What gas pipe sizing is actually protecting
An appliance regulator needs a minimum supply pressure to hold its outlet setting. A residential system typically leaves the meter at 7 inches of water column, and the appliance needs about 5 inches at its inlet under full fire. That is the whole design problem: the pipe may consume only the small difference, and the code writes it down as an allowable pressure drop - normally 0.5 in WC, or 0.3 in WC where the equipment is fussy.
Undersized pipe does not simply deliver less gas. It starves the burner when several appliances fire together, which produces incomplete combustion, sooting, delayed ignition, and in the worst case carbon monoxide. The failure is intermittent and appears only at peak load, which is exactly what makes it dangerous - a system that tested fine with one appliance running can be unsafe on the coldest morning of the year when the furnace, the water heater and the range are all on.
Note that the answer depends on both quantities together. A 1/2 inch line carries 172 CFH over 10 ft and only 50 CFH over 100 ft, because pressure drop accumulates along the run. There is no such thing as the capacity of a pipe size; there is only the capacity of a pipe size over a stated length at a stated drop.
Once you have the flow, converting from heat to volume is arithmetic. Divide the total input rating in BTU per hour by the heating value of the gas. Pipeline natural gas is around 1,030 BTU per cubic foot, so 200,000 BTU/hr is about 194 cubic feet per hour. The CFH to BTU converter handles that step on its own, and the BTU load calculator totals the appliances.
The longest length method, and why it looks wasteful
NFPA 54 offers several sizing routes. The one this calculator implements is the longest length method, which is the simplest and the one most inspectors expect to see on a residential job.
You find the developed length from the meter to the most remote appliance - the longest run in the system - and then size every section of the system using that single length, with each section carrying the total load downstream of it. A branch two feet long feeding a range is sized as though it were at the end of the longest run.
This looks wasteful, and it is deliberately conservative. It exists because it is impossible to get it wrong: any pressure drop calculated on the longest run is the worst case for every shorter path, so no appliance can be starved. The alternative branch length method sizes each branch on its own developed length and produces smaller pipe, at the cost of more bookkeeping and more ways to make a mistake.
Fittings add developed length. The rigorous approach is to look up an equivalent length for every elbow, tee and valve and add them to the measured run. The field approach, and what the percentage field here does, is to add a blanket allowance: 20% for an ordinary residential run, more where the pipe is full of changes of direction. If you have already added equivalent lengths, set the allowance to zero.
One important exclusion: CSST is not sized with this equation. Corrugated stainless steel tubing has a corrugated bore whose friction behaviour is nothing like smooth pipe, and its capacity is published by each manufacturer against an equivalent hydraulic diameter designation rather than a nominal size. Always use the manufacturer's table that came with the listed system, and follow their bonding requirements.
Worked example: 200,000 BTU/hr over a 60 ft run
A house has a 100,000 BTU/hr furnace, a 40,000 BTU/hr water heater and a 60,000 BTU/hr range. The furthest appliance is 60 ft of pipe from the meter, with a normal number of elbows, so use a 20% fitting allowance. The system runs at 7 in WC and the allowable drop is 0.5 in WC. The gas is 1,030 BTU per cubic foot at a specific gravity of 0.60, and the pipe is schedule 40 black iron.
- Total load. 100,000 + 40,000 + 60,000 = 200,000 BTU/hr.
- Convert to flow. 200,000 ÷ 1,030 = 194.2 CFH.
- Developed length. 60 × 1.20 = 72 ft.
- Pressure gradient term. (0.5 ÷ 72)0.541 = 0.0069440.541 = 0.06796.
- Try 3/4 in. Inside diameter 0.824 in, so 0.8242.623 = 0.6019 and capacity = 3987 × 0.6019 × 0.06796 = 163 CFH. Not enough for 194.2.
- Try 1 in. Inside diameter 1.049 in, so 1.0492.623 = 1.1337 and capacity = 3987 × 1.1337 × 0.06796 = 307 CFH. That clears it.
- Answer. 1 inch schedule 40, carrying 307 CFH or 316,000 BTU/hr, leaving 36.8% spare - room for a future dryer or fireplace on the same main without resizing.
Under the longest length method every section of that system is now sized against 72 ft. The main from the meter carries the whole 194.2 CFH and is 1 inch. The branch to the water heater carries 40,000 BTU/hr, or 38.8 CFH, and 1/2 inch pipe carries 78 CFH over 72 ft at a 0.5 in WC drop, so a 1/2 inch branch is adequate.
Reading the answer and knowing when to change the system
Look at the spare capacity figure as well as the size. A run sized with 5% headroom is a run that cannot take a single additional appliance, and adding a gas dryer or a fire pit later will mean replacing pipe rather than tapping into it. Where the next size up is cheap - and between 1/2 and 3/4 inch it usually is - buying the margin now is the sensible economy.
If no size on the list carries the load, you have three real options and only three. Shorten the developed length by rerouting or by moving the meter. Split the load onto two runs from the meter. Or convert to a 2 psi system, in which the meter delivers 2 psi to a small-diameter main and a line-pressure regulator at each appliance drops it to 7 in WC. The 2 psi approach is common in new construction precisely because the allowable drop becomes about 1 psi rather than 0.5 in WC - roughly 55 times more - and the pipe shrinks dramatically.
Check the meter as well as the pipe. A residential diaphragm meter has a rated capacity, commonly 250 CFH for the standard domestic size, and a system whose pipe is comfortable can still overrun the meter or the service regulator. Total the connected load and compare it against the meter rating, which the gas appliance BTU load calculator does directly, and confirm actual delivery by clocking the meter once the system is running.
