Water Tank Refill Time Calculator

How to Use This Calculator

The water tank refill time calculator works with any tank size — domestic storage tanks, overhead roof tanks, garden water butts, swimming pools, hot water cylinders, commercial storage vessels, and more. Simply enter your tank's total capacity and your water supply's flow rate, and the calculator instantly tells you how long filling will take.

If you're partway through filling, enter your current level. If water is draining out while the tank fills (for example a constantly-running outlet), enter the outflow rate so the calculator accounts for net inflow. Results are shown in hours, minutes, and seconds for practical use.

Water Tank Fill Time Formula

The fundamental formula for calculating tank fill time is straightforward. Understanding it helps you estimate refill times without a calculator and check whether your water supply is adequate for the tank size you've chosen.

Units That Work Together

The formula works as long as you keep units consistent:

• Litres ÷ Litres per minute (L/min) → answer in minutes
• Litres ÷ Litres per hour (L/hr) → answer in hours
• Gallons ÷ Gallons per minute (GPM) → answer in minutes
• Cubic metres ÷ Cubic metres per hour (m³/hr) → answer in hours

Worked Example 1 — 1000 Litre Domestic Tank

Worked Example 2 — Tank Filling with Simultaneous Drain

Worked Example 3 — Partial Refill

Tank Fill Time Charts by Flow Rate

These fill time charts show how long it takes to fill common tank sizes across a range of flow rates. All times assume the tank starts empty. Use these to quickly estimate refill time without entering values into the calculator.

500 Litre Water Tank — Fill Time Chart

1000 Litre Water Tank — Fill Time Chart

2000 Litre Water Tank — Fill Time Chart

5000 Litre Water Tank — Fill Time Chart

10,000 Litre Water Tank — Fill Time Chart

Summary Fill Time Chart (All Tank Sizes)

The table below shows fill time in minutes for the most common domestic and commercial scenarios.

How Long Does It Take to Fill a 1000 Litre Water Tank?

The 1000 litre water tank is one of the most common domestic and commercial storage sizes in the UK, India, and across Europe. It's widely used as an overhead tank, loft cold-water storage tank, garden irrigation tank, and emergency backup supply. Understanding its fill time is one of the most searched questions on this topic.

1000 Litre Tank: Detailed Fill Time Examples

1000 Litre Tank: Use Cases

1000 Litre Tank Dimensions

A standard 1000 litre IBC (Intermediate Bulk Container) measures approximately 1200mm × 1000mm × 1160mm (L × W × H). Poly round tanks at 1000L are typically 1200mm diameter × 1100mm tall. Knowing the dimensions helps you identify whether a tank is genuinely 1000L capacity by checking the manufacturer's specification plate.

How Long Does 1000 Litres of Water Last?

At average UK domestic consumption of around 150 litres per person per day, a 1000 litre tank supplies one person for roughly 6–7 days, or a family of four for about 1.5–2 days. This makes refill frequency an important factor in sizing your storage and confirming your supply's flow rate is adequate.

Household Water Tank Refill Times

Domestic water tanks vary widely in type, size, and typical refill rate. Whether you have a cold water storage cistern in the loft, an overhead header tank, or a pressurised unvented cylinder, the refill time depends on how fast your float valve or fill solenoid allows water in.

Average UK Domestic Mains Flow Rates

Cold Water Storage Cistern (Loft Tank)

Traditional UK properties have a cold water storage cistern in the loft, typically 100–230 litres in older homes, or 100–450 litres in modern properties. These are fed via a ball float valve from the mains. At 10–15 L/min fill rate, a 230L cistern takes roughly 15–23 minutes to refill from empty.

Overhead Roof Tank

Common in South Asia, Africa, and the Middle East, overhead tanks typically hold 500–2000 litres and are filled once or twice daily by a submersible pump. At a typical pump rate of 20–30 L/min, a 1000L overhead tank takes 33–50 minutes to fill.

How Long to Fill a Bathtub?

