Question 1

What's the real difference between sizing for natural gas vs propane?

Accepted Answer

Natural gas has about 1,000 BTU per cubic foot while propane packs 2,500 BTU per cubic foot. So if you've got a 100,000 BTU furnace, you need 100 cubic feet per hour of natural gas but only 40 cubic feet per hour of propane. That's why you can often get away with smaller pipes for propane, but you need different regulators and the safety rules are stricter.

Question 2

How much pressure drop can I get away with?

Accepted Answer

Most places follow NFPA 54 or IPC, which lets you lose up to 0.5 inches water column on standard 7" WC systems. If you're running high pressure at 14" WC, you can lose up to 1.0" WC. But if you're in UPC territory, they're tighter - only 0.3" WC for standard and 0.8" WC for high pressure. Check what your local inspector wants before you start.

Question 3

How much gas can different pipe sizes actually carry?

Accepted Answer

For a typical 50-foot run, 1/2" black iron gives you about 56 CFH, 3/4" gets you 115 CFH, 1" handles 215 CFH, and 1-1/4" can do 375 CFH. Double that length to 100 feet and those numbers drop to 38, 80, 150, and 260 CFH respectively. Longer runs kill your capacity fast.

Question 4

Why can't I use this for CSST flexible gas line?

Accepted Answer

CSST is a whole different animal. Each brand - TracPipe, HomeRun, Gastite - has their own capacity charts because the corrugated tubing and fittings create different pressure losses. A 1/2" CSST typically carries 30-40% less gas than regular 1/2" black iron. You really need to use the manufacturer's own sizing charts or software.

Question 5

What safety margin should I build into my pipe sizing?

Accepted Answer

I always aim for at least 20-50% extra capacity if possible. So if I need 100 CFH, I want pipe that can handle 120-150 CFH. This covers future appliance additions, gives you some wiggle room for measurement errors, and keeps the inspector happy. Never size exactly to the load - that's asking for trouble down the road.

Question 6

When do I need permits and professional design?

Accepted Answer

All gas work needs permits and a licensed tech to do the installation - no exceptions. For design, most residential single-run jobs under 100 feet can use simplified methods like this calculator. But anything commercial, over 200 feet, or more than 1,000,000 BTU/hr needs an engineer to stamp the drawings. When in doubt, ask your local inspector what they want to see.

Question 7

How accurate is this calculator compared to engineering software?

Accepted Answer

For straight runs of black iron, this calculator is pretty solid - within about 15% of what you'd get from expensive engineering software. The pros use more complex equations that account for gas temperature, elevation changes, and all sorts of other factors. For most residential jobs, this simplified approach works fine. Complex commercial systems? Get an engineer.

Question 8

How do appliance BTU ratings affect pipe sizing?

Accepted Answer

You add up all the connected appliances - don't try to get fancy with diversity factors unless you really know what you're doing. Most residential furnaces run 80,000 to 200,000 BTU, water heaters are usually 32,000-76,000 BTU, and a typical gas range is around 65,000 BTU total. Add them all up and size your pipe for that total load.

Question 9

Why does pipe length matter so much in gas line sizing?

Accepted Answer

Friction is a killer in gas lines just like water pipes. Take a 1" pipe - at 10 feet it'll give you 520 CFH, but stretch that to 100 feet and you're down to just 150 CFH. That's a 71% drop! Long runs need bigger pipes, period. Sometimes it's worth planning a shorter route even if it costs more in fittings.

Gas Line Sizing Calculator