Hi Lydia, if you just need to see the number, you can click at the bottom of your column and it to "Average", which should get you what you're looking for
If you need to actually use the number (e.g. sending it in an email), I'm afraid you'll need to either:
1. Link all of your records to a single record in another table and use a rollup formula there
2. Use a script (Pro plan only)