Fleet Mass & Wormhole Throughput Calculator
Fleet Mass Calculator spreadsheet. Requires a Google account. File → Make a copy.
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 the Fleet Comp/DScan Checker tab or dscan.info's tonnage readout. Consider taking some exfiltration packs.