Sizing a softener in a manufactured home starts the same way as in any other house. You need your water hardness, your household's daily water use, and the number of days you want between regenerations. But many of these homes have a layout that limits which tank and valve you can install, so the fit questions matter as much as the grain number.
Start with your own numbers
Get a hardness reading from your own water, not from a general figure for your town. Then estimate gallons per day by counting the people in the home, showers, laundry loads, and dishwasher runs in a normal week. Multiply hardness by daily gallons to get the grains removed per day. Multiply that by the number of days you want between regenerations, and you have the grain capacity to ask about. Write this down before you talk to anyone, because it gives you a number to measure each quote against.
Find where the water enters
Manufactured homes often bring the main supply in through the skirting, a crawlspace, or a utility panel rather than straight up through a basement wall. Walk the route from the meter or the well pressure tank to the point where the line enters the house. Note the pipe size, whether there is a shutoff at that point, and whether you can reach the entry without removing skirting. The softener, its bypass, and both connections all need room on that line.
Check the crawlspace and the base
A full tank and its salt are heavy, and the drain has to run downhill to an approved discharge point. Ask whether the home sits on piers, a foundation, or a pad, and whether the crawlspace is tall enough for a technician to kneel and reach the valve. Ask who is responsible for opening the crawlspace access when a repair is needed later, and whether the installer will set the tank on a level base that does not put its weight on a thin floor section.
Questions to ask the installer
Ask which tank size and control type were chosen for your grain number, and why. Ask where the bypass will sit relative to the water entry, and how the drain line will reach the discharge point and whether your local code office needs to see the plan. Ask what will be done with the skirting and the crawlspace opening once the job is finished. Ask who you call for a service visit and whether that person can reach the valve from your crawlspace without taking the unit apart.
If you are comparing companies in the St. Louis area, the St. Louis company ratings page shows how each company scores, and the rating methodology page explains the criteria behind those scores. Use them to build your shortlist, then use the fit checks above to test each company's written estimate.