Sizes low-pressure natural gas or LPG pipework segments by pressure drop, per BS 6891:2015 Annex A ("Guide to sizing pipework installations").
Tender-stage indicative. Verify final design to project spec and BS 6891:2015.
Simplification: segments split the allowance equally rather than BS 6891's full-tree iterative resize; PE/MDPE pipe is out of scope (BS 6891 excludes buried service pipework). For design guidance only. Always verify with a qualified engineer.
About this gas pipe sizing calculator
This free gas pipe sizing calculator sizes low-pressure natural gas or LPG (propane) installation pipework by pressure drop, following BS 6891:2015 Annex A (“Guide to sizing pipework installations”). Enter each segment’s length, a fittings allowance and the downstream appliance demand, and it selects the smallest standard copper or steel bore that keeps the run within the permitted pressure loss, then gives a pass/fail on the whole path. It is aimed at UK Gas Safe engineers, M&E contractors and designers sizing domestic and light-commercial gas carcassing at tender stage. Everything runs in your browser. Nothing is uploaded.
How gas pipe sizing is calculated
Each appliance’s NET demand is converted to GROSS (×1.1) and then to a volumetric flow via the gas’s gross calorific value. Pressure loss per segment uses the BS 6891:2015 equation (A.1) flow relationship, Q = 57.1×10⁻⁵ × √[p·d⁵ ÷ (s·L·f)], rearranged for the pressure drop p, with the smooth-pipe friction factor f from equation (A.2): f = fsp ÷ e², where fsp = (14.7519 + 3.5657X + 0.0362X²)−2 and X = log₁₀(Re) − 5. Each segment is given an equal share of the fixed design allowance (1 mbar natural gas / 2 mbar LPG, clauses 5.3.2 / 5.4.2) and sized to the smallest bore from the copper Table A.1 or steel Table A.2 that meets its share.
Frequently asked questions
Do I enter net or gross appliance demand?
Enter the NET heat input as marked on the appliance data plate. The calculator converts it to GROSS internally (×1.1) before working out the gas flow, because BS 6891 sizing is based on the gross calorific value. Using a gross figure directly would oversize the flow and the pipe.
Why 1 mbar for natural gas and 2 mbar for LPG?
These are the permitted total pressure drops between the meter or emergency control valve and the appliance inlet in BS 6891 (clauses 5.3.2 and 5.4.2). The tool splits that allowance equally across your segments and checks the summed loss along the worst path against it.
What are the limits of this method?
It splits the allowance equally between segments rather than running BS 6891’s full-tree iterative resize, and it covers copper and steel only, buried PE/MDPE service pipework is outside BS 6891’s scope. Treat the output as indicative and verify the final design against BS 6891:2015.