Calculate fill time, required flow rate, or pipe flow rate — with instant conversion between L/min, L/s, m³/h and GPM.
A flow rate calculator turns the basic relationship between volume, time, and speed into a number you can actually use: how fast water (or any liquid) is moving through a pipe, hose, or fixture. Flow rate — also called discharge rate — is the volume of fluid that passes a point per unit of time, usually expressed in liters per minute (L/min), liters per second (L/s), cubic meters per hour (m³/h), or US gallons per minute (GPM).
This calculator covers the three questions people actually ask: how long will it take to fill a tank, pool, or bathtub at a known flow rate; what flow rate do I need to move a certain volume in a given time; and what flow rate does a pipe of a given diameter deliver at a certain water velocity. It's useful for sizing irrigation systems, estimating pool or tank fill times, checking whether a pipe is large enough for a fixture, and converting between the flow rate units used by different manufacturers and countries.
The first two formulas are simple division: flow rate is volume divided by time, and fill time is volume divided by flow rate — the same relationship rearranged. The third formula, used in the "Pipe flow rate" mode, comes from fluid mechanics' continuity equation: the volume passing through a pipe per second equals the pipe's cross-sectional area multiplied by the average velocity of the water moving through it. Since area scales with the square of the radius, even small increases in pipe diameter produce large increases in flow rate at the same velocity.
A small paddling pool holds 300 liters and is filled with a garden hose delivering 18 L/min. Fill time = 300 ÷ 18 = 16.7 minutes, or about 16 minutes 40 seconds.
A drip irrigation zone needs to deliver 500 liters over 2 hours (120 minutes) to stay within the water supply's capacity. Required flow rate = 500 ÷ 120 = 4.17 L/min, well within what a single 1/2-inch line can supply.
A 20 mm pipe carries water at 1.8 m/s. Area = π × (0.010)² = 0.000314 m². Flow rate = 0.000314 × 1.8 = 0.000565 m³/s = 33.9 L/min — enough for a single bathroom fixture but tight for two fixtures running at once, which is why larger supply lines use 25 mm or wider pipe.
| Pipe diameter | Flow rate at 1 m/s | Flow rate at 2 m/s |
|---|---|---|
| 13 mm (1/2") | 8.0 L/min | 15.9 L/min |
| 19 mm (3/4") | 17.0 L/min | 34.0 L/min |
| 25 mm (1") | 29.5 L/min | 58.9 L/min |
| 32 mm (1 1/4") | 48.3 L/min | 96.5 L/min |
| 40 mm (1 1/2") | 75.4 L/min | 150.8 L/min |
| 50 mm (2") | 117.8 L/min | 235.6 L/min |
| 65 mm (2 1/2") | 199.1 L/min | 398.2 L/min |
| 80 mm (3") | 301.6 L/min | 603.2 L/min |
| 100 mm (4") | 471.2 L/min | 942.5 L/min |
Calculated with Q = π×(D/2)²×v. Household plumbing is typically designed for 1-2 m/s water velocity; use the calculator above for your exact diameter and velocity.
Flow rate equals volume divided by time: Q = V / t. For example, 1000 liters in 10 minutes is 1000 / 10 = 100 L/min. If you know pipe diameter and velocity instead, use Q = A × v.
Divide the container's volume by the flow rate: t = V / Q. A 10,000-liter pool filled at 15 L/min takes 10,000 / 15 = 666.7 minutes, about 11.1 hours.
Use Q = A × v, where A = π × (diameter/2)². A 25 mm pipe at 2 m/s delivers about 0.98 L/s, roughly 59 L/min.
Most plumbing codes recommend 1-2 m/s (3.3-6.6 ft/s) for cold water lines to limit noise, erosion, and water hammer; hot water lines run slightly lower, around 0.9-1.5 m/s.
All measure volume per time at different scales: 1 L/s = 60 L/min = 3.6 m3/h = 15.85 US GPM. L/min suits fixtures, m3/h suits pumps, GPM is standard in the US.
A 1/2-inch garden hose delivers roughly 15-20 L/min (4-5 GPM). Kitchen faucets run 6-9 L/min, bathroom faucets 4-8 L/min, and low-flow showerheads about 9.5 L/min (2.5 GPM) or less.
Flow rate scales with area, which grows with the square of diameter. Doubling diameter quadruples area, so at the same velocity a pipe carries roughly four times the flow.
Related but not the same — pressure pushes water, flow rate measures volume passed per time, which also depends on diameter, length, and friction losses. That's why this calculator uses velocity and area rather than pressure alone.
Use the bucket test: time how long a container of known volume takes to fill, then divide volume by time in the "Flow rate needed" mode. A 10-liter bucket in 40 seconds is 10 / (40/60) = 15 L/min.
The volume/time calculations are exact arithmetic. The pipe-diameter mode assumes uniform velocity and no friction losses — standard for quick estimates, but real-world flow can differ slightly in long or restricted pipe runs.