For propane, do not simply change the specific gravity here. LP systems have their own capacity tables in NFPA 58 and NFPA 54, run at different inlet pressures - typically 11 in WC downstream of the second-stage regulator - and with a heating value of about 2,516 BTU per cubic foot, so the flow for a given load is less than half. Use the propane pipe sizing calculator instead.
Schedule 40 steel capacity in CFH, 0.60 gravity, 0.5 in WC drop
| Developed length | 1/2 in | 3/4 in | 1 in | 1-1/4 in | 1-1/2 in | 2 in |
|---|---|---|---|---|---|---|
| 10 ft | 227 | 475 | 894 | 1,835 | 2,750 | 5,296 |
| 20 ft | 156 | 326 | 614 | 1,261 | 1,890 | 3,640 |
| 40 ft | 107 | 224 | 422 | 867 | 1,299 | 2,502 |
| 60 ft | 86 | 180 | 339 | 696 | 1,043 | 2,009 |
| 80 ft | 74 | 154 | 290 | 596 | 893 | 1,719 |
| 100 ft | 65 | 137 | 257 | 528 | 791 | 1,524 |
| 150 ft | 52 | 110 | 207 | 424 | 635 | 1,224 |
| 200 ft | 45 | 94 | 177 | 363 | 544 | 1,047 |
The code's own printed tables are at 0.3 in WC as well as 0.5 in WC. Values at 0.3 in WC are these figures multiplied by (0.3/0.5) to the power 0.541, which is 0.754.
Mistakes that fail an inspection
- Sizing a branch on the branch length. Under the longest length method every section uses the length to the most remote appliance, not its own length. Sizing the two-foot range drop on two feet is the classic error.
- Using the appliance output rating instead of the input. A furnace labelled 80,000 BTU/hr output at 80% efficiency draws 100,000 BTU/hr of gas. Pipe is sized on input, which is the figure on the rating plate.
- Forgetting a future appliance. A gas dryer, a generator or an outdoor kitchen added later will not fit into a run sized with 5% margin. Where the load is likely to grow, size for it now.
- Sizing CSST from steel pipe tables. CSST has its own listed capacities keyed to the manufacturer's equivalent hydraulic diameter. Steel tables will overstate what it carries.
- Applying natural gas tables to propane. Propane is 2.5 times the specific gravity and 2.4 times the heating value, delivered at a different pressure. It has separate tables for good reason.
- Ignoring elevation change on long vertical runs. Natural gas is lighter than air, so a tall riser actually gains a little pressure going up. On a multi-storey building the code has a correction, and this calculator does not include it.
- Skipping the pressure test. Sizing does not prove tightness. The pressure test required by the code, at the pressure and duration your jurisdiction specifies, is a separate and non-negotiable step.
Which edition of the code applies to you
This calculator uses the low-pressure equation given in NFPA 54, the National Fuel Gas Code, which is co-published as ANSI Z223.1 and is reproduced in the International Fuel Gas Code. The equation and its exponents have been stable across recent editions, and the capacities it produces reproduce the printed tables in Chapter 6 to within about half a percent. Editions differ in adopted tables, permitted materials and CSST bonding requirements rather than in the hydraulics. Always work from the edition your jurisdiction has adopted, and treat anything a calculator tells you as preliminary until the plan review confirms it. Gas piping work is licensed work in most jurisdictions.
Key terms
- Developed length
- The measured run of pipe plus the equivalent lengths of the fittings in it, in feet. It is the length the pressure drop calculation uses.
- CFH
- Cubic feet per hour of gas. Divide the load in BTU per hour by the heating value in BTU per cubic foot to get it.
- Inches of water column
- A small pressure unit used throughout fuel gas work. One psi is 27.71 in WC, and a residential system typically runs at 7 in WC.
- Longest length method
- Sizing every section of a system using the developed length to the most remote appliance. Conservative, simple and the usual residential approach.
- CSST
- Corrugated stainless steel tubing. Flexible listed gas tubing whose capacity comes from the manufacturer's tables, not from the smooth-pipe equation.
Why gas sizing uses full connected load, not a demand factor
Anyone who has done an electrical service calculation brings a habit into gas piping that does not transfer: applying a demand or diversity factor to the connected load. Electrical load calculations reduce the sum of nameplate loads because not every circuit draws its rated current at once. NFPA 54 does not do this for fuel gas.
The load this calculator asks for - the sum of every appliance's input rating downstream of a section - is used exactly as entered, with no reduction for the likelihood that the furnace, the water heater and the range all fire at the same time. That is deliberate. A furnace starting the same moment the water heater is already running is not a rare coincidence on a cold morning; it is the condition the pipe has to survive without starving either appliance, and the code sizes for it directly rather than betting against it.
This is also why adding one more appliance to an existing system is not simply a matter of checking whether the meter and service regulator have spare capacity. Every section of pipe between the meter and the new appliance's tie-in point was sized against the old connected load. Increasing that load reopens the sizing calculation for the whole run, not just the new branch - which is exactly what the spare-capacity output on this page is for: it shows the headroom that already exists before anyone picks up a wrench.
The one place diversity does appear is at the meter and the utility's own distribution system, which are sized against a load survey across many customers rather than the sum of every connected appliance. That is the supplier's calculation, not the piping designer's, and it does not change how an individual branch is sized.