Ever stared at a pipe and wondered exactly how much water is sitting inside it? Whether you're draining a system, filling one up, or just trying to figure out why it's taking so long to get hot water to your upstairs bathroom, this calculator gives you the answer in seconds.

Enter your pipe's inner diameter and length, and you'll see the volume in liters along with how much that liquid weighs. The calculator defaults to water, but you can adjust the density for antifreeze, oil, fuel—whatever you're actually working with.

It works for any round pipe: copper, PVC, steel, PEX, or flexible tubing. The math doesn't care about the material—just the dimensions.

Pipe volume is the amount of space inside a pipe. That's it. It's how much liquid (or gas) can fit inside.

Simple concept, but it matters more than you might think. When you drain a water heater, that volume tells you how much water is coming out and roughly how long it'll take. When you're adding antifreeze to a radiant heating system, knowing the pipe volume means you buy the right amount—not too little (system fails) and not too much (money wasted). When your irrigation system takes forever to pressurize in spring, the pipe volume explains why.

The volume directly affects fill times, drain times, and how much stuff you need to buy. Getting it right saves time, money, and frustration.

Pipes are cylinders, so the math is straightforward:

Volume = π × (diameter ÷ 2)² × length

That's pi (3.14159) times the radius squared times the length.

Let's work through a real example:

Say you have a 2-inch diameter pipe that runs 50 feet.

1. Convert length to inches: 50 feet × 12 = 600 inches
2. Find the radius: 2 inches ÷ 2 = 1 inch
3. Plug into the formula: 3.14159 × 1² × 600 = 1,885 cubic inches
4. Convert to gallons: 1,885 ÷ 231 = 8.16 gallons

Water weighs about 8.34 pounds per gallon, so that pipe holds roughly 68 pounds of water when full. That's worth knowing if you're working overhead or calculating structural loads.

Step 1: Enter the Inner Diameter This is the measurement across the inside of the pipe—the hollow part where liquid flows. Not the outside. (More on why this matters in a minute.)

Step 2: Enter the Pipe Length The total length you're calculating. If you have multiple sections of the same diameter pipe, just add the lengths together.

Step 3: Set the Liquid Density (If Needed) The calculator assumes water (997 kg/m³). Working with something else? Swap in that liquid's density. There's a reference table below for common fluids.

Step 4: Get Your Results You'll see the volume in liters and the mass in kilograms. From there, you can convert to gallons, pounds, or whatever units you need.

Here's where people mess up their calculations—and it's an easy mistake to make.

The inner diameter is the hollow space where liquid actually flows. The outer diameter includes the pipe walls. You need the inner one.

The difference isn't small. Take a "1-inch" copper pipe (Type M):

• Outer diameter: 1.125 inches
• Inner diameter: 1.055 inches

Use the wrong number and you'll overestimate your volume by about 13%. On a 100-foot run, that's the difference between 4.5 gallons and 5.1 gallons. Enough to leave you short on antifreeze or confused about why your drain time estimate was way off.

How to find inner diameter when you can't see inside:

Wrap a flexible tape around the pipe to get the circumference. Divide by 3.14159 for the outer diameter. Then subtract twice the wall thickness.

Typical wall thicknesses:

• Copper Type M: 0.035" for ½" pipe, up to 0.058" for 2"
• Schedule 40 PVC: 0.109" for ½", up to 0.237" for 2"
• Schedule 40 Steel: 0.109" for ½", up to 0.154" for 2"

Yes, it's a bit tedious. But it beats buying the wrong amount of material or wondering why your numbers don't match reality.

Don't want to calculate? Here's a cheat sheet for standard Schedule 40 pipes:

Notice how quickly it adds up? A 4-inch pipe holds over 40 times more water per foot than a ½-inch pipe. Diameter matters a lot more than most people expect.

When you blow out pool supply lines before winter, you need to know when they're actually empty. A typical 50-foot run of 2-inch PVC holds about 8 gallons. If you're getting much more water out than that, you might have a low spot trapping water. If you're getting less, the lines might already be partially drained.

Radiant heating loops need precise antifreeze amounts. A 200-foot loop of 1-inch PEX holds roughly 9 gallons. For a 50/50 glycol mix, that's about 4.5 gallons of antifreeze concentrate. Buy 5 gallons to be safe—running short mid-fill means draining and starting over.

Every spring, the same question: why does it take so long for water to reach the far sprinkler heads? A 100-meter run of 25mm drip tubing holds about 49 liters. At a typical residential flow rate, you're waiting 2-3 minutes just to fill the pipe before any water comes out the other end. That's not a problem—it's just physics.

Gravity-draining a 6-foot section of ¾-inch copper? That's only 0.17 gallons—it'll empty in seconds. But 50 feet of 3-inch drain pipe holds over 19 gallons. With minimal slope, expect it to take a while. And all that water has to go somewhere, so have a bucket (or three) ready.

A 6-meter section of 150mm industrial pipe filled with crude oil (density ~870 kg/m³) holds 106 liters weighing 92 kilograms. That matters for everything from material costs to whether your pipe supports can handle the load. Different density, different weight—same volume.

Water is the default, but here's what to use for other fluids:

Density changes with temperature. For most jobs, these numbers are close enough. For precision work—chemical dosing, laboratory applications—look up the specific density at your operating temperature.

Triple-check which diameter you're using. Inner, not outer. This is the #1 source of calculation errors, and it's easy to grab the wrong number from a spec sheet.

"1-inch pipe" isn't 1 inch inside. Nominal sizes are naming conventions, not measurements. A "1-inch" Schedule 40 pipe has a 1.049" inner diameter. Always verify actual dimensions.

Fittings add volume. Elbows, tees, unions, and valves all hold liquid. For rough estimates, add 2-3% to your total. For precise work (like chemical dosing), calculate fitting volumes separately or use published values.

Watch your units. Mixing inches and centimeters is easier than you'd think, especially when you're pulling dimensions from different sources. Double-check before calculating.

Temperature matters (sometimes). Pipes expand slightly when hot. Liquids change density with temperature. For most practical work, you can ignore this. For precision applications at temperature extremes, it might matter.

This calculator gives you the theoretical volume based on the dimensions you enter. Real pipes have manufacturing tolerances. Fittings add volume. Measurements aren't perfect. Your actual volume will be close to the calculated number, but probably not exact.

For most applications—estimating materials, understanding your system, planning a project—that's plenty accurate. For critical work where precision matters (chemical dosing, safety calculations), verify with physical measurements or consult a professional.