or required flow, unit conversions and comparison tables.
9For final pump selection, compare the required flow with the manufacturer's pump curve at the actual total dynamic head.
Input guide and example values
Use values from the same measurement basis and time period. Conditional fields appear only when the related option is selected.
Input
Example value
Why it matters
What do you want to calculate?
Fill Time — I know tank size and pump flow
Select the option that matches the real installation or scenario.
Tank capacity
5000
Enter the total capacity of the tank. Select the correct unit below.
Tank capacity unit
Litres (L)
Select the option that matches the real installation or scenario.
How will you enter the current water level?
Tank level (%)
Select the option that matches the real installation or scenario.
Current tank level
20 %
Enter 0% for an empty tank. Enter the actual level if the tank is partly filled.
Target fill level
100 %
Use 100% for a full tank, or choose a lower target to avoid filling completely.
Pump / water flow rate
50
Use the actual delivered flow at the tank if it is known.
Flow rate unit
Litres per minute (LPM)
Select the option that matches the real installation or scenario.
Estimated real-world flow reduction
0 %
Optional. Use this only when you want to reduce rated flow for head, pipe friction, fittings or other real-world losses.
Pump / motor power
0
Optional. Enter 0 if you do not want an electricity-use estimate.
Formula, derivation and worked example
Tank filling time is controlled by the volume of water still required and the actual flow reaching the tank. A 5,000 litre tank does not always require 5,000 litres: if it is already 20% full, only 4,000 litres are needed to reach 100%. The calculator converts litres, cubic metres and gallons to one internal unit, converts common flow units to litres per minute, applies any user-selected flow reduction, then calculates time or required flow. Pump HP by itself does not determine filling time because actual discharge depends on head, pipe diameter, friction, fittings and the pump performance curve.
Water required = target water quantity − current water quantity
Read the main result together with supporting checks, assumptions, limits and intermediate values.
For a manual check, repeat the first equation, confirm the units and change one input at a time.
Common mistakes when using Tank Filling Time Calculator
Do not mix units for Current tank level (%), Target fill level (%), Estimated real-world flow reduction (%). A unit mismatch changes the input magnitude even when the typed number looks reasonable.
Do not leave What do you want to calculate? on the default choice unless that choice matches the real scenario; the selected option can change the calculation path or factor.
Do not replace the displayed Water required = target water quantity − current water quantity relationship with a different convention without also changing the underlying assumptions; compare like-for-like methods when checking the result.
Do not treat Estimated filling time = 1 hour 20 minutes from the worked example as a universal answer. It belongs to the displayed example inputs and must be recalculated for the actual case.
Next logical calculator
Continue with Water Pump HP Calculator
Water Pump HP Calculator is directly connected from Tank Filling Time Calculator as a source-defined continuation or comparison.
The basic calculation is simple when the actual flow reaching the tank is known. First determine how many litres are still required. Then divide that remaining volume by the flow rate in litres per minute.
For example, a 5,000 litre tank that is 20% full already contains 1,000 litres. To reach 100%, it needs another 4,000 litres. At 50 L/min the calculated filling time is 80 minutes, or about 1 hour 20 minutes.
What Is the Tank Filling Time Formula?
The main formula is Filling Time = Water Required ÷ Effective Flow Rate. The volume and flow units must be compatible. This calculator converts the selected units internally before applying the equation.
For a partially filled tank, Water Required = Target Water Quantity − Current Water Quantity. This is more accurate than automatically dividing the tank's full rated capacity by the pump flow.
How Much Flow Is Required to Fill a Tank in a Target Time?
The same formula can be reversed. Required Flow Rate = Water Required ÷ Desired Filling Time. A completely empty 5,000 litre tank that must be filled in 60 minutes requires an effective flow of about 83.3 L/min.
If you expect the real delivered flow to be lower than the rated pump flow, the calculator can also apply a user-selected flow reduction and show the higher rated flow needed to maintain the target effective flow.
How Long Does a 1000 Litre Water Tank Take to Fill?