A standard bathtub holds 150–200 litres when full (80–120 litres for a half-bath). A typical UK bathroom tap or mixer fills at around 8–12 L/min. This means a full bath takes approximately 15–25 minutes to fill. Walk-in baths and freestanding tubs hold more water (200–350L) and can take 30–40 minutes at typical domestic flow rates.

Understanding Water Flow Rate and Its Impact on Fill Time

Flow rate is the single most important factor in calculating tank fill time. Two tanks of identical capacity can have dramatically different fill times depending entirely on the flow rate of the supply feeding them.

Flow Rate Units Explained

How Pipe Size Affects Flow Rate

The diameter of the supply pipe is a major factor in achievable flow rate. Smaller pipes restrict flow even at high mains pressure. Here are typical maximum flow rates by common UK pipe sizes at standard mains pressure (3 bar):

How to Measure Your Flow Rate

You can measure your tap or hose flow rate at home with a bucket and a stopwatch. Simply time how long it takes to fill a 10-litre bucket. Then: Flow Rate (L/min) = 10 ÷ Time in minutes. For example, if the bucket fills in 45 seconds (0.75 minutes), your flow rate is 10 ÷ 0.75 = 13.3 L/min.

Tank Emptying Time Calculator and Formulas

Just as important as knowing how long to fill a tank is knowing how long it takes to drain or empty one. Whether you're calculating drain time for tank maintenance, estimating how long a leak takes to empty your supply, or comparing inflow vs outflow rates, the same core formula applies in reverse.

Time Required to Empty Common Tank Sizes

Leak Rate Calculation

If your tank has a slow leak, you can calculate how long it takes to empty using the same formula. First, measure how much water is lost over a known period (e.g., mark the tank and check after 24 hours), then divide tank volume by that leak rate.

Inflow vs Outflow — Which Wins?

When a tank receives water while simultaneously draining, the result depends entirely on which rate is higher. If inflow exceeds outflow, the tank fills. If outflow exceeds inflow, the tank drains despite the incoming supply. If they're equal, the water level stays constant — a state sometimes called steady-state or equilibrium filling.

Water Heater Refill Time — How Long for a Hot Water Tank to Refill?

Water heaters and hot water cylinders have two separate time periods to consider: the time for the tank to physically refill with cold water, and the time for that water to heat up (recovery time). Here we cover both.

Hot Water Tank Refill Time (Cold Water Fill)

Most domestic hot water cylinders in the UK hold 120–210 litres. They refill via a mains cold-water connection or a cold-water feed cistern. At a typical flow rate of 10–15 L/min, a 180-litre cylinder takes 12–18 minutes to physically refill with cold water. Unvented cylinders (pressurised) refill directly from mains and are typically faster.

Water Heater Recovery Time (Heating Time)

After the tank refills with cold water, it needs to reheat. Recovery time depends on the heater's power output (kW for electric, BTU/hr for gas) and the volume of water to heat. A gas boiler heating a 180L cylinder typically takes 30–45 minutes to reach 60°C from cold. An electric immersion heater (3kW) on the same cylinder takes approximately 2.5–3 hours.

Reverse Osmosis (RO) Tank Refill Time

Reverse osmosis systems produce purified water much more slowly than a standard tap, which is why they use a small pressure storage tank. A typical domestic RO membrane produces 50–75 litres per day (roughly 0.035–0.052 L/min). Most under-sink RO storage tanks hold 5–12 litres.

As a tank fills with RO water, back pressure builds against the membrane, which progressively slows production. Full tank fill times are therefore always longer than the membrane's rated output rate would suggest — often 2–4× longer once back pressure is factored in.

Swimming Pool and Reservoir Fill Time

Filling a swimming pool is one of the most common large-volume fill time calculations. Unlike domestic tanks where mains supply is usually adequate, pool filling often requires a hosepipe or bowser delivery, and can take many hours or even days.

How Long Does It Take to Fill a Swimming Pool?

A typical UK domestic swimming pool holds 45,000–90,000 litres (10,000–20,000 UK gallons). At a domestic hosepipe flow of 15–20 L/min, filling times range from 37 hours to over 4 days. Most pool owners use a dedicated fill hose at higher pressure or arrange a water bowser delivery to reduce this.

