PEX Pipe Sizing Guide: How Much Pipe Do You Need?

PEX for Radiant Floor Heat: How to Calculate

Radiant floor heating systems embed PEX tubing in the floor (in a concrete slab, gypcrete topping, or between subfloor and finish floor) and circulate warm water through it. The amount of PEX you need depends on the floor area and the spacing between runs. The two most common spacings are 12 inches on center and 8 inches on center — closer spacing delivers more heat per square foot, which is needed in high-heat-loss areas like rooms with lots of glass or in colder climates.

The calculation is: square footage of the heated area divided by spacing (in feet) gives linear feet of tubing. At 12-inch spacing (1 foot between runs), a 200 sq ft room needs 200 linear feet of PEX. At 8-inch spacing (0.67 feet between centers), the same 200 sq ft room needs 200 divided by 0.67 = 300 linear feet. Add 10 to 15 percent for the runs from the manifold to the loop start point and back, plus the curves at each end that eat into straight-run efficiency. A 200 sq ft room at 12-inch spacing needs about 220 to 230 feet of PEX in total.

Each loop should be a single continuous run from the manifold and back — do not splice PEX within the slab or subfloor. Keep individual loops under 300 feet to maintain proper flow and even heat distribution. If your room or zone is large, divide it into multiple loops, each connected to the manifold separately. The ProBuildCalc PEX pipe calculator handles this — enter the room area, choose your spacing, and it outputs total footage and number of loops.

PEX for Plumbing: Sizing by Fixture and Flow

For domestic plumbing, PEX is sized by the fixture load it serves. The main trunk line from the water heater or main shutoff is typically 3/4 inch or 1 inch, depending on the number of fixtures in the home. Individual branch lines to single fixtures (a toilet, a bathroom sink, a washing machine) are typically 1/2 inch. A branch that serves multiple fixtures — say, a bathroom with a toilet, sink, and shower — may start at 3/4 inch before splitting to 1/2-inch branches at each fixture.

The home-run manifold system, where each fixture gets its own dedicated 1/2-inch line from a central manifold, simplifies sizing because every branch is 1/2 inch and you size the manifold supply line (usually 3/4 inch) based on total fixture count. For a typical 3-bathroom house with kitchen and laundry, budget about 400 to 600 feet of 1/2-inch PEX for the branches and 50 to 100 feet of 3/4-inch PEX for the supply trunk lines. Measure the actual run from the manifold to each fixture location — PEX is installed in continuous runs, so add 10 percent for routing around obstacles and framing.

PEX-A vs PEX-B vs PEX-C: Which to Choose

PEX tubing is manufactured three ways, and the differences affect flexibility, freeze resistance, and fitting compatibility. PEX-A (made by the Engel method) is the most flexible, has the best freeze resistance (it can expand instead of splitting in a freeze event), and uses expansion-style fittings that are inserted inside the pipe — these fittings reduce pipe diameter less than crimp fittings and are considered the best long-term solution. PEX-A is the most expensive type.

PEX-B (manufactured by the silane method) is less flexible, not as freeze-resistant, and uses crimp or clamp fittings that compress around the outside of the pipe. It is the most widely available and the most affordable, and it is completely suitable for most residential plumbing and radiant heat applications. PEX-C is the least flexible and least freeze-resistant and is used less frequently in residential work. For radiant heat embedded in slabs, PEX-A is typically specified because of its superior flexibility for bending loops and its better resistance to damage during installation. For plumbing, PEX-B is a practical and cost-effective choice.

Estimating Total PEX and Fittings

For a radiant heat project: calculate each zone's square footage, determine spacing, calculate footage per zone (area divided by spacing in feet, plus 15 percent for headers and bends), and sum the zones. Order PEX in coil lengths that match your zone runs — 300-foot and 500-foot coils are common for radiant applications. For plumbing, measure the distance from the manifold to each fixture location along the intended routing path, add 10 percent, and tally by diameter.

Fittings add cost and need to be counted separately. For a manifold system, count one manifold port and two push-to-connect or compression fittings (supply and return) per fixture. For traditional trunk-and-branch plumbing, count tees, elbows, and couplings based on your pipe routing. Ball valves at the manifold, expansion tanks for closed-loop systems, and air vents for radiant loops are additional line items. A plumbing or radiant heat contractor should itemize fittings in their bid — if they do not, ask for a material breakdown before signing.