Pipe Fill Calculator

Every pipe in a building holds fluid — and every drop adds weight, cost, and complexity. Whether you're figuring out how much water sits in a 40-foot hot water line, how much glycol to order for a radiant heating loop, or how much weight 200 feet of oil-filled steel pipe puts on your supports, you need the volume and mass numbers before you can move forward.

This pipe fill calculator gives you both. Enter the inner diameter, pipe length, and liquid density, and it returns the pipe's internal volume and the total mass of liquid inside — in whatever units you need. Gallons, liters, cubic feet, kilograms, pounds — just pick from the dropdowns.

It handles metric and imperial, and because you can set the liquid density yourself, you're not stuck assuming water. Calculate for oil, diesel, glycol solutions, brine, or any other fluid you're actually working with.

If you've ever had to drain a heating system for maintenance, you already know why pipe volume matters — that fluid has to go somewhere, and "somewhere" needs to be a container big enough to hold it.

But volume calculations come up more often than you might expect. Plumbers need them when sizing new installations. Engineers need them to calculate dead weight on pipe supports. HVAC technicians need them to know how much antifreeze solution to mix before filling a hydronic loop. Pool contractors need them to account for the water trapped in circulation piping when winterizing.

Here's one that catches people off guard: a 2-inch copper pipe running just 100 feet holds roughly 16 gallons of water. That's 136 pounds hanging from your brackets and hangers. Double that diameter to 4 inches and the same run holds about 65 gallons — over 540 pounds. The volume doesn't just double when the pipe gets wider. It quadruples, because diameter is squared in the formula. That's the kind of detail that can make or break a support design.

Knowing the exact volume also saves money. Ordering glycol for a heating system? You don't want to buy 50 gallons when the loop only holds 12. Running a flush on an industrial line? You need to know the pipe's capacity to plan your waste disposal.

The math itself is simple. Getting the right answer depends on using the right measurements — which is where most mistakes happen.

Pipe volume follows the standard cylinder formula:

Volume = π × (d / 2)² × L

Where:

• π (pi) = 3.14159
• d = inner diameter of the pipe
• L = length of the pipe

You can also write this as V = π × r² × L, with r being the inner radius (half the diameter).

Worked example: Say you have a 4-inch inner diameter pipe running 50 feet (that's 600 inches):

• Radius = 4 ÷ 2 = 2 inches
• Volume = 3.14159 × 2² × 600 = 3.14159 × 4 × 600 = 7,539.8 cubic inches
• Convert to gallons: 7,539.8 ÷ 231 = 32.6 U.S. gallons

To find the weight of liquid inside, multiply volume by density:

Mass = Volume × Density

For water at room temperature (8.34 lb per gallon):

• 32.6 × 8.34 = 271.9 pounds

That's the load your pipe supports need to carry when the system is full — and it's easy to underestimate without running the numbers.

1. Enter the inner diameter. Type in the internal diameter of your pipe and pick your unit — meters, centimeters, millimeters, inches, or feet. Use the actual inner diameter, not the nominal pipe size or outer diameter. (If you're unsure of the difference, check the reference table below.)
2. Enter the pipe length. Type in the total length of the pipe run and select the unit. Running multiple segments of the same diameter? Add the lengths together and enter the total.
3. Enter the liquid density. For water, 997 kg/m³ works for most situations. For other fluids — oil, diesel, glycol — use the density reference table further down this page, or look up the specific gravity of your liquid.
4. Read your results. The calculator shows two outputs:

• Mass of Liquid — total weight of the fluid the pipe holds
• Pipe Volume — total internal volume of the pipe

5. Switch output units using the dropdown menus next to each result. Need gallons instead of cubic centimeters? Just select it.

You're wondering why it takes so long for hot water to reach the kitchen. The hot water line from your tank to the faucet is ¾-inch copper, running about 40 feet.

• Inner diameter: 0.75 inches
• Length: 40 feet (480 inches)
• Volume = π × (0.375)² × 480 = 212.1 cubic inches = 0.92 gallons

That's nearly a full gallon of water cooling in the pipe between uses. Every time you turn on the hot tap, that cooled-off gallon has to flush out before the hot water arrives. Now you know exactly why you wait — and if you're thinking about a recirculating pump, you know the volume it needs to keep warm.

A pool contractor is winterizing a system and needs to know how much water is sitting in the plumbing, separate from the pool itself.

• Pipe: 2-inch inner diameter PVC
• Total run: 150 feet (1,800 inches)
• Volume = π × (1)² × 1,800 = 5,654.9 cubic inches = 24.5 gallons
• Weight: 24.5 × 8.34 = 204 pounds

That's 24.5 gallons that need to be blown out with compressed air before temperatures drop, or the pipes risk cracking. Knowing the exact number tells the contractor how long to run the blower and confirms the water is fully cleared.

An engineer needs to calculate how much hydraulic oil a short transfer line holds — both for inventory tracking and structural load calculations.