An empty 1,000 litre tank takes 50 minutes at 20 L/min, 25 minutes at 40 L/min, 20 minutes at 50 L/min and 10 minutes at 100 L/min.
These times assume the stated flow actually reaches the tank. If a pump loses discharge because of elevation or pipe friction, actual filling time will be longer.
Step-by-Step Worked Example: Filling a Standard 1,000 Litre Tank
Step 1 — Start with the tank size: a standard household tank holds 1,000 litres and it is completely empty, so water required = 1,000 litres. Step 2 — Measure or estimate the flow. A timing test shows the supply fills a 10 litre bucket in 20 seconds, so flow = 600 ÷ 20 = 30 L/min. Step 3 — Apply the fill time formula: filling time = water required ÷ flow = 1,000 ÷ 30 = 33.3 minutes, about 33 minutes 20 seconds.
The same steps work for a part-full tank or a different target. If the tank is already 25% full, water required = 1,000 × (100% − 25%) = 750 litres, and at 30 L/min the fill takes 25 minutes. To fill the empty tank in 20 minutes instead, reverse the formula: required flow = 1,000 ÷ 20 = 50 L/min. The calculator above performs each of these steps automatically once you enter the capacity, current level and flow.
How Long Does a 5000 Litre Tank Take to Fill?
From empty, a 5,000 litre tank takes 100 minutes at 50 L/min and 50 minutes at 100 L/min. If the tank is already 20% full, only 4,000 litres are required; at 50 L/min that takes 80 minutes.
This is why entering the current level can produce a much more useful result than using only nominal tank capacity.
Does Pump HP Determine Tank Filling Time?
Pump horsepower alone is not enough to determine filling time. Two pumps with the same motor HP can deliver very different flow rates depending on pump design and operating head.
For filling-time calculations, the most useful value is the actual flow reaching the tank. For pump selection, flow and total dynamic head should be checked together using the manufacturer's performance curve.
Does Pipe Size Affect Tank Filling Time?
Pipe diameter can affect the actual flow delivered by the pump. A smaller pipe generally produces higher velocity and greater friction loss for the same flow, which can move the pump operating point and reduce discharge.
If you already measured the actual flow at the tank, the pipe effect is already reflected in that measured value. In that case, you do not need to add another arbitrary pipe-loss factor to the filling-time formula.
Why Can Actual Filling Time Be Longer Than Calculated?
A pump catalogue may quote a high flow at low head, while the installed pump works against vertical lift, pipe friction, bends, valves and other losses. Voltage conditions, blocked strainers, worn pumps and restrictions can reduce flow further.
The most reliable practical method is to measure delivered flow at the tank or use the manufacturer's pump curve at the actual system head.
Tank Filling Time and Pump Electricity Use
If electrical input power is known, approximate energy use can be found from kW multiplied by operating hours. A pump drawing 1 kW for 90 minutes uses about 1.5 kWh.
When only motor HP is entered, this calculator converts mechanical motor rating to an estimated electrical input using the selected motor efficiency. The result remains an estimate because actual loading and efficiency change with operating condition.
How Long Does It Take to Fill a Tank in Gallons per Minute (GPM)?
In US units the formula is the same: Filling Time (minutes) = Gallons Required ÷ Flow Rate in GPM. A 500 gallon tank filled at 10 GPM takes 50 minutes, and the same tank at 5 GPM takes 100 minutes.
Select US gallons for capacity and US GPM for flow above, and the calculator handles the conversion for you. A typical residential well pump delivers roughly 5–15 GPM, while a garden hose usually delivers about 5–9 GPM.
Common US Tank Sizes and Fill Times
From empty at 10 GPM: a 50 gallon tank takes 5 minutes, a 100 gallon tank 10 minutes, a 275 gallon IBC tote about 27.5 minutes, a 500 gallon tank 50 minutes and a 1,000 gallon tank about 1 hour 40 minutes.
At 5 GPM those times double, and at 20 GPM they halve. For a partly filled tank, subtract the gallons already stored before dividing by GPM — the calculator does this automatically when you enter the current level.
