FREE TOOLS
Gas Pipe Sizing Calculator
Calculate the ideal pipe size for natural gas & LPG systems. Enter the gas supply and section details for a pressure drop value in mb.
Determine pipe sizes section by section with a total pressure loss.
Results:
2025 Heating & Plumbing Report
Exclusive data, trends, and industry insights for heating & plumbing pros.
How to use this gas pipe sizing calculator
Step 1: Enter a gross, net, or gas rate value. The other two will be automatically calculated for your reference.
Step 2: Enter the pipe length and all the fittings you plan on using for this section.*
Step 3: Some initial results will now appear on the right side. If the pressure drop displays red, the pipe diameter is too small. Adjust the pipe type until a green result appears. A green result indicates an acceptable pressure drop of less than 1 mbar for natural gas.
* For gas systems with multiple sections, you will need to add additional sections. Once you’ve added some information, the results will automatically update with separate and combined pressure loss figures.
Once your work is done, you’ll need to issue a certificate to your customer. Create professional gas certificates with Gas Engineer Software to finish your work on site, stay organised, and save time.
This calculator was built by the team at Gas Engineer Software for UK Gas Safe registered engineers. It uses the equivalent length methodology set out in BS 6891:2015 — the British Standard covering installation and maintenance of low-pressure gas pipework in domestic premises.
The pressure drop limit applied is 1 mbar for natural gas, in line with BS 6891:2015 requirements. Equivalent lengths for fittings are taken from the standard’s referenced tables.
This tool is intended as a professional aid for qualified, Gas Safe registered engineers. Gas pipe sizing must always be carried out by a competent person in accordance with the relevant British Standards and Gas Safe requirements. The results from this calculator should be verified against the applicable standard before installation.
Why use a gas pipe sizing calculator?
Save time & money
Our gas pipe sizing calculator will help you find the right size pipe far quicker than any manual calculation.
Get more done
The faster you know what you need to be doing, the quicker you get it done and move onto the next job.
Make fewer mistakes
Calculations done by hand are prone to human error. Get it right every time with a reliable
formula.
Your content goes here. Edit or remove this text inline or in the module Content settings. You can also style every aspect of this content in the module Design settings and even apply custom CSS to this text in the module Advanced settings.
How to manually calculate pipe sizes for gas systems
In fact, our calculator is designed to use the exact same methodology stipulated by the British Standards BS 6891:2015 document fo gas pipe sizing.
The first thing you’ll need is access to some gas pipe size datasheets. Up-to-date tables for copper and steel pipes using natural gas and LPG can be found in the BS 6891:2015 document.
Step 1: Calculate the maximum gas rate of the appliance. To do this, you can multiply the maximum heat input (kW) by 0.095.
Step 2: Add up the total pipe length. This will include straight length of the pipe (m) and any fittings, which have the following equivalent lengths:
| Steel | Copper | 45° Bend | 90° Bend | 90° Elbow | Tee (Entering) | Tee (Exiting) |
|---|---|---|---|---|---|---|
| ≤1/2 inch | ≤15mm | 0.15 | 0.20 | 0.40 | 0.75 | 1.20 |
| 3/4 inch | 22mm | 0.20 | 0.30 | 0.60 | 1.20 | 1.80 |
| 1 inch | 28mm | 0.25 | 0.40 | 0.80 | 1.50 | 2.30 |
| 1 1/4 inch | 35mm | 0.30 | 0.50 | 1.00 | 2.00 | 3.00 |
Step 3: Add up a total equivalent pipe length
Step 4: Cross-reference table A.1 or A.2 in BS 6891:2015 to select an appropriate pipe size and its corresponding pressure loss per metre figure.
Step 5: Multiply the pressure loss per meter number by your total equivalent length (m) for a total pressure loss.
Step 6: If the total pressure loss is greater than 1mbar, repeat the process with a larger pipe diameter.
Common Pipe Sizing Mistakes
Getting pipe sizing wrong isn’t just an inconvenience — it’s a Gas Safe issue. Here are the mistakes that come up most often.
Ignoring fittings
Every bend, elbow, and tee adds resistance. On a 10-metre run with six fittings, the equivalent length could easily reach 13–14 metres. That changes the pressure drop calculation significantly, and it can push you into a larger pipe diameter.
Using nominal bore instead of actual bore
Pipe sizing tables in BS 6891:2015 are based on actual internal diameter, not nominal size. A 22mm copper pipe has an actual internal bore of approximately 19.5mm. Using the nominal figure will give you a slightly optimistic pressure drop, which matters on longer runs.
Not accounting for future appliances
If a customer is likely to add another gas appliance in the future — a gas hob, an outdoor barbecue point, a second boiler — size the pipe to handle the future load now. Going back to upsize pipes after plasterwork is done is extra work.
Treating a multi-section system as one run
If your gas installation has multiple branches serving different appliances, each section needs its own calculation. The pressure drop in each branch must be calculated separately using the flow rate for that branch, not the total system gas rate.
Copper vs steel gas pipe — what’s the difference for sizing?
Both copper and steel are acceptable for domestic gas installations under BS 6891:2015, but they behave differently in a pressure drop calculation.
Steel pipe has a slightly rougher internal surface than copper, which creates more flow resistance at equivalent diameters. That means a steel pipe and a copper pipe of the same nominal size will give you a different pressure drop result — steel will always be marginally worse.
In practice, this difference matters most on longer runs or systems close to the 1 mbar pressure drop limit. For short domestic connections it’s rarely a deciding factor.
Copper is the default choice for most domestic gas installations. It’s easier to work with, widely available, and straightforward to pressure test. Most domestic boiler connections are made in 22mm or 28mm copper.
Steel is more common in commercial settings, where mechanical protection is required, or where pipe runs are exposed and copper would be at risk of damage. Steel is also used for primary meter connections in some installations.
Does pipe sizing work the same for LPG and natural gas?
No. Natural gas and LPG operate at different pressures and have different calorific values, which means the pipe sizing tables are different.
This calculator is built for natural gas installations following BS 6891:2015. Natural gas in the UK is supplied at approximately 21 mbar at the meter outlet, and the maximum allowable pressure drop across the installation is 1 mbar.
LPG systems — whether butane or propane — typically operate at higher supply pressures and use different flow rate tables. The relevant standard for LPG pipework in domestic premises is BS 5482. If you’re sizing pipe for an LPG installation, you should refer to that standard and use the correct LPG tables rather than natural gas figures.
Using natural gas tables for an LPG system (or vice versa) will give you an incorrect result.
Make your job easier with
Gas Engineer Software
Efficient quoting, scheduling & certificates. All in one place.
Gas Engineer Software helps you manage your heating & plumbing business and save hours each week on admin and paperwork.