Show the formula and working
This free pipe insulation calculator works out how many lengths of pipe lagging to buy for a run of copper pipe and estimates the heat and money the insulation saves each year, for homeowners and DIYers in the UK, Ireland and Europe insulating heating and hot water pipes in lofts, garages and floor voids. It covers 15, 22, 28 and 35 mm pipe, any insulation thickness and your own fuel cost.
Quick answer: A pipe insulation calculator adds an allowance for joints to the pipe length, then divides by the piece length. For 10 m of 15 mm pipe, buy 11 one metre lengths. With 13 mm foam, heat loss at 60 C in 10 C air falls from about 33 to 9 W per metre.
What Is a Pipe Insulation Calculator?
A pipe insulation calculator is a tool that tells you how much pipe lagging to buy and how much heat it will stop escaping from hot pipes.
The formula in plain words: multiply the pipe length by one plus the allowance for joints and bends, then divide by the length of each piece and round up. For the saving, the tool estimates the heat lost per metre of bare pipe from standard natural convection and radiation formulas, then the heat lost through the insulation layer using its thickness and conductivity. The difference, times the pipe length and the hours the pipe is hot, gives the kWh saved. Multiply by your cost per kWh of heat for the money saved.
How Do You Use the Pipe Insulation Calculator?
- Choose the Pipe size. Measure the outside diameter of the copper pipe, usually 15 mm or 22 mm in homes.
- Enter the Length of pipe to insulate and pick the Insulation length per piece you will buy.
- Keep or change Extra for joints and bends.
- Enter the Insulation wall thickness and the Insulation conductivity from the product label.
- Set the Water temperature, Air temperature around the pipe, Hours per year the pipe is hot and Cost per kWh of heat.
- Read the result and open “Show the formula and working” to see the pieces, the heat loss per metre and the yearly saving.
How Much Heat Does an Uninsulated Pipe Lose?
A bare 15 mm copper pipe carrying 60 C water through 10 C air loses roughly 33 W per metre, and a 22 mm pipe about 45 W per metre. Over 10 m that is 330 W, similar to a small radiator heating a loft or floor void instead of your rooms. The table gives the tool’s estimates with 13 mm insulation at 0.035 W/mK.
| Pipe size | Bare pipe loss | With 13 mm insulation | Reduction |
|---|---|---|---|
| 15 mm copper | 33.1 W/m | 9.3 W/m | 72% |
| 22 mm copper | 45.3 W/m | 11.8 W/m | 74% |
| 28 mm copper | 55.4 W/m | 13.9 W/m | 75% |
| 35 mm copper | 66.7 W/m | 16.2 W/m | 76% |
Conditions: 60 C water, 10 C still air, horizontal pipe. These are estimates from standard heat transfer formulas, and real losses change with draughts, pipe surface and how well joints are sealed.
What Thickness of Pipe Insulation Do You Need?
For most home pipework, 13 mm foam lagging is a sensible minimum, and 19 or 25 mm gives extra savings and frost protection in cold lofts. On a 15 mm pipe at 60 C, the tool estimates 11.1 W per metre with 9 mm, 9.3 W with 13 mm, 7.8 W with 19 mm and 6.9 W with 25 mm. New and replacement heating and hot water work must meet the pipe insulation requirements in Approved Document L.
Does Pipe Insulation Stop Pipes Freezing?
Pipe insulation slows the loss of heat, so it delays freezing, but it cannot stop a pipe freezing if the water stands still in sub zero air for long enough. Thicker insulation buys more time. For pipes in unheated lofts and outbuildings, combine lagging with loft insulation laid over, not under, the pipes so that warmth from the house reaches them.
Which Pipes Should You Insulate First?
Start with hot pipes that run through spaces you do not heat, such as lofts, garages, cellars and suspended floor voids, because that heat is fully wasted. Next, insulate the first metre or two of pipe from a hot water cylinder, and cold water pipes in cold places to protect them from frost. Pipes inside heated rooms matter least, since their heat still warms the house.
Worked Example: Lagging Heating Pipes in a Loft
Example: 10 m of 15 mm copper heating pipe runs through a loft at about 10 C. The water is at 60 C for 2,000 hours a year, the lagging is 13 mm foam at 0.035 W/mK sold in 1 m lengths, and heat costs an example 8p per kWh. These are the tool’s default inputs.
- Insulation to buy: 10 m x 1.10 = 11 m, so 11 pieces of 1 m.
- Heat loss: about 33.1 W per metre bare and 9.3 W per metre insulated, 72% less.
- Heat saved: (33.1 minus 9.3) W x 10 m = 238 W while the pipe is hot.
- Per year: 238 W x 2,000 h / 1,000 = 475 kWh, worth about 38.04 at 8p per kWh.
For the heat a room needs, try the room heat loss calculator. Any work that involves cutting into a gas or pressurised heating system should be done by a qualified heating engineer.
Pipe Insulation Calculator FAQs
How much pipe insulation do I need?
Measure the pipe run and add about 10% for joints, bends and offcuts. For 10 m of pipe, buy 11 m of lagging, which is eleven 1 m lengths or six 2 m lengths. Buy the size that matches the pipe’s outside diameter.
Is pipe insulation worth it?
Usually yes for hot pipes in unheated spaces. In the worked example, 11 pieces of foam lagging save about 475 kWh a year. Foam lagging is inexpensive, so the payback is often quick, but compare it with your own lagging price and heat cost.
What size pipe insulation fits a 15 mm pipe?
Buy lagging labelled for 15 mm pipe. The size refers to the bore, which matches the pipe’s outside diameter. Lagging for 22 mm pipe on a 15 mm pipe leaves an air gap and slips round, so it works less well.
What does the lambda value on insulation mean?
Lambda is the thermal conductivity in watts per metre kelvin. A lower number insulates better. Many foam pipe insulations are around 0.035 to 0.040 W/mK, but always use the figure on your product’s label.
How do I insulate pipe bends and tees?
Cut the lagging into short segments at 45 degree angles for bends, and cut a notch for tees, then tape every joint. That is why the calculator adds an allowance for joints and bends to the measured length.
Should I insulate cold water pipes too?
Yes, where they run through lofts, garages, outside walls or floor voids that can drop below freezing. Insulation will not save energy on cold pipes, but it slows freezing and reduces condensation drips.
Checked October 2026. Sources: GOV.UK, Approved Document L: conservation of fuel and power, Wikipedia, Pipe insulation, Wikipedia, Stefan Boltzmann constant.