How to make calculation field read only?

Hi, how do I make a calculation field read-only?  

Thanks,

Rebecca

Here's how:

1. Open the calculation field settings

2. Click the ...  to see more options

3. Go to Options tab

4. Check "Result field is read only"

Thanks.  I didn't realise there were any more options.  They are very hidden!

Is is possible for me to make read only the default for this field?

Yes, checking that option should make the calculation field read-only by default. :)