Skip to content
Calcapedia
Menu
Trades

Pipe Flow Calculator

Find how fast water moves in a pipe, how much a pipe carries, or what size you need, with standard copper, PEX, PVC and steel sizes.

Level: Grades 7–8

  • Common Core standard 7.G.B.4Know the formulas for the area and circumference of a circle and use them to solve problems; give an informal derivation of the relationship between the circumference and area of a circle.
  • Common Core standard 8.EE.A.2Use square root and cube root symbols to represent solutions to equations of the form x² = p and x³ = p, where p is a positive rational number. Evaluate square roots of small perfect squares and cube roots of small perfect cubes. Know that √2 is irrational.
Printable worksheet

Pipe Flow Calculator

Know the answer you need? Pick an input to work backward.

Inputs
Find

Result

Velocity

3.858 ft/s

10 gpm through 1 in Schedule 40 PVC moves at 3.858 ft/s (1.176 m/s).

Velocity
1.176 m/s

See the step-by-step solution ↓

Step-by-step solution

  1. Find the open area inside 1 in Schedule 40 PVC, since that's the space the water fills (inside diameter 1.029 in = 0.026137 m)

    A=π4×0.0261372=0.0005365227 m²

  2. Change the flow to cubic meters per second so all units match (1 gpm = 0.0000630902 m³/s)

    10×0.0000630902=0.0006309 m³/s

  3. Divide the flow by the area to get the water's speed

    0.00063090.0005365227=1.175909 m/s

  4. Show the speed in feet per second (1 ft = 0.3048 m)

    1.1759090.3048=3.85797 ft/s

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

What this calculator does

Find out how fast water moves through a pipe, how much a pipe can carry, or what size pipe you need. All three are tied together by one simple rule. The amount of water passing each second equals its speed times the area of the bore. The bore is the open space inside the pipe.

Plumbers, irrigation designers and pump installers use this rule all the time. It checks that a line isn't running so fast it bangs and whistles. It shows what flow a pipe can deliver at a sensible speed. And it helps pick the smallest pipe that will carry a given flow.

The calculator works the rule in any direction. Find the speed (velocity) for a flow through a chosen pipe. Or find the flow a pipe carries at a given speed. Or find the smallest inside diameter for a flow at a target speed. It also names the smallest standard size of your chosen material that's big enough.

It holds published bores for copper Types K, L and M, PEX, Schedule 40 and 80 PVC and Schedule 40 steel. It accepts flow in gpm (gallons per minute), L/min, L/s, m³/h or ft³/s.

Where it's used

Why this math matters outside the classroom:

  • Irrigation zones. A landscaper checks that a 12 gpm zone in 1 in Schedule 40 PVC flows at about 4.6 ft/s, under the 5 ft/s caution line.
  • Pump lines. Pump installers size suction pipe so the water moves slowly enough to avoid noise and cavitation at the pump.
  • Upsizing a main. A plumber finds 10 gpm at 5 ft/s needs at least a 0.904 in bore, so 1 in Schedule 40 PVC is the smallest that works.
  • Metric systems. An engineer finds that 1 L/s through a 25 mm bore moves at about 2.04 m/s.
  • Algebra practice. Students rearrange Q = V × πd²/4 to solve for d, taking a square root at the end.

Grade level and standards: Grades 7–8; Common Core State Standards for Mathematics 7.G.B.4, 8.EE.A.2. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.

How to use it

  1. Choose what to find: velocity, flow rate, or pipe size.
  2. Pick the pipe material and its size name, called the nominal size. Or enter your own inside diameter. You don't need this when finding the size.
  3. Enter the flow rate, the velocity, or both, with their units.
  4. Read the result in both US and metric units. When finding a size, you'll also see the smallest listed size that works and the water speed in it.

What the inputs mean

Pipe or tube
The bore sets the speed, not the size name printed on the pipe. A 1 in Schedule 80 PVC pipe has thicker walls, so a smaller bore than 1 in Schedule 40. The same flow moves faster through it.
Inside diameter
The width of the hole inside the pipe, not its outside or the size it's sold as.
Flow rate
Typical figures: a faucet 1 to 2.2 gpm, a showerhead up to 2.5 gpm, a garden hose 5 to 10 gpm, a small irrigation zone 5 to 15 gpm.
Velocity
When finding a size, enter the highest speed you're willing to accept. A lower target gives a larger pipe and less friction loss (pressure lost as water rubs along the pipe).