Rainwater Tank Fill Time

A rainwater harvesting tank fills based on rainfall intensity, roof catchment area, and tank volume. The formula for estimating fill time from rainfall is: Volume collected = Catchment Area (m²) × Rainfall depth (mm) × Runoff coefficient (0.85–0.95 for most roofs). A 50m² roof in a 25mm rainfall event collects roughly 1,062–1,187 litres — enough to fill a 1000L rainwater tank in a single decent downpour.

Common Refill Time Examples

Frequently Asked Questions

• How long does it take to fill a 1000 litre water tank? +
  A 1000 litre water tank takes approximately 1 hour 7 minutes to fill at a typical UK domestic mains flow rate of 15 litres per minute. At a slower 10 L/min it takes 1 hour 40 minutes, while a pump delivering 40 L/min can fill the same tank in 25 minutes. Use the calculator above to enter your exact flow rate for a precise answer.
• How do I calculate water tank fill time? +
  Use the formula: Fill Time = Tank Volume ÷ Flow Rate. For example, a 2000 litre tank with a 25 L/min supply: 2000 ÷ 25 = 80 minutes. If the tank is already partially full, subtract the current volume first. If there's a simultaneous outflow, subtract the drain rate from the inflow rate before dividing.
• What is the formula for tank filling time? +
  The standard tank filling time formula is: T = V ÷ Q, where T is fill time (in minutes if Q is in L/min), V is the volume to fill in litres, and Q is the net inflow rate in litres per minute. For a simultaneous drain: Q = Inflow − Outflow.
• How long does a cold water tank take to fill up? +
  A typical UK loft cold water storage cistern holds 100–230 litres and refills via a ball float valve at around 10–15 L/min. From empty, this takes 7–23 minutes. Most loft tanks don't fully empty in normal use and only top up by a fraction of their capacity at a time, making refill times in practice much shorter.
• How long does it take to fill a bathtub? +
  A standard bathtub holds around 150–200 litres when full. At a typical bath mixer tap flow of 10–12 L/min, a full bath takes 13–20 minutes to fill. Most people run a half-depth bath of around 80–100 litres, which takes 7–10 minutes. Freestanding or oversized baths can take 25–40 minutes.
• How long does it take for a water heater to refill? +
  The cold water physical refill of a hot water cylinder (180L) takes approximately 10–15 minutes at domestic mains flow rates. However, the water then needs to heat: a gas boiler takes 30–45 minutes to reheat a fully cold cylinder, while an electric 3kW immersion heater takes 2.5–3 hours for the same volume.
• How long does it take to fill a swimming pool? +
  A typical domestic in-ground swimming pool holds 45,000–90,000 litres. Using a standard hosepipe at 15–20 L/min, filling takes 37–100 hours (1.5–4+ days). Using a higher-capacity fill hose at 100 L/min reduces this to 7.5–15 hours. Large pools are often part-filled using a water tanker delivery for speed.
• How long does an RO tank take to fill? +
  A typical domestic reverse osmosis storage tank (5–12 litres) takes 20–60 minutes to fill when empty, depending on membrane output (usually 50–200 GPD). As the tank pressurises, fill rate slows. Actual time to reach full pressure can be 2–4× longer than expected from the membrane's rated daily output.
• How long does it take to empty a water tank? +
  Use the same formula in reverse: Empty Time = Volume ÷ Drain Rate. A 1000L tank draining at 20 L/min empties in 50 minutes. A slow leak of 0.1 L/min would take roughly 7 days to drain the same tank. For tanks draining under gravity, the rate slows as the head pressure drops — the actual time is longer than a constant-rate calculation suggests.
• For the tank to be full, how long should the tap be running? +
  This depends on the tank's remaining empty volume and the tap's flow rate. Measure your tap's flow rate (litres per 10 seconds × 6 = L/min), calculate how much volume remains, and divide: Time = Remaining Volume ÷ Flow Rate. For example: 200L remaining at 12 L/min = 16.7 minutes of running tap.

Related Water and Pipe Calculators

These calculators work alongside the tank refill time tool to help you plan and size your water system correctly.