Winterizing Antifreeze and Sanitizing Calculator
How the antifreeze estimate works
Start with a base of one gallon for the pump, the main cold and hot trunk lines, and the city-water inlet on a trailer up to about 20 feet, then add for length and for every place water can sit:
| Item | Added | Why |
|---|---|---|
| Each foot over 20 ft | 0.05 gal | More PEX between the pump and the far fixtures. |
| Each sink | 0.25 gal | Hot and cold supply to the faucet plus a cup in the P-trap. |
| Each shower | 0.3 gal | Two supplies, a mixing valve, a long hose, and the trap. |
| Each toilet | 0.25 gal | Supply valve plus enough in the bowl to cover the blade seal. |
| Outdoor shower | 0.25 gal | Usually the longest, coldest run in the rig. |
| Washer hookups | 0.5 gal | Two valves and hoses that hold water. |
| Ice maker / fridge line | 0.25 gal | Thin line, easy to forget, expensive to split. |
| No water heater bypass | Tank capacity + 1 gal | Antifreeze has to fill the whole tank before any reaches the hot lines. Install a bypass instead. |
| Blown out first | −25% | Lines are mostly empty so antifreeze displaces less water and reaches fixtures sooner. |
The full procedure, in order, including the water heater drain and both methods, is in how to winterize a travel trailer. If the water heater will not light in spring, the bypass valve left in the winter position is the first thing to check; see the water heater guide.
Sanitizing math
The common manufacturer and extension-service recommendation is one quarter cup of regular household bleach (5 to 6 percent sodium hypochlorite) for every 15 gallons of fresh-tank capacity, mixed into a gallon of water first, pumped through every fixture until you smell it, and left for at least four hours (overnight is typical) before draining and flushing twice. Concentrated 8.25 percent bleach needs about 70 percent as much. The calculator scales from that ratio.