Roughly 6.7x its face value at a 10% return. A $500 receipt held for 20 years represents about $3,360 of tax-free withdrawal capacity.
The receipt itself doesn't grow. The money you didn't withdraw does — and the receipt is what lets you take that grown money out tax-free later.
At a 10% annual return, $1 left invested becomes:
| Years | Value of $1 | Value of a $500 receipt |
|---|---|---|
| 5 | $1.61 | $805 |
| 10 | $2.59 | $1,297 |
| 20 | $6.73 | $3,364 |
| 30 | $17.45 | $8,724 |
Read the last column as: the $500 you paid out of pocket, left invested instead, is worth that much later — and your receipt lets you withdraw it without tax.
Compare that to swiping the HSA card for the same $500. The expense is covered, and the compounding never happens.
Run your own numbers with the savings calculator. The assumption here is a 10% nominal return, which is optimistic in any given decade — the shape of the conclusion doesn't change at 7%.