One formula, six very different answers
Change in length = coefficient × original length × change in temperature
Everything on this page comes out of that. The coefficient is in inches per inch per degree Fahrenheit, so keep the length in inches and the arithmetic works without conversion. Sixty feet is 720 inches; a 75 degree swing on copper at 9.4 millionths gives 720 × 75 × 0.0000094, which is 0.51 inches. Half an inch on a run you would not have thought moved at all.
| Material | Coefficient (in/in/°F) | Movement per 100 ft per 100 °F | Relative to copper |
|---|---|---|---|
| Cast iron | 5.8 × 10-6 | 0.70 in | 0.6x |
| Steel | 6.5 × 10-6 | 0.78 in | 0.7x |
| Copper | 9.4 × 10-6 | 1.13 in | 1.0x |
| PVC | 2.8 × 10-5 | 3.36 in | 3.0x |
| CPVC | 3.4 × 10-5 | 4.08 in | 3.6x |
| PEX | 8.0 × 10-5 | 9.60 in | 8.5x |
These are typical figures for each material class rather than exact constants. Plastics in particular vary between formulations, and a manufacturer's data sheet for the specific product beats any general table — the custom field exists for that. But the ordering is stable and it is what governs how each material gets installed. Metals move a little; rigid plastics move a few times more; PEX moves nearly ten times what copper does, which is why it is installed the way it is.
Anchors, guides and slack
Expansion only becomes a problem when the pipe is not allowed to expand. A run held loosely at both ends, free to slide through its supports, grows and shrinks without complaint. A run clamped hard at both ends has to put the movement somewhere, and the options are: bow sideways, push the fittings apart, or grind against whatever is holding it.
Rigid materials are handled by deliberately deciding where the pipe is anchored and where it is merely guided. Between two anchors the movement has to be absorbed by geometry — a change of direction in the run, a purpose-built offset, or a loop — and the dimensions of that geometry come from the pipe manufacturer's expansion data, since they depend on the material's stiffness as well as its movement. That is a lookup, not a guess, and it is the point at which the calculation stops and a data sheet starts.
PEX takes the opposite approach: rather than absorbing the movement in a designed feature, it is run with slack in it and supported in a way that lets it move. That is why a PEX run looks untidy compared to a copper one and why clamping it taut between two tight fasteners is a mistake. The material is flexible enough to accommodate its own movement, provided nobody removes its room to do so.
Support spacing is a code table, not a physics result
How far apart hangers go is not something you derive from the expansion figure. It comes from the pipe's own stiffness, the weight of the water in it, and the sag anyone is willing to accept, and the numbers are published as a table in whichever code your jurisdiction adopted. Those tables differ between codes and are sometimes amended locally.
The values this page uses are the ones in common circulation so the hanger count has something to run on. As orientation, metals span the furthest — steel a good deal further than copper — rigid plastics rather less, and PEX considerably less again at intervals measured in feet rather than tens of feet. Vertical runs are usually allowed longer intervals than horizontal ones because the pipe is carrying its weight in compression rather than in bending, with a support commonly required at each floor. Read your table.
Two things the count does not include. Fittings, valves and anything else heavy get supported individually rather than being treated as part of a span, and a hanger placed right at a change of direction is doing a different job from one in the middle of a run. And the weight the hangers carry is the pipe plus the water in it — the pipe volume calculator gives the water figure directly, and on large horizontal runs it is the larger of the two.
The noises
Most of what people notice about thermal movement arrives as sound. A ticking or crackling in a wall or floor as hot water runs is a pipe sliding through a tight hole in a joist or against a hanger, expanding a fraction of an inch at a time. Boring the hole slightly oversized, or sleeving it, is the usual fix, and it is trivial before the wall closes and awkward afterwards.
A bang is a different problem. That is water hammer — the pressure spike when a fast-closing valve stops moving water dead — and it has nothing to do with temperature at all. It responds to arrestors and to securing the pipe, not to expansion allowances, and confusing the two leads people to fix the wrong thing. Both, though, are easier to address while the framing is open. If you are chasing a noise in a finished wall alongside a damp patch, the water leak guide covers tracing the water rather than the sound. Plumbing work is regulated, and the requirements are not the same from one town to the next. There are several model codes, jurisdictions adopt different ones and then amend them, and the local authority having jurisdiction decides what applies to your building. Most permanent work needs a permit and an inspection. Anything that touches the water service, the gas line, or a backflow-prevention point is licensed work in essentially every jurisdiction. What is on this page is common working practice and arithmetic, not a requirement you can hold up to an inspector.
Questions people ask
How much does PEX expand compared to copper?
About eight and a half times as much for the same length and the same temperature change. PEX runs around 8.0 × 10-5 inches per inch per degree Fahrenheit against copper at 9.4 × 10-6. Over 100 feet with a 100 degree swing that is roughly 9.6 inches of movement against 1.1 for copper. It is the single most important practical difference between installing the two materials, and it is why PEX is run with slack and supported loosely rather than clamped tight and straight the way a copper line can be.
Does pipe diameter change how much it expands?
No. Linear thermal expansion depends on the material, the length and the temperature change, and not on the diameter. A 4 inch pipe and a half inch pipe of the same material, the same length and the same temperature swing grow by exactly the same amount along their length. Diameter matters for other things in this calculation — support spacing, because a larger pipe is stiffer but heavier, and the geometry of an expansion offset, because a stiffer pipe needs a longer leg to flex through the same distance — but not for the movement itself.
What temperatures should I use for the calculation?
The full range the pipe will actually see, not the operating temperature. For a hot water line the cold end is the pipe sitting unused at ambient, which might be 55 degrees in a basement or considerably less in an unconditioned crawl space in winter, and the hot end is the delivery temperature. The swing is what matters, and it is larger than people assume because they think of the hot state only. For a cold line in an unheated space the interesting swing may run the other way entirely, from summer ambient down to near freezing.
Do I need an expansion loop in a house?
On a typical residential run, usually not, because houses rarely have long straight uninterrupted runs between two hard anchors — the pipe changes direction every so often and those bends absorb movement on their own. Where it does come up is a long straight run through a chase or along a basement ceiling, particularly in CPVC or in copper on a hot line, with the pipe held firmly at both ends. If the calculated movement is a meaningful fraction of an inch and there is no change of direction between the anchors, the run needs somewhere to put it, and the offset dimension comes from the pipe manufacturer's data rather than from a general formula.
Is the ticking in my walls thermal expansion or something worse?
A rhythmic tick or crackle that starts when hot water runs and stops shortly after is almost always thermal movement — pipe sliding against a joist, a hanger or a hole that is too tight. It is a nuisance rather than a fault, although the abrasion is not doing the pipe any favours over decades. What is not thermal is a single loud bang when a tap or an appliance valve shuts, which is water hammer and a pressure problem. And a hiss, a running sound with nothing open, or a tick accompanied by a stain is a different investigation altogether.