Propane tank sizes, explained
Portable propane tanks are named by how many pounds of propane they hold when properly filled (to 80% of their water capacity, leaving room for the fuel to expand safely). Here's the common lineup:
20 lb tank — the grill standard
Holds about 4.6 gallons. This is the tank under almost every backyard grill, patio heater, and portable fire pit in America, and it's the only size the exchange cages carry. Every refill station fills these.
30 lb tank — the RV and heater workhorse
Holds about 7 gallons. Common on travel trailers (often in pairs) and for jobsite heaters and larger portable equipment. Exchange cages don't carry this size — a refill station is the way to fill it.
40 lb tank — food trucks and forklifts' bigger cousin
Holds about 9.4 gallons. Used by food trucks, roofing and construction crews, and big portable heaters. Again, refill only — no exchange option exists at this size.
100 lb tank — the cabin and farm tank
Holds about 23.6 gallons. Stands about four feet tall and serves cabins, workshops, farm buildings, and homes as a semi-permanent supply. Many stations on this site fill up to 100 lb — check the “largest tank filled” note on each location page — but call first about how you'll transport it: a full 100 lb tank weighs roughly 170 lb and must ride upright and secured.
The 12-year rule: recertification
Every portable DOT cylinder has its manufacture date stamped on the collar. 12 years after that date, the tank expires and no refill station may legally fill it until it's been requalified by an authorized facility — typically a visual inspection and re-stamping that extends it for a further period (commonly 5 years). An expired tank is the most common reason a refill gets refused, and it's a safety rule, not a station being difficult. Check the date on your collar before you haul a tank across town; if it's expired, your practical options are requalification, recycling the tank, or replacing it.
Which size should you buy?
Match the tank to the appliance's fitting and appetite: grills are built around the 20 lb, trailers around the 30 lb, and anything bigger is about how often you want to refill. Owning two 20 lb tanks — one in use, one full spare — is the cheapest insurance against a mid-cookout empty.