Formula and method

Q = V × A,   A = π ÷ 4 × d²
V = Q ÷ A
d = √(4 × Q ÷ (π × V))
US shortcut: V (ft/s) = 0.4085 × Q (gpm) ÷ d² (in)
1 gpm = 0.0000630902 m³/s;  1 ft = 0.3048 m

In the formulas, Q is the flow rate (volume per second) and V is the water's average speed. A is the area of the bore, and d is the inside diameter. π is pi, the number used for circles.

This is called the continuity equation for a full pipe. Whatever water goes in each second must pass every point along the pipe. So a smaller bore forces the same flow to move faster. The calculator changes everything to metric (SI) units, solves, and changes back. So any mix of units works.

The US shortcut comes from the same formula. A gallon is 231 cubic inches. So 1 gpm through a 1 in bore moves at 231 ÷ 60 ÷ (π ÷ 4) inches per second. That's 0.4085 ft/s. Irrigation charts round this number to 0.408.

Worked example

10 gpm flows through 1 in Schedule 40 PVC. Its average bore (inside diameter) is 1.029 in.

  1. Bore: 1.029 in = 0.0261366 m, so the area is π ÷ 4 × 0.0261366² = 0.000536523 m².
  2. Flow: 10 gpm × 0.0000630902 = 0.000630902 m³/s.
  3. Velocity: 0.000630902 ÷ 0.000536523 = 1.175909 m/s.
  4. In feet per second: 1.175909 ÷ 0.3048 = 3.857970 ft/s.

Result: The water moves at about 3.86 ft/s (1.18 m/s). That's comfortably under the 5 ft/s that pipe charts flag for caution.

Understanding the result

Speed is a quick health check on a water line. Slow water means the pipe is bigger than it needs to be, but it's quiet. Fast water is noisy and raises friction loss sharply. It also makes the pressure surge worse when a valve shuts quickly (water hammer, the banging you hear).

When the calculator suggests a size, it's the smallest listed bore at least as big as the minimum. Pressure lost along the run also matters. So for long runs, check the friction loss before you settle on a size.

Assumptions and limitations

  • The pipe runs full, and the velocity is the average across the bore.
  • Bores are the published standard or average inside diameters for each material.
  • It doesn't account for pressure, friction loss, height changes, or the fixtures and pump at either end. Use the friction loss calculator for pressure drop.
  • Drain lines that run partly full (gravity flow) behave differently. The plumbing code sizes them by fixture units, a rating for how much each fixture drains.
  • Plumbing systems must follow local code. A licensed plumber or engineer should design or approve them.

Frequently asked questions

How do I convert gpm to feet per second in a pipe?

Divide the flow by the bore area. Or use the US shortcut, V = 0.4085 × gpm ÷ d². Here V is the speed in ft/s and d is the inside diameter in inches. 10 gpm in a 1.029 in bore gives 0.4085 × 10 ÷ 1.0588 = 3.86 ft/s.

What velocity should water flow at in a pipe?

Design guides set their own limits for each use. They're often lower for hot water and for quiet home systems. Irrigation pipe charts mark speeds above 5 ft/s (about 1.5 m/s) for caution. Check the guide or code you're designing to.

Does a smaller pipe increase water pressure?

No. A smaller pipe makes the same flow move faster, and that increases friction loss. So the pressure at the far end is usually lower, not higher. A nozzle raises the speed of the water leaving it, which is a different effect.

How much flow can a 3/4 in pipe carry?

It depends on the speed you allow and the bore. 3/4 in Type L copper (0.785 in bore) at 5 ft/s carries about 7.5 gpm. At 8 ft/s, it carries about 12 gpm. Pressure loss over the run may limit it further.

Sources

How this calculator is checked

Method: Continuity Q = V·A for a full circular pipe; exact unit factors (1 US gal = 3.785411784 L, 1 ft = 0.3048 m). Matches the velocity columns of Rain Bird's PVC, steel and copper pipe charts (V = 0.408 Q/d²). The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

Embed this calculator

Put this Pipe Flow Calculator on your own website or blog for free. Paste this code where you want it to appear: