There are tables for getting a ballpark guess. In most cases, 10RM is about 75% of your 1RM I think. Reversing the math, 100 lb for 10RM would be about 133 lb for 1 RM (100 / 0.75). Of course, individual factors and composition will come into play as well.
This page has an interactive calculator and some of the formulae to use.
However, there are differences based on the exercise itself. Here is another chart to show a different conversion factor for squat, bench press & deadlift.
| Repetitions | Squat | Benchpress | Deadlift |
|---|
| 1 | 1.0 | 1.0 | 1.0 |
| 2 | 1.0475 | 1.035 | 1.065 |
| 3 | 1.13 | 1.08 | 1.13 |
| 4 | 1.1575 | 1.115 | 1.147 |
| 5 | 1.2 | 1.15 | 1.164 |
| 6 | 1.242 | 1.18 | 1.181 |
| 7 | 1.284 | 1.22 | 1.198 |
| 8 | 1.326 | 1.255 | 1.232 |
| 9 | 1.368 | 1.29 | 1.232 |
| 10 | 1.41 | 1.325 | 1.24 |