• Pipe: 6-inch inner diameter steel
• Length: 200 feet (2,400 inches)
• Liquid: Hydraulic oil (density: 870 kg/m³ = 7.25 lb/gal)
• Volume = π × (3)² × 2,400 = 67,858 cubic inches = 293.8 gallons
• Mass: 293.8 × 7.25 = 2,130 pounds

Nearly 294 gallons of oil at over a ton of weight. That number feeds directly into the structural engineer's support calculations and the procurement team's inventory planning. Getting it wrong in either direction causes real problems.

An HVAC technician is filling a radiant floor system and needs to know how much propylene glycol solution to prepare.

• Pipe: ½-inch inner diameter PEX
• Total loop: 300 feet (3,600 inches)
• Liquid: 50% propylene glycol (density: 1,040 kg/m³ = 8.67 lb/gal)
• Volume = π × (0.25)² × 3,600 = 706.9 cubic inches = 3.06 gallons
• Mass: 3.06 × 8.67 = 26.5 pounds

Three gallons of glycol solution — that's the exact amount to pre-mix. No guessing, no wasting a $40 jug of glycol, no running short mid-fill and having to stop the job.

A fire protection engineer needs the standing water volume in a main supply line for a commercial building.

• Pipe: 4-inch inner diameter steel (Schedule 40, actual ID: 4.026 inches)
• Length: 250 feet (3,000 inches)
• Liquid: Water
• Volume = π × (2.013)² × 3,000 = 38,162 cubic inches = 165.2 gallons
• Weight: 165.2 × 8.34 = 1,378 pounds

That's over 165 gallons and nearly 1,400 pounds of water sitting in a single pipe run. The structural team uses this for dead-load calculations, and the maintenance crew uses it to plan drain-down procedures during inspections.

Most pipe calculators assume everything inside is water. This one doesn't. Because the liquid density field is open, you can calculate mass for any fluid — from gasoline to molasses.

Here's a quick reference for the fluids people work with most:

Why does this matter in practice? Take a 100-foot run of 3-inch pipe. The internal volume is the same regardless of what's inside — about 23.3 gallons. But fill it with water and it weighs 194 pounds. Switch to diesel and it drops to 162 pounds. Fill it with a 50% glycol solution and it climbs to 202 pounds. When you're designing supports or calculating pump loads, those differences change your specs.

Temperature also plays a role. Water at near-freezing (4°C) is at peak density — 1,000 kg/m³. Heat it to 80°C for a high-temperature system and it drops to about 972 kg/m³. That's a 3% difference, which adds up in large systems.

One of the most common sources of error in pipe volume calculations is using the nominal pipe size instead of the actual inner diameter. They're not the same.

Nominal pipe size is a naming convention — a label that was loosely based on inner diameter decades ago but has since drifted. Here are the actual inner diameters for the pipe types you'll encounter most:

Notice how a "½-inch" steel pipe actually has an inner diameter of 0.622 inches — over 24% larger than you'd expect from the name. Use the nominal size in your volume calculation and you'll underestimate by roughly 55%. That's a significant error, especially when multiplied across long runs or entire systems.

When in doubt, measure the actual inner diameter with calipers. It takes ten seconds and avoids the most common mistake in pipe volume calculations.

Grabbing the outer diameter instead of the inner one. It's the single most frequent error, and it always overestimates your volume. The outer diameter includes the pipe wall, which doesn't hold any fluid. For a 2-inch Schedule 40 steel pipe, the outer diameter is 2.375 inches but the inner is only 2.067 inches. Using the OD would overestimate your volume by about 32%.

Trusting the nominal pipe size. That "1-inch pipe" on your spec sheet isn't 1 inch inside. It's a label, not a measurement. Always look up the actual inner diameter for your specific material and schedule — or better yet, measure it directly.

Ignoring fittings and valves. Elbows, tees, reducers, and valves all add small volumes. For a quick estimate, the straight-pipe number is usually close enough. For precision work — like calculating exact glycol fill quantities — adding 5-10% for fittings is a good rule of thumb.

Mixing units without converting. Diameter in inches and length in meters will produce nonsense. The calculator's unit selectors handle this for you, but if you're doing the math by hand, keep everything in the same system until the final conversion.

Forgetting that water density changes with temperature. Room-temperature water at 998 kg/m³ is fine for most jobs. But a high-temperature heating system at 80°C uses water at about 972 kg/m³ — roughly 3% lighter. On a large system with thousands of gallons, that 3% affects pump sizing and expansion tank calculations.

Formulas Used:

Pipe volume (cylinder volume): V = π × r² × L

Liquid mass: m = V × ρ

Where V is volume, r is the inner radius, L is the pipe length, and ρ (rho) is liquid density.

Common Unit Conversions:

Disclaimer: This calculator provides results based on the standard cylinder volume formula and assumes straight, uniform pipe. Actual capacity may differ slightly due to manufacturing tolerances, fittings, bends, valves, and internal surface conditions. For critical engineering applications — fire protection, structural load calculations, pressure systems — consult the relevant codes and a qualified engineer.