How Long to Fill a Pool or Large Tank From a Garden Hose?
A garden hose at about 8 GPM delivers roughly 480 gallons per hour. Filling a 5,000 gallon pool from that hose therefore takes on the order of 10 hours, and a 10,000 gallon pool about 21 hours.
Hose flow drops with longer hose runs, smaller diameter and lower supply pressure, so measure the actual flow by timing how long it takes to fill a known container and enter that value for a realistic result.
Worked Example: Part-Full 500 Gallon Tank at 12 GPM
A 500 gallon poly tank is one-quarter full and must be topped up from a well pump measured at 12 GPM. Water required = 500 − 125 = 375 gallons, so filling time = 375 ÷ 12 = 31.25 minutes, about 31 minutes 15 seconds.
To finish the same fill in 20 minutes you would need 375 ÷ 20 = 18.75 GPM at the tank inlet. In metric terms that is 1,419 litres at 71 L/min, which is why capacity and flow units can be mixed freely above — the conversion is handled internally.
How to Measure Your Real Flow Rate in One Minute
Time how long the supply takes to fill a container of known volume. Flow in L/min = 600 ÷ seconds to fill a 10 litre bucket; flow in GPM = 300 ÷ seconds to fill a 5 gallon bucket. A 10 litre bucket filled in 12 seconds means 50 L/min.
Measure at the time of day you normally fill, because municipal pressure and borewell yield both vary. Entering a measured flow removes the need for any flow-reduction allowance and gives the most accurate filling time this calculator can produce.
Formula
Water required = target water quantity − current water quantity
Tank filling time is controlled by the volume of water still required and the actual flow reaching the tank. A 5,000 litre tank does not always require 5,000 litres: if it is already 20% full, only 4,000 litres are needed to reach 100%. The calculator converts litres, cubic metres and gallons to one internal unit, converts common flow units to litres per minute, applies any user-selected flow reduction, then calculates time or required flow. Pump HP by itself does not determine filling time because actual discharge depends on head, pipe diameter, friction, fittings and the pump performance curve.
Common Water Tank Filling Times
Approximate time from empty with constant delivered flow and no additional flow reduction.
Tank capacity
20 L/min
40 L/min
50 L/min
100 L/min
500 L
25 min
13 min
10 min
5 min
1,000 L
50 min
25 min
20 min
10 min
2,000 L
1 h 40 min
50 min
40 min
20 min
5,000 L
4 h 10 min
2 h 5 min
1 h 40 min
50 min
10,000 L
8 h 20 min
4 h 10 min
3 h 20 min
1 h 40 min
Formulas explained
Water required = (Tank capacity × Target %) − Current water
A partially filled tank does not need its full rated capacity. The calculation uses only the water required to move from the present level to the selected target level.
Tank capacity
Total usable capacity converted internally to litres.
Target %
Required final tank level expressed as a fraction of full capacity.
Current water
Water already stored, entered either as a percentage or quantity.
The target level must be greater than the current level when calculating filling time or required flow.
Fill time (min) = Water required (L) ÷ Effective flow (L/min)
Flow rate states how many litres reach the tank each minute. Dividing the remaining litres by litres per minute gives the required number of minutes.
Water required
Remaining volume to the selected target level.
Effective flow
Actual or adjusted flow reaching the tank in litres per minute.
If the flow entered is already a measured flow at the tank, keep Flow Reduction at 0%.
Required flow = Water required ÷ Desired time
This reverses the fill-time formula and calculates the flow that must actually reach the tank to achieve the selected filling duration.
Desired time
Maximum desired filling duration converted internally to minutes.
If a flow-reduction percentage is selected, the displayed rated-flow requirement is increased so the reduced effective flow still meets the target.
Energy (kWh) = Electrical input power (kW) × Time (h)
Electrical energy depends on actual input power and how long the pump runs.
Electrical input power
Entered directly in kW/W, or estimated from motor HP and motor efficiency.
Time
Pump operating time in hours.
This is an energy estimate only. Real input power varies with motor loading, voltage, efficiency and pump operating point.
