Find out exactly how long it takes to fill or drain your tank, how much water your pipes can deliver, and how much horsepower your pump really needs — six free tools, each showing its formula and working.
Managing a water supply system means understanding not just how much water a tank holds, but how quickly it fills, how long it will last before running dry, and whether your pipes and pump can keep up with demand. The Refill, Flow & Time category covers the moving parts of that picture — from tank fill and drain times, to how much a pipe can carry, to the pump power needed, to reading your water meter and estimating the bill.
These tools are used by homeowners troubleshooting low water pressure, plumbers designing residential and commercial pipe networks, irrigation engineers balancing supply to multiple zones, and facilities managers monitoring consumption and controlling utility bills. Each calculator uses a standard formula, works in metric and US units, and shows its working, a worked example and its sources, so you can check the numbers yourself. All six were reviewed by John Mac, PE.
🧭 Which Tool Do I Need?
How to Choose the Right Calculator
Start with the Tank Refill Time Calculator if your core question is "how long will it take to fill my tank?" You need to know your tank's volume (use our Volume calculators if you haven't measured it yet) and your supply or pump flow rate. The calculator converts both to matching units and gives you fill time in hours and minutes.
Use the Water Flow Rate Calculator when you need to measure or verify how much water a pipe, tap, or pump outlet is actually delivering. This is particularly useful for checking a pump or tap against its rated flow — a bucket and a stopwatch are all you need — or for checking the velocity of a known flow in a pipe.
Reach for the Tank Drainage Time Calculator in two scenarios: when planning maintenance (how long until I can work in the tank?) or when assessing whether gravity feed is fast enough for your application. It handles both a pump or outlet with a known flow and gravity drainage, where it uses Torricelli's law because the flow slows as the level falls.
The Pipe Size & Flow Rate Calculator is the right choice when designing or upgrading a pipe run. Try different inner diameters to see how much each carries at your pressure and length — an undersized pipe starves the outlet, an oversized one costs more than it needs to. It shows the friction loss and flags high velocities; the Water Pressure Calculator gives the pressure a tank height provides.
The Water Meter Reading Calculator is your go-to tool for billing verification, leak detection between readings, and tracking consumption trends over time. If consumption suddenly jumps between two readings, it may indicate a hidden leak — use the calculator to quantify the discrepancy before calling a plumber.
Choose the Pump Horsepower Calculator when selecting or replacing a pump motor. An undersized motor runs overloaded and hot, shortening its life. Enter the total head — vertical lift plus friction losses — and the pump efficiency from its curve, and the calculator gives the brake horsepower in HP and kW and suggests the next standard motor size.
Each calculator in this category uses a standard formula and shows its working on the page, so you can check the numbers yourself. The main references are:
Hazen-Williams equation for flow in water pipes (SI form Q = 0.2785 C D2.63 S0.54), as given in standard water-supply hydraulics texts.
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.
Last updated
❓ FAQ
Frequently Asked Questions
How do I calculate how long it takes to fill a water tank?
Divide the tank's total volume (in litres) by your supply or pump flow rate (in litres per minute). For example, a 2,000-litre tank fed by a 25 L/min pump fills in 80 minutes. If water is being used while the tank fills, subtract that from the supply rate first. Our Tank Refill Time Calculator handles the unit conversions, a partial start level and water used during filling.
What is a normal water flow rate for a home?
There is no single normal figure — it depends on your supply pressure, pipe sizes and meter. Fixtures give a guide: in the US, new showerheads are limited to 2.5 GPM (9.5 L/min) and WaterSense models to 2.0 GPM (7.6 L/min), and kitchen faucets to 2.2 GPM (8.3 L/min); WaterSense bathroom faucets use no more than 1.5 GPM (5.7 L/min). A garden hose on a good supply usually runs well above any of these. The simplest way to know is to time how long a bucket takes to fill — our Water Flow Rate Calculator does the maths.
How long does it take to drain a water tank by gravity?
Gravity drainage speed is determined by the outlet pipe diameter and the height of water above the outlet (hydraulic head). As water drains and the head drops, the flow rate slows — so drain time is not simply volume divided by a fixed rate. For a straight-sided tank the time to empty is exactly twice what it would be if the starting flow held steady. A 1,000-litre tank 1 m deep, draining through a 25 mm sharp-edged hole (discharge coefficient 0.61), starts at about 80 L/min and empties in about 25 minutes; a short pipe stub (0.82) is faster, about 19 minutes. A long outlet pipe with bends adds friction and slows it. Our Tank Drainage Time Calculator uses Torricelli's law for gravity drainage.
What pipe size do I need for a given flow rate?
Pipe sizing balances two competing factors: too small a pipe causes high velocity and pressure loss; too large a pipe is expensive and may cause low-velocity sediment buildup. A common design range for water supply lines is roughly 0.6–2.0 m/s. At 1.5 m/s, a pipe with a 20 mm bore carries about 28 L/min, 25 mm about 44 L/min, 32 mm about 72 L/min, 40 mm about 113 L/min and 50 mm about 177 L/min (flow = bore area × velocity). Then check your pressure can push that flow along your pipe length with our Pipe Size & Flow Rate Calculator.
How do I read a water meter and calculate my consumption?
Record the meter reading at two points in time — both in the same unit, typically cubic metres (m³) or kilolitres (kL), where 1 kL = 1,000 litres. Subtract the earlier reading from the later one to get total consumption. Divide by the number of days between readings for your daily average. Our Water Meter Reading Calculator converts between units, computes daily and monthly averages, and can estimate your bill based on a per-litre or per-m³ tariff rate.
How do I calculate pump horsepower for my water system?
Pump power depends on three things: the flow rate required, the total dynamic head (vertical lift plus pipe friction losses), and the pump's efficiency rating. In metric: shaft kW = (flow in L/min × total head in m) ÷ (6,116 × efficiency), or HP = (L/min × m) ÷ (4,562 × efficiency); in US units, HP = GPM × ft ÷ (3,960 × efficiency). Read the efficiency from the pump curve — small household pumps are often 40–55%, larger centrifugal pumps 60–80% near their best point. Then pick the next standard motor size, leaving a margin of 10–20%. Use our Pump Horsepower Calculator to step through this calculation with your own numbers.
Can a high flow rate damage my pipes or tank?
It can. High velocity wears pipes and fittings — copper tube makers, for example, commonly recommend no more than about 8 ft/s (2.4 m/s) in cold-water lines, and erosion-corrosion at bends is a known failure in copper. Fast flow also makes water hammer worse — pressure spikes when a valve closes quickly. In a tank, a fast inlet jet can stir up settled sediment, so the inlet is usually placed or diffused to fill gently, and a float valve shuts the supply off when the tank is full. Use the Pipe Size & Flow Rate Calculator to check velocity.
