The short answer
A TDS meter reads total dissolved solids, not hardness; use hardness strips or a drop-count kit to size a softener.
The difference
| TDS meter | Hardness test | |
|---|---|---|
| Measures | All dissolved solids | Calcium and magnesium |
| Sizes a softener | No | Yes |
| Reads after softening | About the same | Near zero |
Hardness tests
Drop kits: Culligan 1035250 Hardness Test Kit, HYCHEK-21W Hardness Test Kit. Strips: On The Go 481108 Hardness Test Strips.
Why TDS doesn't drop
A softener swaps calcium and magnesium for sodium, so dissolved solids stay about the same while hardness falls.
Products mentioned
Questions readers ask
Can I use a TDS meter to test my water softener?
No; TDS stays about the same after softening. Use a hardness test.
What test do I need to size a softener?
A hardness test in gpg or mg/L, plus iron on well water.