GPM quick answer
From empty at 10 GPM: 100 gal = 10 min, 275 gal IBC tote ≈ 28 min, 500 gal = 50 min and 1,000 gal ≈ 1 h 40 min.
Typical US flow rates
Garden hose ≈ 5–9 GPM, residential well pump ≈ 5–15 GPM, water-hauling truck fill ≈ 100+ GPM. Measure if you can.
1000 L tank quick answer
From empty: 20 L/min = 50 min, 40 L/min = 25 min, 50 L/min = 20 min and 100 L/min = 10 min.
5000 L tank quick answer
From empty: 50 L/min = 100 min, 75 L/min ≈ 67 min and 100 L/min = 50 min.
Measured flow is best
If you know the flow actually entering the tank, use it directly and leave the optional flow-reduction setting at 0%.
HP is not the same as flow
Pump HP indicates power, not litres per minute. Actual discharge depends on the pump curve and system head.
2Current level = 20%, so the tank already contains 1,000 litres.
3Target level = 100%, so 4,000 litres are required.
4At an actual flow of 50 L/min, filling time = 4,000 ÷ 50 = 80 minutes = 1 hour 20 minutes.
Assumptions
Tank capacity represents usable liquid volume.
Flow remains approximately constant during the calculated filling period.
The entered current level reasonably represents the water already stored.
If flow reduction is used, it is a user-supplied planning allowance rather than a calculated hydraulic loss.
Electricity calculations are approximate and depend on the power information entered by the user.
Tips
Measure actual delivered flow at the tank with a calibrated container and stopwatch when practical.
If real flow is much lower than expected, check pump head, pipe size, fittings, valves and the pump performance curve.
Use Required Flow Rate mode when you know the tank size and the maximum acceptable filling time.
Use a suitable automatic level controller or float arrangement where unattended filling could cause overflow.
Warnings
Do not assume that a pump's maximum catalogue flow will be delivered at your actual installation head.
Very high inlet flow should be checked against the tank inlet, float valve, overflow arrangement and water source capacity.
Electrical energy and cost results are estimates and are not substitutes for measured motor input power.
Frequently asked questions
How do I calculate tank filling time?+
Subtract the current water quantity from the target water quantity, then divide the remaining litres by the actual flow in litres per minute. The result is the filling time in minutes.
How long does a 1000 litre tank take to fill?+
From empty, a 1,000 litre tank takes 50 minutes at 20 L/min, 25 minutes at 40 L/min, 20 minutes at 50 L/min and 10 minutes at 100 L/min.
How long does a 5000 litre tank take to fill?+
From empty, a 5,000 litre tank takes 100 minutes at 50 L/min or 50 minutes at 100 L/min. A partially filled tank takes less time because only the remaining volume is required.
How much flow is needed to fill a 5000 L tank in one hour?+
From completely empty, 5,000 ÷ 60 gives an effective flow requirement of about 83.3 L/min. If real-world flow is expected to be lower than the rated value, the required rated flow will be higher.
Does pump HP determine tank filling time?+
No. Pump HP is a power rating. Filling time depends mainly on the actual flow reaching the tank. Flow itself depends on the pump curve and the system head.
Why does my tank take longer to fill than the calculator predicts?+
Common causes include using a catalogue flow instead of actual delivered flow, high elevation head, undersized pipes, pipe friction, fittings, restrictions, low voltage or pump wear.
Can I calculate filling time for a partially filled tank?+
Yes. Enter the current water level as a percentage or quantity and select the desired target level. The calculator uses only the remaining volume.
Can I calculate the flow rate required for a target filling time?+
Yes. Select Required Flow Rate mode, enter the tank details and desired filling time, and the calculator returns L/min, L/h, m³/h and gallons per minute.
This calculator provides preliminary filling-time, flow and energy estimates. Actual tank filling depends on delivered pump flow, total dynamic head, pipe friction, fittings, water source conditions, pump condition and electrical supply. Verify final pump selection and installation details using manufacturer data and site measurements.