Fleet Mass Calculator spreadsheet. Requires a Google account.

WARNING: this sheet doesn't magically know if a hole in your chain has been massed prior to your use of it. If you get rolled out, get right with Bob.

Pick ships and amounts and you're good. Change the hole mass reduction percentage if you want more or less leeway in the conservative transit counts.

If you have a fleet out, use Fleet Comp/DScan Checker subsheet. Also, dscan.info gives a mass readout in tonnes at the bottom-right (works with dscan and fleet comps).

If you're going through JS or filamenting around NS to explore/roam, consider taking some exfiltration packs.

An example of what the sheet does: